Mixture screws not changing anything.

[dburrows6735]

Hey guys,

I recently noticed that I was running a bit lean and want to remedy it. I have stock intake so I started adjusting the mixture screws. I was able to adjust them to their full range in and out with no change in how the engine was running.

I did a small test to see if it was changing the fuel mixture by starving it and then pulling the air filters expecting it to die but it did not.

What am I doing wrong? Are my mixture screws so dirty they don’t do anything anymore? The carbs are in good shape, and recently had a once over, nothing deep though.

Thanks

David

 

[HoughMade]

Could be that the tips are broken off in the holes. Could be the carbs are dirty and fuel isn't getting to the screws in the first place.

 

[dburrows6735]

Ill check the tips. Will the gas bypass the screws if they are dirty? The fuel dosent pass through them just passes by them right?

 

[drewpy]

If this is an SOHC engine; take the screw out completely, if they are blunt, the tips are broken. If they are nice and sharp pointed your in luck.

 

[cutlass400]

If your idle is set too high (to make up for another possible problem with the engine), the mixture screws will become useless because the throttle plates are open past the point that the idle fuel feed holes are supplying fuel and the carb is getting it's fuel from the main fuel circuit. This happens alot with car engines with larger cams that need alot of air to idle because of decreased vacuum so the idle screw has to be turned up to open the plates for more air. One cure for this on modified engines is to drill 1/8" holes in the throttle plates for more air. Since I would have to imagine your engine is stock this probably is not the case. Try turning the idle down as low as it will stay running then turn the mixture screws if the idle changes now then you know you have a problem elsewhere (ignition timing, vacuum leak, valve adjustment) and you are compensating for it with a wider throttle angle at idle.

 

[dburrows6735]

My idle screw is set very high, maybe due to an air leak. All this is coming together though. I'm replacing my boots this weekend so that will tell the tail. Ill keep you posted.

 

[HoughMade]

Can you change the idle speed with the idle air screw at all?

 

[dburrows6735]

Yes, I can but I have it set so high to keep the bike alive during idle. This is what leads me to believe I have a major air leak in my boots (which I'm replacing this weekend), making my mixture screws usless. Ill check out the tips of the mixture screws this weekend too.