Adjusting Carburetor

Whether it's on a small two-stroke or a big block engine, adjusting carburetor settings is a common maintenance requirement when doing a tune up or changing performance measures.

Be aware, however, that adjusting the carb is not a "fix all" measure and should only be done when other common performance problems have been repaired.

What Should Be Fixed BEFORE Carburetor Adjustments

Be sure your engine's timing is correct and tuned. Many common problems mis-associated with carb issues are actually timing. If the timing on your engine is wrong, adjusting the carb can only make up for it by a fraction and will not eliminate the engine wear and efficiency problems you're having.

Carbon buildup on exhaust baffles, pipes, pistons, etc. cannot be remedied with carb adjustments, only masked.

Air leaks, bad seals, low compression, and related issues are not going to go away because you've raised the fuel mixture in the carburetor. Always do a simple compression test as a part of your tune up!

A dirty carb will not clean itself just because you've raised the fuel-air mixture or opened the choke. The only way to clean a carburetor is with elbow grease.

Adjusting the Carburetor

While carbs will vary from engine-to-engine, most run in the same basic way. If you have an electronically-controlled carb, then most of your adjustments will be through the control circuits or computer. This requires specialized equipment and knowledge.

Manual carburetors can be adjusted with only a screwdriver in most cases. With the engine warm, adjust screws on each carburetor to the highest idle RPM recommended by the engine's instruction and maintenance manual. This usually means reducing the idle screw(s) to their furthest open point. On most automotive engines, this is about 2,000 RPM.

Once there, begin air screw adjustments in very small increments (usually 1/4 turns). If the air screw is out more than 1-3/4 turns, it's likely that a smaller pilot jet should be installed. If out less than 1-1/4 turns, a larger pilot jet is needed. The air screw is used to fine-tune the carb intake or pilot jet.

Then adjust the idle screw inward slowly, in half-turn increments, watching idle speed as you do so. Cylinders will usually be at normal on a gasoline engine somewhere between 1250 and 1400 RPM. Again, check your manual and use the numbers given. Lower RPM is generally better if the engine is not running roughly.

Once adjusted, synchronization between carbs is required. The slide position and idle stop screw setting will do this.

Adjust the slide so that when the throttle is in any given position, all carbs are opened by the same amount. This adjustment usually does not require re-adjusting unless the carb has been removed and replaced or disassembled.

Finally, the carb's needle clip position should be adjusted to give proper mixture at higher positions (1/4-3/4 throttle open). Too rich or too lean will mean bad performance.