How to tune a carburettor…
This guide will only cover problems associated with carburetion; any other problems cannot be fixed by adjusting the jetting but may appear as carburetion problems. So before you start make sure there are no air leaks where the carb attaches to the manifold – run the bike and spray the area with wd40 or any other aerosol, if the idle speed drops or increases then there is a leak and this must be fixed before any other changes are made. Also check that there is a good spark (fat and blue) by holding the spark plug by the HT lead against the head and kicking the engine over. A further test is to ensure there is compression by checking for resistance as the engine is kicked over.
By using this guide and any others your research may come up with you can get your bike running better than so-called experts and save time and money on mechanics aswell as develop a useful talent that is a source of pride when you get it right or solve a problem.
Now for jetting changes…
I usually start with the pilot circuit; problems here have an impact at surprisingly high throttle openings. Firstly adjust the idle, on a fully warmed up bike, to the required rpm. From here the mixture is adjusted for the fastest idle by adjusting the fuel and/or air screws. A fuel screw regulates the amount of air/fuel mixture moving into the carb while an air screw regulates the amount of air moving into the pilot (idle) circuit. Fuel screws are always found, IMHO, underneath the carb on the side of the intake manifold. Turning in this screw (gently!) limits fuel and leans the mixture while turning out richens the mixture. An air screw is found on the opposite side of the carb and not necessarily underneath. By turning in this screw the air movement into the pilot circuit is limited and the mixture is richened while turning out this screw leans the mixture. These differences can be seen in the attached pictures (Fuel Screw and Air Screw) with the following key: (blue area is air, orange area is fuel, green area is air/fuel mixture).
To tune turn in or out the screws independently of each other so that the idle is at its fastest then turn the idle back down to where it is required, blipping the throttle occasionally while adjusting. Repeat until any adjustment of either screw results in the idle decreasing, the idle is stabile and throttle response to a blip is clean with a quick return to idle. If the fuel screw is more than 4 turns out a larger pilot jet is needed, while if it’s in less than half a turn out a smaller pilot jet may be needed. An air screw out more than 2 turns needs a smaller pilot jet while if the screw is in only half a turn or less a bigger jet may be necessary. Both screws are measured in 360degree turns from fully seated, but be gentle the seats are delicate and turning in too hard will do damage.
The main jet is the next circuit I tune for. Warm the engine up with a fresh spark plug, then under load (high gear up a hill, decent revs) with full throttle run the bike for as long as possible (100m at least) then kill the engine with the kill switch, pull in the clutch and brake to a stop all at the same time – known as a plug chop. Take the plug out (before any other running of the engine) and examine the colour of the porcelain area – black means the main jet is too large (rich), while white means it is to small (lean). Change the jet size and repeat until spark plug colour is coffee coloured.
A similar and complementary way to tune the main jet is to run at a constant speed up to an area where a mate can time you. When you cross the 1st mark go to full throttle, the jet that consistently gives you the fastest time between points A and B at full throttle is the right jet.
The needle circuit is the most important area of carb jetting (because even on an underpowered pitbike you spend most of your time here) and probably the easiest to get right. You can perform a plug chop as above using half throttle or use the time technique described above. Adjust the needle by moving the E-clip up or down the notches on the needle. By moving the clip down the needle the needle is lifted out of the needle jet, richening the mixture at part throttle. By putting the clip higher on the needle the needle is lowered causing leaner conditions. Very lean needle settings will result in ‘stuttering’ power delivery while slightly rich or lean settings feel unresponsive. The ideal setting is that which feels ‘sharp’ without any hiccupping. Try all settings to get an idea of which is best, but it should be clear after only a few changes which dierction to move is the right one.
As a final note, changes in one of these three areas of jetting makes a negligable difference in the other areas. DO NOT try and fix a problem in one area with adjustments in the other two, fix the problem dont mask it. This should result in a carb that is 'seamless' to operate, that is you can do almost anything to the throttle and the bike will respond in a smooth and consistent way. There is a limit of course as to how seamless these old carb designs can be made to perform but it will certainly be better than stock. Any problems in power delivery require the tuning process be restarted or if the problem area can be identified adjusted directly.
TROUBLESHOOTING
This cannot be a comprehensive guide to problem solving but highlights some obvious and common problems.
Bike idles very high – too lean on pilot circuit, throttle piston the wrong way around, air leak at manifold.
Bike hiccups at idle – If the hiccup slightly slows the engine but doesn’t cut it out is a sign of rich pilot circuit (eight stroking). A louder hiccup that can stall the engine may be lean conditions spitting burning fuel back through the carb.
Black smoke out of exhaust – rich conditions at the throttle opening it occurs at.
Stuttering – If the bike stutters at full throttle but is alright at part throttle the problems lies in the main jet, if the bike is fine at full throttle but bogs down at part throttle the needle needs to be adjusted. By understanding which jet area of the carb is functioning at which throttle opening jetting problems are more easily fixed.
What jet sizes are best? - No one can tell you above a best guess, jetting is based on how and where you ride as well as the exact specifications of your bike. Variability between bikes is quite high with chinese imports making a blanket jetting setting impossible but even on better quality bikes good jetting may differ between otherwise identical bikes.
Please note that incorrect jetting or improper mechanical practice on your bike can result in damage, I take no responsibility for any problems you cause either directly or indirectly from what is in this guide. In other words if your unsure get someone knowledgeable to help if your not sure. Enjoy
Gawain 2008
This guide will only cover problems associated with carburetion; any other problems cannot be fixed by adjusting the jetting but may appear as carburetion problems. So before you start make sure there are no air leaks where the carb attaches to the manifold – run the bike and spray the area with wd40 or any other aerosol, if the idle speed drops or increases then there is a leak and this must be fixed before any other changes are made. Also check that there is a good spark (fat and blue) by holding the spark plug by the HT lead against the head and kicking the engine over. A further test is to ensure there is compression by checking for resistance as the engine is kicked over.
By using this guide and any others your research may come up with you can get your bike running better than so-called experts and save time and money on mechanics aswell as develop a useful talent that is a source of pride when you get it right or solve a problem.
Now for jetting changes…
I usually start with the pilot circuit; problems here have an impact at surprisingly high throttle openings. Firstly adjust the idle, on a fully warmed up bike, to the required rpm. From here the mixture is adjusted for the fastest idle by adjusting the fuel and/or air screws. A fuel screw regulates the amount of air/fuel mixture moving into the carb while an air screw regulates the amount of air moving into the pilot (idle) circuit. Fuel screws are always found, IMHO, underneath the carb on the side of the intake manifold. Turning in this screw (gently!) limits fuel and leans the mixture while turning out richens the mixture. An air screw is found on the opposite side of the carb and not necessarily underneath. By turning in this screw the air movement into the pilot circuit is limited and the mixture is richened while turning out this screw leans the mixture. These differences can be seen in the attached pictures (Fuel Screw and Air Screw) with the following key: (blue area is air, orange area is fuel, green area is air/fuel mixture).
To tune turn in or out the screws independently of each other so that the idle is at its fastest then turn the idle back down to where it is required, blipping the throttle occasionally while adjusting. Repeat until any adjustment of either screw results in the idle decreasing, the idle is stabile and throttle response to a blip is clean with a quick return to idle. If the fuel screw is more than 4 turns out a larger pilot jet is needed, while if it’s in less than half a turn out a smaller pilot jet may be needed. An air screw out more than 2 turns needs a smaller pilot jet while if the screw is in only half a turn or less a bigger jet may be necessary. Both screws are measured in 360degree turns from fully seated, but be gentle the seats are delicate and turning in too hard will do damage.
The main jet is the next circuit I tune for. Warm the engine up with a fresh spark plug, then under load (high gear up a hill, decent revs) with full throttle run the bike for as long as possible (100m at least) then kill the engine with the kill switch, pull in the clutch and brake to a stop all at the same time – known as a plug chop. Take the plug out (before any other running of the engine) and examine the colour of the porcelain area – black means the main jet is too large (rich), while white means it is to small (lean). Change the jet size and repeat until spark plug colour is coffee coloured.
A similar and complementary way to tune the main jet is to run at a constant speed up to an area where a mate can time you. When you cross the 1st mark go to full throttle, the jet that consistently gives you the fastest time between points A and B at full throttle is the right jet.
The needle circuit is the most important area of carb jetting (because even on an underpowered pitbike you spend most of your time here) and probably the easiest to get right. You can perform a plug chop as above using half throttle or use the time technique described above. Adjust the needle by moving the E-clip up or down the notches on the needle. By moving the clip down the needle the needle is lifted out of the needle jet, richening the mixture at part throttle. By putting the clip higher on the needle the needle is lowered causing leaner conditions. Very lean needle settings will result in ‘stuttering’ power delivery while slightly rich or lean settings feel unresponsive. The ideal setting is that which feels ‘sharp’ without any hiccupping. Try all settings to get an idea of which is best, but it should be clear after only a few changes which dierction to move is the right one.
As a final note, changes in one of these three areas of jetting makes a negligable difference in the other areas. DO NOT try and fix a problem in one area with adjustments in the other two, fix the problem dont mask it. This should result in a carb that is 'seamless' to operate, that is you can do almost anything to the throttle and the bike will respond in a smooth and consistent way. There is a limit of course as to how seamless these old carb designs can be made to perform but it will certainly be better than stock. Any problems in power delivery require the tuning process be restarted or if the problem area can be identified adjusted directly.
TROUBLESHOOTING
This cannot be a comprehensive guide to problem solving but highlights some obvious and common problems.
Bike idles very high – too lean on pilot circuit, throttle piston the wrong way around, air leak at manifold.
Bike hiccups at idle – If the hiccup slightly slows the engine but doesn’t cut it out is a sign of rich pilot circuit (eight stroking). A louder hiccup that can stall the engine may be lean conditions spitting burning fuel back through the carb.
Black smoke out of exhaust – rich conditions at the throttle opening it occurs at.
Stuttering – If the bike stutters at full throttle but is alright at part throttle the problems lies in the main jet, if the bike is fine at full throttle but bogs down at part throttle the needle needs to be adjusted. By understanding which jet area of the carb is functioning at which throttle opening jetting problems are more easily fixed.
What jet sizes are best? - No one can tell you above a best guess, jetting is based on how and where you ride as well as the exact specifications of your bike. Variability between bikes is quite high with chinese imports making a blanket jetting setting impossible but even on better quality bikes good jetting may differ between otherwise identical bikes.
Please note that incorrect jetting or improper mechanical practice on your bike can result in damage, I take no responsibility for any problems you cause either directly or indirectly from what is in this guide. In other words if your unsure get someone knowledgeable to help if your not sure. Enjoy
Gawain 2008
