Carb & vacuum question

[Dano]

OK, got the "twinkie" (oil cooler?) redone and installed. And the bike actually started and ran after that!!!! However, as some of you know, I've been having a problem with the bike back-firing under load below about 3k on the tach. Just for the heck of it, and checking the pressure (boost) sensor, I pulled the sensor hose off of #2, held my finger over the hole, raised my rpm's to 2500-3000, applied vacuum and the rpm's changed. Sensor good.

Put the hose on and decided to check the vacuum on the other cylinders. #2 dropped a little with open tube, same as 3 & 4 (Maybe 100 rpm). However, and this is what is mystifying, #1 when opened up, the idle INCREASED dramatically, almost 500 rpm! HUGE amounts of vacuum on that carb?

My question to you'all then, is #1 OK and the other 3 are weak? Or is #1 too high and the other 3 are OK?

Could this be tied into my back-firing problem?

 

Inquiring minds NEED to know!!!!!

 

Stupified Dan

[MiCarl]

Sounds like you need to synchronize the carburetors.

 

#1 has high vacuum (low pressure) because it's butterfly is too far closed relative to the others. It isn't getting fuel/air and the other three are dragging it along. Basically the other three cylinders are working hard to pump a vacuum on #1. When you open up the air flows and the engine isn't working as hard. Engine speeds up.

[Dano]

Thanks MiCarl, I'll hook-em up and see. I had a sneaky suspicion, but wanted to see what diagnoses was.

 

Thanks,

 

Dan