Messed-up Miku­nis

Practical Sportsbikes (UK) - - Fettling -

Q My 1990 Yamaha FZR1000 EXUP was laid up last year as part of a re­build with the valve clear­ances set but the en­gine oth­er­wise un­touched. I’m now hav­ing real trou­ble with the Mikuni BDST38 carbs. I’ve cleaned them but not ul­tra­son­i­cally.

The bike won’t run right just off idle, say at 3000 to 4000rpm de­spite re­set­ting float heights, fit­ting new pi­lot jets, carb to man­i­fold rub­bers, gas­kets and seals, check­ing for air leaks and so on. It runs rough but it’s hard to de­scribe – flat, but I can’t tell if it’s weak or too rich.

Do you know any Mikuni ex­perts who could ad­vise or in­deed help with the set-up? Andy Tip­per, email

A Steve Pan­ter says: These carbs have a plethora of in­ter­nal O-rings and gas­kets. Have you checked and re­placed them all? There’s a weird con­vo­luted gas­ket un­der the jet block that’s of­ten over­looked. Check for miss­ing or per­ished O-rings un­der the mix­ture screws and that their tips are un­bro­ken. Then there are the O-rings in the white plas­tic slide blocks that go on top of the nee­dle jets and the O-rings seal­ing the push in main jets. Have you bal­anced the carbs? Check that the slides are re­turn­ing at the same rate in case there’s a bent nee­dle or burrs on slide or body. The slides can wear as can the plas­tic slide blocks that go into the body. Also check the O-rings on the float mounts and the plas­tic choke plungers.

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