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New member here on some advice from Vulcan Forums. Members of both forums may recognize my post.
My VN750 is an '86. It ran perfectly when I bought it a couple of months ago. Then one day when I started it, it smoked like crazy, then cleared up after about 20 seconds. Blue smoke, smelled like burning oil. Next morning, same thing. Next morning, wouldn't start at all. Former owner said it might be carbs. Made appt. to have carbs cleaned (looked scary to do them myself). I did however flush the tank myself with evaporust before carb appt.

Bike shop also rebuilt petcock but when I got the bike back, I noticed they had misrouted the vac hoses. (Guy also said he had some trouble with the diaphragm in the petcock.)

The bike ran fine for a couple of days after returning from the shop but gradually things got screwy again. The smoke, it turns out, comes from the front cylinder, and only when the vacuum line from the front carb is hooked up to the petcock for vacuum. Sometimes, at idle, that hose being there will stall that cylinder completely and it won't fire. It will run cold and just pass unburnt fumes out the tailpipe. At higher RPMs it will fire and then smoke like crazy down the road. But then it will stumble and I'll be lucky to make it home.

Alternatively, I can disconnect the hose and run it on Prime and it will run fine for a while but then it will start to flood. Last time I did this, gas even began spurting out the vacuum nipple and onto the hot engine even after I shut it off.

BTW, when it's running poorly in any of these scenarios, it backfires bad.

I have checked the hoses and routing to the vacuum switch under the seat (many times) and ensured it's fine.

Plugs are new, spark is good, compression is good, air filters are clean.

I've read here in the forums that the bike is gravity feed so technically doesn't that mean I SHOULD be able to run it on Prime without gas overflowing the floats in the carbs? Or, put another way, can I rule out the petcock as being my problem (or at least MAIN problem) as far as the way the bike is running?

And what about the vacuum switch? Can I rule that out? Even if that or the "emissions system" per se was faulty, could it be causing these kinds of problems as I've described?

In the course of researching my fuel and vacuum lines and comparing them to the factory service manual, I realized I am missing this item, a "fuel vent valve", normally positioned between the carbs, which I then found a picture of on eBay:

In place of this valve, there is only a small length of hose that goes from the carbs to just above the rear cylinder head where it sits open-ended.

Interestingly, I also see NO place whatsoever on the back of my right side air filter for a hose from this switch to attach.

Is it a problem not to have this "vent switch"?

More broadly, does anyone see any red flags in anything I've mentioned so far as to what may be causing my problems?

Any help -or pointers - would be most appreciated.


7,983 Posts
First... Sounds like it has a needle/seat/float problem... Floods on prime, engine off.

Second ... Run it with the vacuum connected, petcock set to on. ... Shut it off and make sure the vacuum line is dry. If it's wet petcock diaphragm is leaking. But the needle and seat/float problem is causing flooding all the time.

Third... Both air filters have a spot for a carb vent, and that's where it needs to be, to keep it out of the wind.

Toss that vent valve, get a hose tee, hook two together, run one to the filter housing. You just have to find the hole. Search for a pic, it's out there.

Download the manual from my signature.
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