Motorcycle Repair: carburetor, honda trail 90, fuel air mixture

Question
I have a 1971 Honda Trail 90 and I would like to know the correct procedure to adjust the carburetor.   It idles fine.  It stalls out when the gas is applied.  The air filter is clean. Looking from the left side of the bike there are two adjustments.  One on the left and one on the right side of the carburetor.
Thank You

Answer
Trevis, short answer: There is a screw that changes the fuel air mixture ratio at idle... aka idle mixture screw.
Second screw is idle speed screw and it just pushes the slide up and down to adjust the amount of air being drawn into the engine. If you take the air filter tube off, you can watch the slide and see what happens when you turn the speed screw. If the idles okay, then the mixture circuit must be working and the adjustment close to correct.

LONG ANSWER: Turning the throttle raises the slide, transitions the fuel system from idle circuit to needle/main jet circuit. The clip on the needle can be adjusted up and down to achieve a different mixture at part throttle. This only works well if the main jet and main jet holder are clean in all the passages and orifices.
If the bike has been modified by changing the exhaust system or removing the baffle, then the mixture will need correction. Any changes from stock configuration to air filter or exhaust side will affect carburetion....

On top of all this, the valves need to be checked and set to .002" so the engine has full benefit of compression available.

Before messing with the carb settings (assuming a clean carburetor!), the ignition system needs checking. Points need to be cleaned, gapped to .014"-.016" at maximum opening, then set the backing plate so points just OPEN at the F mark on the flywheel.

Before you do that, take the point plate off, inspect the mechanical spark advancer for proper function, advancing against the springs and then returning fully. If not, then disassemble and clean/lube the advancer. Then reinstall the point plate and set gap and timing again.

NOW you can start being concerned about the stalling condition, as it pertains to the carburetor. Again, the idle mixture seems to be in the ballpark, but the transition accelleration depends on a combination of full compression, correct ignition timing/advance and carburetor fuel mixture settings.

If the actual year of production was 1971 (check the sticker on the frame!), then the bike is a K3 model and should have 20mm float level setting, #85 main jet, #35 idle jet and the clip on the middle of the 5 notches on the needle. One turn out, from a gentle turn in to stop) is specified as idle mixture setting. If the float level is incorrect, it will influence the whole fuel system range.

Although the engine is only a simple single cylinder machine, the laws of physics and thermodynamics apply to it as much as a F1 racer. Everything has to be right in the foundation part of the machine (compression, spark, full battery voltage and fuel circuits), before you get proper performance throughout the rpm range.

A shop manual will be a good investment for you... and some good tools, if you don't have them.

Bill Silver