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Guest Message by DevFuse

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Help with CV Carbs (motorcycle)

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2 replies to this topic

#1 lostinthe202


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Posted 21 March 2010 - 01:07 PM


I've got this winter project in the form of a 1987 Yamaha FZ700. I've got it running but I'm having some carb trouble that I just can't lick. There's a few riders here so maybe someone has some suggestions.

Here's the particulars,

The first time I got it started, it started easily enough (after 5 years of sitting) but it wouldn't idle down and it sounded even but rough like it wasn't running on all four. If I touched the throttle it would rev way up and take for ever to come back down. The throttle cable is free moving so no stuck throttle as is the throttle plate for each carb.

I chased down the ignition issue and fixed that and restarted. This time the revs shot up to to about 7k and stayed there. I shut it down after a few minutes. I restarted and it "idled" at about 4k and wouldn't drop below that. Verified that it was running on all four.

I pulled off the airbox and restarted and it idled at a more normal 800-1000k rpm. If I goosed the throttle a hair it would behave as expected, but if I twisted any more then a smidge it would shoot up to 7k or so and take a long time idling back down.

So work I've done, I cleaned the carbs. Actually they were pretty clean, no clogged jets, I replaced the needle valves and checked all of the diaphragms for general condition and holes (as best as I could see). When I put them back together the first time I did pinch one of the diaphragm gaskets which I thought was my problem, but while I was diddling with the electrical I fixed that issue as well but clearly I still have a problem.

Do I still have an air leak in the carbs someplace? Could there be pinprick holes in the diaphragm? I'm pretty sure the intake boots are fine. I'm wondering if I could check the diaphragm tops somehow maybe?

Any suggestions??????



#2 john in KY

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Posted 21 March 2010 - 03:31 PM

Where is all the air coming from to support idling at 7K rpms? I'd determine if the butterflys are open or still in a nearly closed position. If the butterflys are in a normal position, then major air leak downstream of the carbs. If butterglys wide open, then determine why.

#3 lostinthe202


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Posted 22 March 2010 - 02:28 PM

Thanks for the reply John,

The butterflys are operating properly. I'm going to pull the intakes off and see about changing the o-rings and see if that helps. The intake boots are still nice and soft so I don't imagine it's those, but at this point I'm pretty clueless.


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