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Discussion Starter #1
Have noticed that when switching to RES on the fuel petcock I lose 1 of the cylinders for a brief period and several times lost power completely. The bike will eventually start back up after sitting a few minutes.

I also noticed that upon cold start only 1 of the 2 cylinders is firing during idle and corrects as the engine warms up or the throttle is opened

I pulled the petcock and the screens are clear and undamaged. The vaccuum diafram looks and seems to work, on the bench. Was thinking of pulling the carbs but don't think that will help and besides that seems to be a royal pain in the ass. I did not disassemble the lever side of the petcock but probably will to see if dirt clogged.

the bike has 24000 miles, the VH cruizers, been marbled, and KN filters but otherwise is stock.

Thoughts/ suggestions??
 

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Mine did that once back in February after not riding for a couple weeks. I pulled over and added some seafoam to the tank and after a couple more miles it was running fine. Hasn't done it since. I keep seafoam in the saddlebag. I'd try that first, before thinking about pulling things apart. Give it a good dose, maybe half a can on a full tank. Give it a few days, riding and sitting, and see what happens.
 

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Giggity!
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Mine was doing that at the end of last season. I had to rebuild the petcock due to a leak. In there I noticed my diaphram was SHOT, I thought that was the problem. With everything rebuilt it still does it. A tad annoying... But I know it will always fire back up.

Although, I wouldn't mind a solution.
 

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Discussion Starter #5
Thanks for the feedback. Sounds like just one of those quirks we all learn to live with. And I already an using 3oz of seafoam per tank. Will try a 'dose' to see if it will clean up any. Anything is better than trying to mess with the carbs.

:rockon:
 

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Newb with a Bullet!
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While we're talking petcocks, a newb question. The petcock is vacuum draw, right? So that means if you forget to turn it to off when you park it the gas should not flow, ie continue to run into the carbs and overflow, right?

Just wondering because I used to have a Yamaha that if you didn't turn off the petcock it would run gas gravity feed and it went into the crankcase, into the oil.

Will that happen with the VN750? Or does the vacuum draw preclude that?
 

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The float valve in the carb bowl is supposed to shut off after the bowls fill to a pre-defined level, so if your petcock were to keep running fuel, the float needle should shut off the flow. If the float gets sticky or the needle valve gets some crud in it, or is badly adjusted, it can keep letting fuel in and overflow into the crank case. Another thing you will often see on a petcock with a vacuum diaphram, if it gets where it won't shut off, it usually will let gas in through the vacuum line, right into the intake and your bike will run rich, plugs get fouled etc. Just some things to be aware of. If all is well, you shouldn't get any fuel running past your petcock when you bike is off, and furthermore, it should be stopped by the carb float valve if it does get past the petcock.

And yes, the vn750 petcock is vacuum operated.
 
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