‘86 Virago 700 running on front cylinder only

[Streetfighterkz]

Been working on a 700 Virago and having a tough time diagnosing a dead cylinder.

The bike came to me with a smoking issue. I started the bike and it sounded like a thumper (1 cyl). When it first starts, it seem to run fine for the most part, then it would slowly drop the revs until it died. I noticed that the front carb manifold’s o-ring was torn and you could see it sticking out. So I ordered 2 new ones. When I went to remove the rear cylinder’s manifold, one of the mounting screws was slightly loose. While I had the carbs off, I gave them a quick clean as the inline fuel filter that was installed had sediment in it. Put it all back together and no change. I tested compression after getting the engine warmed up. The front was about 155 and the rear 165 or so. Threw new plugs in (DPR7ES). No change. Tested spark by unplugging the lead and using an old plug laid on the cylinder head. Both cylinders seem to fire fine. I can start the bike and remove the rear cylinder plug wire while it’s running and no change. I remove the front and it dies immediately.

Right now I’m at a complete loss. Any help would appreciated.

Later, Doug

 

[cxman]

take the rear vacuum port cap off the rear cylinder manifold start the bike

squirt a bit of fuel in the vacuum port

if the cylinder picks up and fires you have a carb/fuel delivery issue

if it doesnt you have a mechanical or ignition issue

 

[Streetfighterkz]

Seems to be an electrical gremlin somewhere. Changed nothing and the cylinder is now running (at least for the time being).

Another problem arose: the carb feeding the front cylinder is overflowing to the point that I have to remove the spark plug and spin the engine over to get the gas out of the cylinder. I never touched the float level but I pulled the carbs to bench test them.

This is turning into a never ending repair.

Later, Doug

 

[Streetfighterkz]

Update: Got the fuel overflow problem solved. Also got it running on both cylinders after checking over just about everything. After more research, it turns out the Viragos are finicky with the spark plugs. They can’t be resistor type. I bought a pair of NGK BPR7ES as the parts store did not have the non-resistor style. Went back and bought a set of Autolites that were non-resistor and magically the bike ran on both cylinders without any other change being made.

I’ll have to remember that in the future.

Later, Doug