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Tech Corner

Technical Q and A for classic motorcycle maintenance and repair.

Mikuni Carburetor Mixture Setting

Motorcycle Classics tech expert Keith Fellenstein

Editor’s note: If you’re having trouble with that old Suzuki, BSA or BMW, Keith Fellenstein is your guy. From motorcycle tuning tips to detailed motorcycle engine repair, he can draw from a wealth of experience to help guide you to success. Send questions to: Keith’s Garage, 1503 SW 42nd St., Topeka, KS 66609, or send an e-mail with “Keith’s Garage” as the subject.

Mixture setting

Q: I am in the process of restoring a 1970 Yamaha AT1 125. Can you give me some advice on setting the fuel/air mixture screw on a Mikuni carburetor? — Phill/via email

A: There are two screws to adjust when setting the idle on a carburetor. One adjusts the slide height and is called the idle speed screw, while the other adjusts the air-to-fuel ratio and is called the idle mixture screw. The manual for the AT1 says start the idle mixture screw at 1-1/2 turns counterclockwise from fully screwed in (lightly seated; don’t screw it tight), then adjust either way in 1/2 turn increments to find the fastest, smoothest idle. If the fastest idle is too fast, turn the idle speed screw out to slow it down. Again turn the idle mix screw a 1/2 turn clockwise and counterclockwise seeking the fastest, smoothest speed. Once you determine which direction helps, refine it to 1/4 turns either way to fine-tune the mixture. If you find that the bike stumbles a bit when you open the throttle, 1/4 to 1/2 turn tighter will richen up the mixture a bit and compensate for the added air in the mix from the opening slide. MC