Moped Troubleshooting Guide

Bringing 'em back to life!


Starting Problems     Storing Your Moped    Moped (PUCH) won't spark
Timing Adjustments   Clutch Problems  2-Speed Clutch Cable install
   Rust in Gas Tank  2-Speed Clutch Removal    1 Speed Puch Clutch Tuning  
Points and Plug Gap Settings and Puch Spark Plug Types
Carb Maintenance   Carb Constantly Leaks   Cylinder Head Replacement 
Puch 1980 6 Wire Stator Wiring Diagram
1976 - 1977 Complete wiring harness diagram 
Puch Engine Exploded Views

  Performance Tuning Tricks

Puch Engine Exploded Views

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Starting Problems: (directed towards Puch mopeds, but some info may pertain to others as well)

It is easiest to work on your Puch by removing the long side cover screws, medium side cover screws, and then the plastic side covers first.

1. Most important!!! Make sure the fuel has the correct mixture of 2-stroke oil in it!!!! Is this a new moped for you? If it is fuel that you personally did not place into the moped, don't use it. Don't take any chances by using it, or trust that the prior owner had a correct mixture, or that the fuel isn't too old. It's not worth losing the motor over a few bucks. The parts are very hard to come by. Drain it first by, turning off the fuel at the petcock. It is on the right side of the lower portion of the fuel tank. It should be in the full "down" position for a full fuel tank of fuel, or in the "straight up" position for a low fuel tank, (your reserve fuel) (also depending on what model you have). Place it in the off position, (horizontal) Take the fuel line off at the carburetor, and drain it out into a container. (Not a Tupperware! Avoid home troubles!) Replace the fuel with a 50-1 mixture of Regular Unleaded gasoline and 2-stroke oil. (disregard the "Do Not Use Unleaded Fuel" notice on the moped, this was placed on the bikes prior to the dissolution of leaded gasoline) You can use use Premium if you'd like. Try to start it again.  

2. When the switch is in the "on" position,  hold the "small" left clutch handle, on the left brake/clutch assembly, (opposite the throttle assembly) in tightly There is a brake handle looking arm on the left side, under the rear brake handle that needs to be pressed when starting it. It will make it hard to pedal when this handle is depressed, and you should here the engine turning over like it is trying to start. Crank the pedals as necessary for a few rotations. If it sounds like it's trying to start, repeat a few times. If it doesn't sound like it's "popping", continue to the next step. Also, if it does "pop" and you've done this a few times, continue to the next step. Don't wear yourself out trying to start it yet, you'll get discouraged quickly! Just continue onto the next step.  

3. Is the "fuel petcock" turned on?  Then,  go to the carburetor, and find the spring loaded, black plastic button on it. It is about 1/4 inches long. Not the 2" long, black rod, that is the "choke". With the "petcock" turned on, press the black plastic button down, several times, until fuel starts dripping from the bottom of the carb. (If you're not familiar with the "Priming" of the moped, it may start dripping from under the cylinder before you notice where it actually drips from on the carb, which will drip a bit more fuel out of it, so make sure you are doing this somewhere that it won't matter that fuel spills on it. Push down the 2" rod, the choke, until it is fully down. This choke will reset itself, when you turn the throttle after it starts. (in the up position, if not, pull it up manually after it is running for a minute or so). Go back to Step 2, and try again. Do this a few times, if it doesn't start after 5-10 times, go to the next step.  

4. If you are doing the above and it won't "fire", take the round cover off the side of the engine with the plastic "PUCH" emblem on it. (If you happen to have one in good shape). It will be held on with 2 small screws. DO NOT REMOVE the clutch cover, it has a cable going to a small arm on it. This will cause a real mess if you remove that one, unless the crankcase oil (automatic transmission fluid) is removed first. After you remove the "magneto cover", remove the spark plug from the cylinder. Place the spark plug back into the spark plug wire and "lay" the plug electrode on the cylinder, so that it gets grounded. Next, using your hand, turn the magneto, the round wheel inside the cover you just removed, clockwise as fast as possible. While it is turning, you should see the spark plug send a spark to the cylinder, or at least some kind of spark. Make sure you are not touching the plug or cylinder, that spark will give you an unreal awakening! That is, if it is sending a spark to the plug. If it is sparking, your in an easier position. If not, we have to go deeper. Replace the spark plug, being careful not to break the plug. Replace the plug wire, making sure it is on tight. Go back to step 2.

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Storing Your Moped

Depending on how long you may be storing your moped, there are a few items you should attempt before "sticking back in the shed/garage for an extended amount of time. Depending on what that "little bit of time" refers to, there are a few things you may want to do. Some a little more involved, if you plan on storing it for over 6 months or so. These are just some personal tips that I have done the past 20 years, so do not assume these are manufacturer's requirements that I am placing here.

In regards to the gas tank, your best bet would be to fill the tank with a very oil rich fuel, and then drain it, and discard, if it is going to be for over a month or so. The inside of the tank tends to rust, and this is one problem you need to try to avoid. 

If the moped will be sitting for over 6
months, I would recommend removing the carb, and carefully put a little 2-stroke oil into the intake, and turn the engine over by turning the magneto. (obviously you'll have to remove the magneto cover first, this is the round cover on the left side of the moped). Turn the engine over several times, to insure the oil spreads throughout the cylinder/piston. This will help preserve the rings from rusting and seizing to the cylinder wall.

And lastly, when you replace the carb on the intake, (IN FACT, IN ANY CASE), make sure the carb is
drained and void of any leftover fuel. Fuel that sits for an extended amount of time, turns into a mineral spirits type liquid, and may even thicken, clogging the jet and fuel inlet port. If it is to be stored in a damp area, a little WD-40 never hurts to put some in the points/magneto area, and possibly the complete engine, to preserve the exterior from pitting.

Cover the moped with a plastic sheet or moped/motorcycle cover.

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Timing

Remove the left cover (round, with plastic Puch emblem on it, if it's still attached, and held on by two screws.) Look inside the magneto, between the openings, and find the 3 screws holding the coils (stator, the part with the copper windings and points attached, which by the way, may be the problem, points do need to be changed on occasion). What you'll be doing is loosening the screws holding the stator to the block, and sliding the stator clockwise (retarding the timing) , or counterclockwise (advancing the timing) to adjust the timing. BE SURE TO MARK WHERE IT IS CURRENTLY, so that if you want to go back to the initial setting, you'll know where that was.
Mark it with a small screwdriver at one of the screws, by making a small scratch on the stator, right where the screw is, so you can find that place again.

Now, once the screws are loose, but not too loose, as it may not stay where you lave it, due to the magnets, so keep a little tension on the screws, but loose enough to "turn" the stator. Very small adjustments should be made at
a time, retighten the screws, and try out the moped. You don't need to replace the cover for testing purposes, unless you are riding in alot of dirt or something. Keep trying until you get it the way you want. There is
really no way to tell, other by performance of the moped, if the timing is right, unless you have special tools for a single cylinder, 2 stroke engine. Top Dead Center (TDC) is however, when the mark on the magneto (a line), is
at the 3 o'clock position, for most models, then you can adjust from there, but it should be pretty close to that position, since there really isn't much adjustment for the timing.

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Clutch Problems:

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Clutch cable installation for MKII (2-speed):

How to install the clutch cable (the cable outside the engine):
1. Turn the adjustment nut (for the short starting cable within the
engine) to the right until the rear wheel is dragging. 
2. Mark the position on the nut and sleeve.
3. From this mark, turn the adjustment nut 6-7 turns to the left.
If your clutch pads are severly worn, go another 1.5 turns  My clutch used to always slip and be a pain to start until I turned the adjustment nut a bit more

Make sure the inner clutch cable is in working condition.

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Rust in Gas Tank:

Rust in your gas tank can be one of the most irritating and most difficult to remedy, depending on the type of frame you have. If your gas tank is a separate unit, then the task is a bit easier. Tanks built into the frame is a bit more challenging. The best way to remove rust from the gas tank is by sandblasting. There are some chemicals that can be used, for example, Kreem, that can remove the rust, and a second chemical that seals the inner portion of the tank. Frames with the tank built in usually needs to be taken completely apart, removing the engine and so forth, in order to manipulate the moped more easily, as well as keeping the sand out of the nicks and crannies of the engine. If sandblasting is not possible, you can use small nuts and bolts and shake the daylights out of the tank, or the frame if it is built in. Removing the engine makes this easier in regards to the weight of shaking the moped. Once you have loosened the rust, flush well with cleaning fluids, mineral spirits, gasoline, (but be very careful if using flammables, dropping the frame or tank can cause sparks if dropped on a hard surface like cement or concrete. Once the rust has been flushed, replace any and all parts that were removed and completely fill tank with fresh gasoline and 2-stroke oil. The oil will help prevent future rusting. If you have a separate oil injection system, I would suggest to remove the oil from the reservoir, and mix the gas and oil with a 50:1 ratio, and once the tank has been emptied from use, go back to the oil injection system, if you prefer to. 

 

Clutch removal for MKII (2-speed):

Removing the 2-speed clutch is about the most difficult part to remove, and without special tools, it's almost impossible without assistance. It's a reverse nut, so you have to tighten it to loosen it. Bring it to a service station with a real fast and powerful air ratchet to break and vibrate it free. It can't be done by hand because the gear slips. There are special tools needed to hold the gears but sometimes the air wrench works.

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Clutch Tuning: (1-speed):

Adjusting the clutch can be a time consuming task, depending on how many times it takes to get it right. It's basically set upon your taste once adjustments are needed after the clutch breaks in, or gets alot of use.

Tools needed:
Phillips screwdriver (large)
Flathead Screwdriver (large)
17mm wrench (box end)
wheel puller (or use a hammer if YOU ARE familiar with removing flywheels or clutches with them. Serious damage can occur if you have never done this before. Wheel puller is HIGHLY recommended.

You'll need to start off with draining the transmission fluid from the block. Remove the large screw on the bottom of the block, having a catch pan there when the screw is removed, oil is gonna be everywhere if you don't. Remove the clutch cable from the clutch arm on the right side of the motor. Remove the 4 Phillips screws (as long as the original screws are still there) from the clutch cover. It's spring loaded, so it's a bit tricky to maneuver on and off the block. Move the clutch lever on the cover to remove and replace cover to the block. Remove the nut & washer holding the clutch onto the shaft. Pull clutch from shaft with wheel puller.

There is one flathead screw on each clutch pad. These are your adjusting screws. When adjusting, tightening the screw (clockwise, I believe) will give you more rpm's at take-off, loosening it will grab the clutch plate sooner, causing slow pick-up, bogging the engine down. You can make both (or 3 on some clutches) pads the same tension, (the same amount of turns on all screws from fully tight, or as I do, I turn the screws out, "counting the turns" until the screw is flush with the metal portion of the pad, and going back to initial settings, and adjusting each from there) giving an even take-off, but if you have one a little tighter than the others, having each at a different tension, you can get good results getting similar to a two speed transmission. One pad will engage earlier, and when the rpm's pick up a bit, the send pad will then engage, giving you more power at midrange. I haven't messed with a 3 pad series clutch in probably 20 years, so i don't recall if having all 3 pads at different tensions, if it worked similar to a 3 speed. ( I imagine it would if set properly, I'd actually like to find out, now that I think about it) Replace the clutch after you make a change, and don't adjust it too much, a single turn on the screws can make a big difference to its operation, either being too tight, or too loose. Replace all parts as they were removed, using silicon gasket sealer if you don't have a good gasket on the clutch cover, I use it anyway for security. DON'T FORGET to replace the tranny fluid with regular ATF transmission fluid. Fill on the side of the clutch cover, until it starts to flow back out, when the motor is in an upright position. Try it out and see how the change worked out. If not satisfied, try it again.

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Points and Plug Gap Settings

The points for Puch Mopeds should be set to 0.014 - 0.018 inches, or 0.35 - 0.40 mm.

The plug should be gapped at 0.016 - 0.02 inches, or 0.4 - 0.5 mm.

Version Bosch NGK Champion
17 mph W95T1 B-4H L-90
20 mph W95T1 B-4H L-90
25 mph W145T1 B-4H l-90
30 mph W175T1 B6H or B-6HS L-86

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Carb Maintenance:

Click here for an exploded view of the carb if needed

Remove the carb from the intake manifold with the screw holding to the manifold. Then, remove the two top screws of the carb, and carefully pull out the slide and choke. Careful, as the choke (a small, flat metal plate, about an inch long, will fall off, so before it does fall off, look to see how it is mounted to the choke button, (the 2-3 inch black plastic rod sticking out of the top of the carb) so you'll know how to replace it. Remove the fuel line. (make sure the fuel is either turned off at the petcock (fuel valve) or that the tank is empty, as the fuel will start running all over the place. Now, you should have the complete carb in your hand, other than the top part, the throttle slide (round thimble looking part, connected to the throttle cable) and the choke plate. Looking at the bottom of the carb, remove the round bowl with a wrench or a tool that will fit on the nut shaped object on the bowl, turning counterclockwise to remove. Clean out the complete bowl, and check the inside of the carb itself, where the float portion is located. (round marshmallow part) check the jet (brass colored screw, with a very small hole in it). See if you can see light through it. If not, try to blow air through it to clean it out. Remove the fuel nipple where the fuel line was connected. Inside the banjo bolt that you have removed holding the nipple, should be a brass colored part, which is a filter. It may not be there, as some folks remove it, instead of cleaning it and replacing it. Replace it, and replace the nipple. Make sure to get the round gaskets, on each side of the actual nipple, when replacing, or fuel will leak all over the place. Replace the bowl, once you are satisfied that it is clean. Replace the carb back on the 12mm or 14mm intake manifold, by leaning the carb sideways, in order to maneuver it into its correct location. Place the choke plate back on the choke rod, inside the top of the carb, where the cable goes through. Replace the slide (thimble looking part) into its place, making sure that the slot on the side of the slide, goes down over the little knot inside where the slide goes, and put the choke slide in at the same time. Tighten the screws at the top of the carb, replace the fuel line, turn the carb back up, top facing up, and tighten the screw holding the carb to the intake manifold. Turn on the petcock, (fuel valve), but make sure you have completely drained the tank, and replaced it with fresh fuel, with a 50:1 2 stroke oil mixture, added to the gas. Replace the gas cap. (locking gas cap)

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Carb Constantly Leaks:

Click here for an exploded view of the carb if needed

If you find that your carb is constantly leaking, this is one procedure you can try, if all other attempts fail, but be sure that you have looked the carb over, for dirt or sludge build-up. Before jumping to conclusions, thinking that you need a new carb, there may be a little trick to fixing it. You have to be somewhat careful not to overdue this procedure, or you may damage the float beyond repair.

Remove the float and float needle valve (small piece with rubber end, that stops fuel flow) from the carb. (Of course, you'll need to pull the float retaining pin out to remove the float first) Keeping the float needle valve in it's slot on the float, imagine how this piece actually works. The float floats up, the needle goes up, pushing the rubber into the fuel flow hole. Hold the float and valve in it's normal position when it is sitting in the carb, in it's upright position. Now, "slightly bend" the brass slip where the valve slides into, into the direction which would push the needle valve up further, (remember, only very small increments), and the direction should be in the up direction, if you are holding the float in it's correct position. Replace the float into the carb, replace the fuel line on the carb, but you shouldn't have to put the carb back on, in fact, you can probably do this from the petcock (right) side of the moped, just by pulling the fuel line from the other side of the ped. Turn on the petcock, holding the carb in an upright position, and see if the leak stops. Make sure the carb is level, or it won't do any good to do this test. If it stops leaking, replace the carb onto the intake manifold, replace the fuel line, and ride it around for a minute or two, stop, then shut it off, and insure that the leaking has stopped. If it hasn't stopped leaking during the initial test, remove the float, and bend the brass part up a little more. These bends should be VERY SMALL moves, not to bend it too far. Retest the carb, and continue until either the leak stops, or you have confirmed that the carb will not stop leaking, no matter what you do. Another important factor, is the actual float needle valve. Inspect the rubber nipple on the end of the valve pin, and see if it may be cracked, or worn considerably. If it is, it may be cheaper to try and replace this part first, before attempting to purchase a new carb.

If you have any further questions regarding this test, feel free to contact us here.

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Cylinder Head Replacement

Install the washers and nuts. Finger-tighten all nuts and then torque all nuts to 7 ft lbs. Tighten the top right nut first, then the bottom left, then the bottom right, then the top left, in that order. Right and left is determined by sitting on the moped, facing forward. Right is right, left is left. Make sure the cylinder mounting bolts are in good condition.

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Wiring Schematic

(for most Puch models, pre 1980 color codes are the same though)

Click here to download a complete wiring schematic for 1976-1977 Puch Maxi and other models.  (5 WIRES)


1977 Puch Maxi Sport Complete Moped wiring Diagram
(May be used for other models as well. Thanks to Daniel Herron


 

 

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Moped just wont spark


Mostly related to Puch mopeds

Some of the older models, older meaning pre-1980, had a silly problem that if any of the light bulbs burned out, the moped would not spark. I have no idea why they would have made the moped like this, but it is a reality. Check the tail light bulbs, make sure they are getting a good contact, that no corrosion has built up. Inspect the filament in the bulbs, make sure they look OK. The best way to make sure is to get a 6 volt battery and test it that way, or put 4 D batteries together and test with that. (may have to tape the batteries together, it's a bear holding 4 D's together, as well as wires at the ends.)

Check the brake light switches, located behind the left brake lever and right brake lever. Make sure they are working properly. You might want to use an OHM meter, (voltmeter), and insure they open and close watching the meter as you open and close the switch.

Sometimes the horn can cause this as well, if it has rusted out or gone bad. Try to test it as well when you test the bulbs. If you hear anything at all when you test it, it should be good.

Check the points, make sure they are clean. Use a fingernail file if necessary, to remove any surface rust. Also make sure that the condenser is in place.

You can take it to a motorcycle shop that works on 2-stroke bikes, like motocross type bikes. They are basically a larger version of the moped, and works about the same way. If the shop mentions that they can't get parts to repair these old Puch's, send them to us, or ask if you get the parts for them, will they work on it.

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Puch Engine Exploded Views

Puch 1 Speed Maxi & Maxi Luxe exploded view
Puch 2 Speed Newport II & MKII exploded view
Puch 2 Speed Magnum II exploded view
Puch 2 Speed Magnum with Oil Injection System exploded view

 

Performance Tuning Tricks:

BASIC TWO-STROKE TUNING

By Eric Gorr

Changing the power band of your dirt bike engine is simple when you know the basics. A myriad of different aftermarket accessories is available for you to custom tune your bike to better suit your needs. The most common mistake is to choose the wrong combination of engine components, making the engine run worse than stock. Use this as a guide to inform yourself on how changes in engine components can alter the powerband of bike's engine. Use the guide at the end of the chapter to map out your strategy for changing engine components to create the perfect power band.

TWO-STROKE PRINCIPLES

Although a two-stroke engine has less moving parts than a four-stroke engine, a two-stroke is a complex engine because it relies on gas dynamics. There are different phases taking place in the crankcase and in the cylinder bore at the same time. That is how a two-stroke engine completes a power cycle in only 360 degrees of crankshaft rotation compared to a four-stroke engine which requires 720 degrees of crankshaft rotation to complete one power cycle. These four drawings give an explanation of how a two-stroke engine works. 

1) Starting with the piston at top dead center (TDC 0 degrees) ignition has occurred and the gasses in the combustion chamber are expanding and pushing down the piston. This pressurizes the crankcase causing the reed valve to close. At about 90 degrees after TDC the exhaust port opens ending the power stroke. A pressure wave of hot expanding gasses flows down the exhaust pipe. The blow-down phase has started and will end when the transfer ports open. The pressure in the cylinder must blow-down to below the pressure in the crankcase in order for the unburned mixture gasses to flow out the transfer ports during the scavenging phase. 

2) Now the transfer ports are uncovered at about 120 degrees after TDC. The scavenging phase has begun. Meaning that the unburned mixture gasses are flowing out of the transfers and merging together to form a loop. The gasses travel up the back side of the cylinder and loops around in the cylinder head to scavenge out the burnt mixture gasses from the previous power stroke. It is critical that the burnt gasses are scavenged from the combustion chamber, in order to make room for as much unburned gasses as possible. That is the key to making more power in a two-stroke engine. The more unburned gasses you can squeeze into the combustion chamber, the more the engine will produce. Now the loop of unburned mixture gasses have traveled into the exhaust pipe's header section. The gasses aren't lost because a compression pressure wave has reflected from the end of the exhaust pipe, to pack the unburned gasses back into the cylinder before the piston closes off the port. This is the unique super-charging effect of two-stroke engines. The main advantage of two-stroke engines is that they can combust more volume of fuel/air mixture than the swept volume of the engine. Example: A 125cc four-stroke engine combusts about 110cc of F/A gasses but a 125cc two-stroke engine combusts about 180cc of F/A gasses.

3) Now the crankshaft has rotated past bottom dead center (BDC 180 degrees) and the piston is on the upstroke. The compression wave reflected from the exhaust pipe is packing the unburned gasses back in through the exhaust port as the piston closes off the port the start the compression phase. In the crankcase the pressure is below atmospheric producing a vacuum and a fresh charge of unburned mixture gasses is flowing through the reed valve into the crankcase. 

4) The unburned mixture gasses are compresses and just before the piston reaches TDC, the ignition system discharges a spark causing the gasses to ignite and start the process all over again.


CYLINDER PORTING

The cylinder ports are designed to produce a certain power characteristic over a fairly narrow rpm band. Porting or tuning is a metal machining process performed to the cylinder ports (exhaust & transfers) that alters the timing, area size, and angles of the ports in order to adjust the power band to better suit the rider's demands. For example, a veteran trail rider riding an RM250 in the Rocky mountain region of the USA will need to adjust the power band for more low end power because of the steep hill climbs and the lower air density of higher altitudes. The only way to determine what changes will be needed to the engine is by measuring and calculating the stock engine's specifications. The most critical measurement is termed port-time-area. This term is a calculation of a port's size area and timing in relation to the displacement of the engine and the rpm. Experienced tuners know what the port-time-area values of the exhaust and transfer ports should be for an engine used for a particular purpose. In general, if a tuner wants to adjust the engine's power band for more low to mid range he would do the following things. Turn down the cylinder base on a lathe to increase the effective stroke (distance from TDC to exhaust port opening). This also retards the exhaust port timing and shortens the duration and increases the compression ratio. Next the transfer ports should be narrowed and re-angled with epoxy to reduce the port-time-area for an rpm peak of 7,000 rpm. The rear transfer ports need to be re-angled so they oppose each other rather than pointing forward to  the exhaust port. This changes the loop scavenging flow pattern of the transfer ports to improve scavenging efficiency at low to mid rpm (2,000 to 5,000 rpm). An expert rider racing mx in England would want to adjust the power band of an RM250 for more mid to top end power. The cylinder would need to be tuned radically different than for trail riding. Here is an example. The exhaust port would have to be raised and widened to change the port-time-area peak for a higher rpm (9,000 rpm). For either of these cylinder modifications to be effective, other engine components would also need to be changed to get the desired tuning effect.

 

CYLINDER HEAD

Cylinder heads can be reshaped to change the power band. Generally speaking, a cylinder head with a small diameter and deep combustion chamber, and a wide squish band (60% of  the bore area). Combined with a compression ratio of 9 to 1 is ideally suited for low to mid range power. A cylinder head with a wide shallow chamber and a narrow squish band (35-45% of bore area) and a compression ratio of 8 to 1, is ideally suited for high rpm power. There are many reasons why a particular head design works for certain types of racing. For example; a head with a wide squish band and a high compression ratio will generate high turbulence in the combustion chamber. This turbulence is termed Maximum Squish Velocity, MSV is rated in meters per second (m/s). A cylinder head designed for supercross should have an MSV rating of 28m/s. Computer design software is use to calculate the MSV for head designs. In the model tuning tips chapters of this book, all the head specs quoted have MSV ratings designed for the intended power band changes.

 

CRANKSHAFT

There are two popular mods hop-up companies are doing to crankshafts; stroking and turbo-vaning. Stroking means to increase the distance from the crank center to the big end pin center. There are two techniques for stroking crankshafts; weld old hole and re-drill a new big end pin hole, or by installing an off-set big end pin. The method of weld and re-drilling is labor intensive. The off-set pin system is cheap, non-permanent, and can be changed quickly. In general, increasing the stroke of a crankshaft boosts the mid range power but decreases the engine's rpm peak. The term "Turbo-Crank" refers to a modification to the crankshaft of a two-stroke engine, whereby scoops are fastened to the crank in order to improve the volumetric efficiency of the engine. Every decade some hop-up shop revives this old idea and gives it a trendy name with product promises that it can't live up to. These crank modifications cause oil to be directed away from the connecting rod and often times the vanes will detach from the crank at high rpm, causing catastrophic engine damage. My advice, don't waste the $750!

 

CARBURETOR

In general a small diameter carburetor will have high velocity and a good flow characteristic for a low to mid rpm power band. A large diameter carburetor works better for high rpm power bands. For 125 cc engines a 34mm carburetor works well for supercross and enduro and a 36 or 38 mm carburetor works best for fast mx tracks. For 250 cc engines a 36 mm carburetor works best for low to mid power bands and a 39.5 mm carburetor works best for top end power bands. Recently there has been a trend in the use of air-foils and rifle-boring for carbs. These innovations are designed to improve air flow at low throttle openings. Some companies sell carb inserts, to change the diameter of a carb. Typically a set of inserts is sold with a service of over boring the carb. For example; a carb for a 250cc bike (38mm) will be bored to 39.5mm and two inserts will be supplied. The carb can then be restricted to a diameter of 36 or 38mm.

 

REED VALVE

Think of a reed valve like a carburetor, bigger valves with large flow-areas work best for high rpm power bands. In general, reed valves with six or more petals are used for high rpm engines. Reed valves with four petals are used for dirt bikes that need strong low end and mid range power. There are three other factors to consider when choosing a reed valve. The angle of the reed valve, the type of reed material, and the petal thickness. The two common reed valve angles are 30 and 45 degrees. A 30-degree valve is designed for low to mid rpm and a 45 degree valve is designed for high rpm. There are two types of reed petal materials commonly used, carbon fiber and fiberglass. Carbon fiber reeds are lightweight but relatively stiff (spring tension) and designed to resist fluttering at high rpm. Fiberglass reeds have relatively low spring tension so they instantly respond to pressure that changes in the crankcase, however the low spring tension makes them flutter at high rpm thereby limiting the amount of power. Fiberglass reed petals are good for low to mid power bands and carbon fiber reeds are better for high rpm engines. Boyesen Dual Stage reeds have a  large thick base reed with a smaller thinner reed mounted on top. This setup widens the rpm range where the reed valve flows best. The thin reeds respond to low rpm and low frequency pressure pulses. The thick reeds respond to higher-pressure pulses and resist fluttering at high rpm. A Boyesen RAD valve is different than a traditional reed valve. Bikes with single rear shocks have off-set carbs. The RAD valve is designed to redistribute the gas flow to the crankcases evenly. A RAD valve will give an overall improvement to the power band. Polini of Italy makes a reed valve called the Supervalve. It features several mini sets of reeds positioned vertically instead of horizontally like conventional reed valves. These valves are excellent for enduro riding because of improved throttle response. In tests on an inertia chassis dyno show the Supervalve to be superior when power shifting.  However these valves don't generate greater peak power than conventional reed valves. Supervalves are imported to America and sold by Moto Italia in Maine. 

 

EXHAUST PIPE

The exhaust pipe of a two-stroke engine attempts to harness the energy of the pressure waves from combustion. The diameter and length of the five main sections of a pipe, are critical to producing the desired power band. The five sections of the pipe are the head pipe, diffuser cone, dwell, baffle cone, and the stinger. In general, after market exhaust pipes shift the power band up the rpm scale. Most pipes are designed for original cylinders not tuned cylinders. Companies like MOTOWERKS custom computer design and fabricate pipes based on the cylinder specifications and the type of power band targeted.

 

SILENCER

Silencers come in all sorts of shapes and sizes. A long silencer with a small diameter enhance the low to mid power because it increases the bleed-down pressure in the pipe. A silencer with a short length and a large core diameter provides the best bleed-down pressure for a high rpm engine. Too much pressure in  the pipe at high rpm will radically increase the temperature of the piston crown and could cause the piston to seize in the  cylinder.  

FLYWHEEL WEIGHTS

The flywheel is weighted to improve the engine's tractability at low to mid rpms. There are two different types of flywheel weights, weld-on and thread-on. A-Loop performs the weld-on flywheel weight service. Steahly makes thread-on flywheel weights. This product threads onto the fine left-hand threads that are on the center hub of most Japanese magneto rotors. normally the threads are used for the flywheel remover tool. Thread-on flywheel weights can only be used if the threads on the flywheel are in perfect condition. The advantage to weld-on weights is they can't possibly come off. External rotor flywheels have a larger diameter than internal rotor flywheels so they have greater flywheel inertia. Internal rotor flywheels give quicker throttle response.

 

AFFECTS OF THE IGNITION TIMING

Here is how changes in the static ignition timing affects the power band of a Japanese dirt bike. Advancing the timing will make the power band hit harder in the mid range but fall flat on top end. Advancing the timing gives the flame front in the combustion chamber, adequate time to travel across the chamber to form a great pressure rise. The rapid pressure rise contributes to a power band's "Hit". In some cases the pressure rise can be so great that it causes an audible pinging noise from the engine. As the engine rpm increases, the pressure in the cylinder becomes so great that pumping losses occur to the piston. That is why engines with too much spark advance or too high of a compression ratio, run flat at high rpm. Retarding the timing will make the power band smoother in the mid-range and give more top end over rev. When the spark fires closer to TDC, the pressure rise in the cylinder isn't as great. The emphasis is on gaining more degrees of retard at high rpm. This causes a shift of the heat from the cylinder to the pipe. This can prevent the piston from melting at high rpm, but the biggest benefit is how the heat affects the tuning in the pipe. When the temperature rises, the velocity of the waves in the pipe increases. At high rpm this can cause a closer synchronization between the returning compression wave and the piston speed. This effectively extends the rpm peak of the pipe.

 

HOW TO ADJUST THE TIMING

Rotating the stator plate relative to the crankcases changes the timing. Most manufacturers stamp the stator plate with three marks, near the plate's mounting holes.  The center mark is the standard timing. If you loosen the plate mounting bolts and rotate the stator plate clockwise to the flywheel's rotation, that will advance the ignition timing. If you rotate the stator plate counterclockwise to the flywheel's rotation, that will retard the  ignition timing. Never rotate the stator plate more than .028in/.7mm past the original standard timing mark. Kawasaki and Yamaha stator plates are marked. Honda stators have a sheet metal plate riveted to one of the mount holes. This plate insures that the stator can only be installed in one position. If you want to adjust the ignition timing on a Honda CR, you'll have to file the sheet metal plate, with a 1/4in rat-tail file.

AFTERMARKET IGNITIONS

The latest innovation in ignition systems is an internal rotor with bolt-on discs that function as flywheel weights. PVL of Germany makes these ignitions for modern Japanese dirt bikes. Another advantage to the PVL ignition is that they make a variety of disc weights so you can tune the flywheel inertia to suit racetrack conditions. MSD is an aftermarket ignition component manufacturer. They are making ignition systems for CR and RM 125 and 250. MSD's ignition system features the ability to control the number of degrees of advance and retard. These aftermarket ignition systems sell for less than the OEM  equivalent.

 

TIPS FOR BIG BORING CYLINDERS

In the mid nineties, European electro-plating companies started service centers in America. This made it possible to over bore cylinders and electro-plate them to precise tolerances. This process is used by tuners to push an engine's displacement to the limit of the racing class rules, or make the engine legal for a different class. When you change the displacement of the cylinder, there are so many factors to consider. Factors like; port-time-area, compression ratio, exhaust valves, carb jetting, silencer, and ignition timing. Here is an explanation of what you need to do when planning to over bore a cylinder.

Port-Time-Area - This is the size and opening timing of the exhaust and intake ports, versus the size of the cylinder and the rpm. When increasing the displacement of the cylinder, the cylinder has to be bored to a larger diameter. The ports enter the cylinder at angles of approximately 15 degrees. When the cylinder is bore is made larger, the transfer ports drop in height and retard the timing and duration of those ports. The exhaust port gets narrower. If you just over bored and plated a cylinder, it would have much more low end power than stock. Normally tuners have to adjust the ports to suit the demands of the larger engine displacement. Those exact dimension changes can be determined with TSR's Time-Area computer program.

Cylinder Head - The head's dimensions must be changed to suit the larger piston. The bore must be enlarged to the finished bore size. Then the squish band deck height must be set to the proper installed squish clearance. The larger bore size will increase the squish turbulence so the head's squish band may have to be narrowed. The volume of the head must be increased to suit the change in cylinder displacement. Otherwise the engine will run flat at high rpm or ping in the mid range from detonation. 

Exhaust Valves - When the bore size is increased, the exhaust valve to piston clearance  must be checked and adjusted. This pertains to the types of exhaust valves that operate within close proximity of the piston. If the exhaust valves aren't modified, the piston could strike the valves and cause serious engine damage. 

Carb - The piston diameter and carb bore diameter are closely related. The larger the ratio between the piston size and the carb size, the higher the intake velocity. That makes the jetting richer. Figure on leaning the jetting after an engine is over bored. 

Ignition Timing - The timing can be retarded to improve the over rev. Normally over bored engines tend to run flat on top end. 

Pipe and Silencer - Because only the bore size is changed, you won't need a longer pipe only one with a larger center section. FMF's line of Fatty pipes work great on engines with larger displacement. Some riders use silencers that are shorter with larger outlets to adjust the back-pressure in the pipe for the larger engine displacement. 

to be continued.........

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