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· Registered
4,350 Posts
Discussion Starter · #1 ·
BFD, I was wrong yesterday. It DOES make a difference how far in/out the idle mixture screw is.

I got the carbs, John, Thanks. But guess what? stupidity strikes again. First thing I noticed was how far in your idle mixture screws were in. I too k them out, compared them with mine, same length screw and spring.

SO I put the left (JB's) carb on the bike, the one that was not firing. I screwed in the idle mixture screw till it seated, and then out one turn. I noticed how far Fucking out the other screw was on the right side. So i decided to try to screw in the OLD carb idle mixture screw in farther. Sure enough, it kept going with a lot of screwing pressure. finally it seated where the newer carbs were approximately seated. So i did it to both sides, and sure enough, the other side went in a lot farther too. It seems to have come down to the fact that there was enough corrosion and shit in the idle mixture holes/threads that I could not get an accurate setting because I kept screwing it in till I felt it had bottomed out, when in fact it had another 3/16 " or so to actually seat!

NOW i have a real race bike that works. I am going to take it out to a secluded spot and run it for a full power run to see what happens.

Thanks to everyone for helping and putting up with this, but it was only on seeing another example that made all this happen. I should have looked at BFD's bike closer yesterday.

Happy Scott

· Premium Member
7,411 Posts
Whatever you do don't swap any parts from the carbs I sent with the old ones. The carbs I sent should have nice clean idle air screws and passages. Because these carbs are meant for a 350 they barely work at an idle on a 175, very sensitive.

Bear in mind that these old Keihins use an Idle AIR screw, which means that backing out the screw will lean the mixture. Usually a symptom of it being too lean on the idle circuit is that it will stumble or die when you try to give it throttle from an idle (assuming it will idle). Because these carbs have no acclerator pump the engine depends on idle mixture for a fraction of a second when you increase throttle from idle. I like to run them a tad on the rich side...usually about 1 1/4 turn out to 1 3/4 turn out. Back out the screw until the bike starts to miss at an idle...then back in till it reaches the highest rpm...then in just a tad more like 1/8 turn.

Usually when the idle screw is hard to turn in it's the seat that is messed up...hard to believe you left enough dirt in there to cause a problem.

I'd put both new carbs on...I'm big on keeping a good set together.
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