Mike's Keihin carb tuning

[650andy]

Gents,
I'm relatively new to the engine tuning scene, had my 81' XS650 for a few years now and just did the Mike's XS carb swap out on my original Mikuni's. Didn't have any issues setting up the fuel lines or the vent/drain lines, but as far as tuning the carbs themselves, does anyone have some insight for the air/fuel mix downward angle screw with spring or adjusting the idle? I'm sitting around 1900rpm on idle and I lowered it down to 1000 with the mix screw but I've a feeling that's not right. Just a novice here, any help is greatly appreciated.

 

[5twins]

I think you're mixing up your terminology here. The angled screw with the spring on it that pushes upward against the slide is your idle speed adjustment screw. The smaller brass screw that goes in at a right angle to the body just above where the float bowl fits on is your mix screw. Yes, the angled screw with the spring is what you adjust for idle speed. You want about 1200 RPMs, and both carbs running at the same speed. The mix screws have a certain best setting as well, usually somewhere between 1 and 3 turns out from lightly seated, where the idle is smoothest and fastest.

 

[650andy]

Whats the best way to equalize both idle screws at the new lower rpm?

 

[5twins]

Well, best way is to use a manometer to read the vacuum signal from each carb, or vacuum gauges. There is also an "old school" practice called the "dead cylinder" method, but it can be tricky on your later electronic ignition model. It consists of removing one spark plug wire and grounding it so the bike is only running on one cylinder. But, you have to be careful to not run it without the removed plug wire properly grounded or you can fry your ignition. You set the idle speed of the running cylinder as low as you can without stalling. Then do the reverse, reconnect the loose plug wire, remove the other one, and set the other cylinder like the first. Personally, I don't use this method because I have a manometer, and I don't like the possibility of damaging the ignition.

 

[Jack]

To set both slides equally use anything like feeler gauges, sewing needles ,etc to set slide height until desired idle speed is achieved and make sure slides pull equally by the adjuster at top of carbs where the throttle cable ends rest. Also found this video that might be helpful for you in synchronizing the carbs. As for adjusting low speed air,turn in clockwise till engine runs rough or begines to die, then back off counterclockwise till engine roughness smooths out

 

[jetmechmarty]

There is also an "old school" practice called the "dead cylinder" method.

This is the factory service manual method for the early bikes. The intakes had no barb for a manometer and they had a simple breaker point ignition. I concur that it's risky to do this with electronic ignition.