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Discussion Starter · #1 ·
Hello all. I’m having some serious trouble with an ‘86 VT500C. I’ve read lots and learned a ton, but I feel like my problem is a little unique. It starts and runs great for a couple miles, then misfires, seems to run on one cylinder, and will only (barely) stay running with throttle wide open. It sputters and misses and dies a lot. After it sits for a while it does this exact same thing—every time. It did this before and after a local vintage bike shop did an ultra sonic clean and rebuild to the carb. I’ve replaced the plugs, fuel petcock, battery, and checked voltage. The tank has been cleaned of all rust.

I pulled the plugs and can see arc on all four, but they look really rich.
Hand Automotive tire Spark plug Finger Nail


Appreciate any advice you guys can give!!
 

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1983 Honda vt750 Shadow
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Yes pretty rich. Was it that way before the carb work? Bigger jets?
Try to adjust the mixture screws in 1/2 turn.
Or you can tweek them= go back and forth side to side, to get the fastest smoothest idle.
Then reset the idle speed.
Maybe a choke problem if they are not seating because the cables won't relax when the lever is supposed to be off.
Is there any air filter restriction?
 
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Discussion Starter · #3 ·
Yes plugs looked the same before carb work. We adjusted mixture back and forth (1/2 turns at a time) and it seemed to help at first, then back to misfire and same old thing. I’ve played with the idle too, doesn’t make a difference out on the street. When I choke it—plungers? —it dies immediately. Haven’t checked air restrictions at all. Also, I’m running straight headers, no mufflers—not by choice, just weren’t on the bike when I got it.
 

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Check on the choke operation and make sure the carb bowl vent hose is open to atmosphere hanging down behind the engine.
If it has an auto petcock fuel valve, check for a diaphragm leaking into the vacuum hose at the cylinder
 

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OP. When I read your read your description about the problem occuring when the engine warms up the first thing I thought about was the coil. Coils and high tension leads can act differently under hot cold conditions. The pic is confusing though as it sure seems rich. Could it be appearing rich because the plug is misfiring and there is an excess of unburned fuel.
 

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Discussion Starter · #6 ·
I’ve been thinking about coils too since it only seems to do it when it’s hot, but I haven’t removed them to test resistance—just made sure the plugs were firing.
Adjusting mixture screws helped with richness, I had it running smooth, then adjusted idle. But it immediately started sputtering again and doing the same thing.

@swifty2014 I don’t think I have an auto fuel petcock, do I? It’s just the manual one coming out of the tank, and it’s new. The carburetor vent hoses have a tee fitting and then a single hose runs out the right side of the frame, sort of hang down behind the shock.
 

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I couldn't quite see if the tach was bouncing. The tach signal comes from the front cylinder yellow wire on most of these. So if the front coil or ignition was faltering, you may see the tach react when it starts misfiring.
 

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Discussion Starter · #14 ·
New coils just installed—hoping that would fix it. It didn’t, but I’ve discovered that it is only running on the rear cylinder. I’m getting spark on all four plugs, but the front cylinder is not firing at all.
 

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How do know that it's not firing on the front. Are the front plugs still the same sooty. When you say there is spark on all plugs what colour is the spark. Should be blue. Maybe the front is flooded. Maybe try pinching the fuel to its carb and party draining the bowl then see if it fires. If the plug sparks but does not fire then there is a problem with the fuel. Either none or possibly flooded.
 

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Discussion Starter · #16 ·
The front cylinder is cold, only cold air coming from that pipe, and the bike is running horrible—actually worse than ever. I see spark when I remove the plugs, it’s mostly blue. It shocked me pretty bad when I pulled the wire off (wearing shorts, leg touching frame) so I think it’s strong. The plugs look the same, covered in black soot.

If I see spark, am I correct in thinking that the igniters, coils, wires etc. are all fine? How can I identify a fuel problem to only the front cylinder?
 

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Start by draining the float bowl for the front cylinder and see if any fuel comes out. If not you have a restriction somewhere either fuel line or carb.
 
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Yes if you have spark and fuel you should get combustion in the cylinder. If sooty apparently too much fuel as opposed to too little. Also you need the spark at the right time. Are you sure the plug wires aren't mixed up. Is there some other contamination in that cylinder. Oil? The sooty would imply too rich. Or flooded. Such as could be too high level of fuel in the carb. That it why i suggested restricting the fuel to that cylinder to test. Or maybe a problem with the choke or insufficient air to it. Maybe a balancing problem but others here know more about that then me.
 

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Nick - If the front cylinder is indeed getting fuel AND the plug wires are configured correctly AND the plugs are as sooty as the photo, I would guess you indeed have an over-rich issue that's fouled the spark plugs. Replace all four plugs and try it again. Would hazard to guess the spark in the front cylinder is getting shorted through the soot on the electrodes (carbon).
From another post on here, do check your air filter to make sure it isn't oil-soaked or dirty... either will cause the over-rich condition and fouling you're experiencing - Kevin
 
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