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.