Tuning A Walbro Carb

We are having problems tuning my bro's carby aswell. It seems to be starving for fuel... any ideas?

Mixture: Mixture refers to how much of each. example): How much sugar added to your coffee. How much oil added to your gas. In this case how much fuel being added to the air passing through the carb. This is the fuel / air mixture.

Rich: Rich means a lot or in this case more than perfect. (example): If your coffee is too sweet, it is to rich with sugar. If your engine is running rich, it is getting to much fuel added in with the air that the engine is bringing it.
"The mixture is rich".
Lean: Lean means little or less than perfect. (example): If your coffee is not sweet enough then it is to lean, there isn't enough sugar. If the engine is running lean, there is not enough fuel being added to the air the engine is bringing in.
" The mixture is lean".
Bog: This is the sound you hear due to a lean mixture. When you open the throttle by squeezing the trigger to the bars, you are letting the engine draw in all of the air it can. If there is not enough fuel added to that air, the engine will not accelerate, it will bog and usually die.

Chutter: This is the sound an engine makes when it is getting a lot of fuel. You may hear this sound at any throttle position including at wide open. Go-Peds® run better with a little "Chutter" at low to mid throttle settings


The H.P. Carb includes 3 adjustment screws. These are the idle adjustment screw, low throttle adjustment screw & high throttle adjustment screw. Some people call them the Low speed & High speed screws, but they actually operate based on throttle opening not engine speed.

Idle Screw: The idle screw keeps the throttle slightly open allowing enough air to pass for the engine to idle. Turning this screw in or clockwise allows more air to pass. Turing it out or counter-clockwise decreases airflow.
Clockwise = more air = faster idle.
Counter-Clockwise = less air = slower idle.
Low Throttle Screw: This screw adjusts the amount of fuel added to the air passing through the carb at partial throttle, or up to 1/3 throttle openings.

High Throttle Screw: This "T" Screw adjusts the amount of fuel added to the air passing through the carb from 1/3 throttle to full open.

NOTE: The fuel added by the Low Throttle Screw is still being added at 1/3 to full open. Adding more fuel with the Low Throttle Screw will affect the High Throttle mixture slightly



In order to begin tuning your carb you must first get the engine started so you can listen for Rich or Lean mixture. We recommend these initial settings to start with:

Idle Screw - All the way clockwise
Low Screw - 1 1/4 turns out (counter-clockwise from closed)
High "T" Screw - 2 turns out (counter-clockwise from closed)




Step 1: If you just installed your H.P. Carb Kit. Push the primer until fuel can be seen returning to the tank in yellow return line. (Primer Note: The primer brings fuel from the tank to the carb. Priming the carb not the engine. Raw fuel is not actually being pumped into the engine. Pumping will not help start a cold engine).

Step 2: If the engine is being started for the 1st time with an H.P. Carb, we recommend you pull off the filter and place your finger over the carb opening so no air can pass. Now pull the pull start. After each pull check to see if your finger is wet with fuel. Once your finger is wet, the engine is ready to fire. Hold the throttle 1/2 to wide open* and pull start. Once the engine starts keep it running at the lowest possible RPM. (* without holding your finger on the carb)

Step 3: Warm the engine up all the way. When you are sure it is warm let off the throttle to see if it will idle. If the engine idles slow, chutters & dies then turn the Low Throttle adjuster in 1/4 turn and try again until it idles steady.

Step 4: Once the engine idles steady, use the low throttle screw to get a slow chuttering idle. If the engine slowly chutters and dies, then it is to rich and the low throttle screw must be turned in, to lean the mixture a bit (1/8 to 1/4 turn). If the idle speed gets too fast with the leaner low throttle setting use the Idle screw to get a slower idle speed. The ideal low setting is as rich as possible without loading up (Chuttering & Dying). Once the low throttle mixture and idle speed are set, you are now ready to move on to the High Throttle Adjustment.

Step 5: The object of High Speed adjustment is to obtain the highest possible RPM under load with the richest possible setting. The richer you can keep the mixture while still getting peek RPM the better. The engine will accelerate, it will live longer, run cooler and respond better. Just adjusting the high throttle screw for max RPM on the kick stand would leave the engine very lean and when the engine is under load it probably won't accelerate well at all and could seize.

Instead, start rich and lean the mixture out in small increments (1/8 of a turn). Testing each new setting under a load. Once the acceleration starts to suffer, you have gone to far. (Go back 1/8 of a turn). It is better to depend on cylinder porting to get peak

RPM than on ultra lean mixture settings. Optimum settings for typical stock engines is about 1 1/8 to 1 3/4 turns out.



Problem: Engine Chutters at full throttle and doesn't rev high.
Solution: The "T" screw is to far open. Close 1/8 turn and test under load until acceleration suffers and then open 1/8 turn. NOTE: Engines with stock porting tend to chutter at high RPM.

Problem: Engine bogs from low speed.
Solution: The "T" Screw is to far closed. Open it 2 or more turns and tune for chutter. NOTE: Worn parts, tall gear ratio with heavy porting, high intake duration, restricted exhaust and low cylinder compression can cause low speed bog.

Problem: Engine idles but slowly dies.
Solution: The low speed screw is to far open. Turn in 1/8 turn at a time until idle cleans up. If idle speed is to fast after adjustment slow the idle down using the idle screw.

Problem: Engine idles smooth but runs poor under load unless throttle is full open.
Solution: The low speed screw is to far in. Turn idle screw all the way clockwise and set a low idle using the low throttle screw.

Problem: Engine idles fast and stalls when you try to slow idle speed down.
Solution: These are the symptoms of an intake leak. Go-Peds® usually leak at the manifold to cylinder gasket. Make sure the carb mount screws arent' going to far through the manifold and causing an air leak. Screw marks or cuts in the gasket are a tell-tell sign of screw interference. You can check for leaks with the ped running. Spray WD-40 around the manifold, if the idle speed changes you have a leak.

I hope that it can be to any help.

Great Post! When I worked as a small engine mechanic a few years back I always found the walbro carb very reliable & easy to tune, if you do it properly as said in bobbe's last post. With the adjustable H & L jets it allows you to fine tune your engine easily after any mods or different weather & temperature. What I found is that a lot of people tune them on the H screw to get peak revs, in the false belief that they are getting more power. This is far to lean & will overheat & sieze the engine.

i still have trouble tuning mine. i have tried two of them. when i accelerate, it stalls straight away, but on the stand it runs fine. no matter what i try, it still stalls

Sounds like it is running to lean. The fact that you have tried 2 carbs & are still having problems seems to suggest that you have an air leak somewhere. Double check everything, all the joints, clean out & check the carb, set the H & L screws correctly. There must be something causing the lean mixture, so be thorough. Millions of small motors uses these carbs successfully so there is no reason why it shouldn't run well. Good luck

i installed mine today and followed bonegrafts advice and fired it up on the second pull 8) runs pretty good, will test it at the track tomorrow
only problem im having at the moment is a high idle due to the throttle not returning all the way (nothing an extra return spring wont fix :wink: ). i thought it might have been the throttle cable binding but it seems to glide verywell so im thinking its just a poor std return spring

Yea, some good basic there, great post. I think the high throttle jet is blocked or something because we had it screwed alllll the way out (almost falling out) the bike would rev hard then slowly die until it stalled. Going to take it apart and try and clean the high throttle jet and see if that makes a diff. Also check manifold for air leak, but i cant see that being the prob coz it idles fine.