BS34 Mikuni problems

Biffo

XS650 Enthusiast
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Have just had the first run out of an '81 SH that I am restoring. Idling was shocking, with loud "popping" from the left exhaust. Checked the exhaust system for leaks and all OK, but noticed that the left carb intake was wet with fuel. While poking around I found that blocking the pilot air jet hole with my finger immediately gave perfect idling. Take the finger away and the problem is back, with the engine revs rising and falling dramatically as well. Have dismantled and checked the carbs. Float heights are good, floats are intact, jets are all clear. Both pilot jet mixture screws are 2 turns open. All ports appear to be clear. Any ideas, anyone?
 
Well something is still amiss in that left carb. did you sync them with a gauge?
 
The U.S. spec BS34s usually need 3 to 3.5 turns out on the mix screws. 2 turns out would cause lots of popping.
 
were the float valve o-rings replaced? gotta do that on BS34s or gas leaks right around the valve. check that the diaphragm in the petcock isn't leaking into the the intake.
 
were the float valve o-rings replaced? gotta do that on BS34s or gas leaks right around the valve. check that the diaphragm in the petcock isn't leaking into the the intake.
My float valves have the Viton tips. I must admit I thought of the petcock last night. It is connected to the left carb, and that's the only one that's popping. Have also ordered new carb mounts. Mine are probably OK but a bit old and hard. Thanks to 5twins as well. I will increase the mix screws.
 
My float valves have the Viton tips.
there is an o-ring between the valve and the carb body. it is often hard and leaks. If you have replaced the entire valve you would have a new o-ring.
You can block the vacuum port and run the petcock on prime to test the petcock diaphragm theory

bs34floatvalve.jpg
BS34Jet.JPG
 
May not be related, but on my newly acquired free 1980 XS650, I was showing lean on the right cylinder, and lots of backfiring and such. Was tearing my hair out, had the carbs on and off a number of times thinking I was missing something. Turned out that the vacuum petcock was completely toast and had been "fixed" by the PO. The vacuum line to the right cylinder intake boot was sucking air straight through the petcock causing my issues. A new petcock rebuild kit and it is running great!
Great thread here - http://www.xs650.com/threads/rebuilding-yamaha-vacuum-operated-fuel-valves-petcocks.20662/
 
May not be related, but on my newly acquired free 1980 XS650, I was showing lean on the right cylinder, and lots of backfiring and such. Was tearing my hair out, had the carbs on and off a number of times thinking I was missing something. Turned out that the vacuum petcock was completely toast and had been "fixed" by the PO. The vacuum line to the right cylinder intake boot was sucking air straight through the petcock causing my issues. A new petcock rebuild kit and it is running great!
Great thread here - http://www.xs650.com/threads/rebuilding-yamaha-vacuum-operated-fuel-valves-petcocks.20662/
Thanks for the info. I think it may be related, as I have done everything possible and not found any faults. My petcock is on the left. I have bought a new non-vacuum petcock but can't try it out yet as the tank's in for painting. Will update you when I get the chance to run it again.
Cliff
 
I've been searching the web for seals, rings, etc. and can find many of them but apparently no one-stop shop. Any recommendations for a source of BS34 parts?
Have you checked out SUDCO?

May or may not help, but GS1100s also used BS34 carbs.
 
Thanks for the tip. Strange that they didn't come up on any of my Google searches. Have looked online and nothing for BS34s, but I'll contact them.
 
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