150 Pro V top cylinders not sucking... which sucks.

[iammarcuse]

I have a 1990 PROV150LD. that stumbles at idle and often coughs and stalls. I rebuilt the carbs and set every thing to spec. The motor is running great off idle and turning 5500 rpm at wot. After tinkering with the idle adjustments I noticed I was getting almost no vacuum in the top 2 cylinders, I can put my hand over the throttle body and there is little suction and the motor does not change rpm at all. As a result there is little fuel entering the throttle body. Basically I am idling on the bottom 4 cylinders. Once I crack the throttle I get good vacuum and the motor runs fine.

I ran a compression test and I am 95-100 across all 6. I looked in the intake and the reeds that I could see looked like they were sealing. Any ideas would be greatly appreciated?

 

[99yam40]

follow the link and sync procedure every time you put the carbs back on

 

[iammarcuse]

yes sir I do

 

[99yam40]

Sounds like top plates are closed off more than the others if cracking open the throttle will get the top ones to finally show vacuum,

Only other thing besides the throttle plate shaft in top carbs I can think of in common to both cylinders is the labyrinth seal between the 2 cylinders on the crank

 

[iammarcuse]

Sounds like top plates are closed off more than the others if cracking open the throttle will get the top ones to finally show vacuum,

That's a good thought. I am going to re do the link and sync. It's possible I didn't get it right.

 

[iammarcuse]

Well good call 99yam, the carbs were out of sync. They didn't want to close properly on there own and needed a little pressure while tightening the screws to keep every thing lined up. Once I was sure they were all in sync, it idled perfectly. I was surprised at how such a tiny bit off affected the run quality. Thanks for your help!

 

[99yam40]

and a lot of people do not even know they need to do this each and every time the put the carbs back on.

glad you got it ironed out

Problems can sometimes be simple and cheep to fix