My Wife's Bike Is Driving Me Nuts

[ThomWV]

My wife has a 1994 Yamaha Virago 535. Its a very nice little bike when its running right, a mean little bastard when it isn't. The problem is the rear cylinder (V-twin) quits firing. Generally all I have to do is replace the plug and its just fine again, but that only lasts for about a week, and then it quits again. So I'm thinking its clearly an electrical problem, not enough juice from the coil probably and as the plug's internal resistance increases with age there is no longer enough voltage to jump the gap. OK, I can live with that while I keep an eye out for a replacement coil.

The thing is the bike has a strange fuel tank system and even though its a carburated bike (two Mikunis) the thing has an electric fuel pump. That is because the tank you see at the top is really just there for show, it holds about a gallon. The main tank is down below the battery, behind the engine and below the carburetors. This second tank holds another couple of gallons, I'm not sure how much and I don't much care.

When I was riding the bike back home yesterday, a trip of about 20 miles, it was running for shit while I was in town. It was missing and popping, rear cylinder was running intermittently. Problem was it would not run right under constant power and was acting like (backfiring) it was running lean as hell when decelerating. I noticed that the switch on the grips was set to axillary tank so I switched it back to what I believe to be main (it actually says "Res" and "On"). Then about a mile or two later I had to climb a steep hill for about 3/4 mile. The bike started to run flawlessly. When I got to the top it flattened out and so I was riding under more or less constant power (there was traffic) and decelerating often in the up and down of the West Virginia hills. Everything it had been doing wrong before was gone and it continued to run flawlessly.

This morning I checked the bike over to see what I have to do to make it good for her to ride this year, air pressure, fluids, that sort of thing, and took it for a short ride while I was at it. Once again, it started instantly (something you never expected from that cold blooded son of a bitch) and ran perfectly. I had a spare spark plug gapped and at the ready but I put it aside, not wanting to screw up something that was working just like it ought to.

So tell me, what do you know about intermittent missing? I presume the problem is not the carburetors, not after the outrageous amount of money I had to put out for parts when the Dealer rebuilt them last year. Got any ideas?

[HamstersFromHell]

1. Going to take a wild assed guess here...

Haven't worked on a 535 Virago, but did several of the Liter+ ones a few years back...similar systems.

Missing on the reserve tank but smoothing out on the main tank leads me to think there's water in the gas tank. Only picking it up when it's on reserve and of course, pulling from the very bottom of the tank where the water would be. Probably in the fuel system piping as to why it affects the rear cylinder more than the front. (Fuel line from the pump splits vertically at some point and feeds the rear cylinder first? Water being heavier, makes the first downhill "turn" it can take and ends up mostly in the rear carb.)

Might be 100% wrong here, but I'd try to pull the tank if possible and drain it, or else toss in some alcohol to absorb the water and burn it out.

One thing I have definitely noticed in the last two years: Since gas prices skyrocketed, quality of gas overall sucks to no end, and I see a LOT more water in fuel systems nowdays.

Hope this helps!

[ThomWV]

2. Probably a good guess

The fuel line splits at a fitting under the tank. There is a single line up to the split and then an individual line runs to each carb. I will reverse them and flip it back to the reserve tank and see if the problem reoccurs, but with the front cylinder this time. Otherwise it might just be my best bet to go ahead and override the fuel pump cut off (5 seconds of ignition on without engine start shuts off the fuel) and just pump it empty.

When we bought the bike, last year, it had been sitting for 3 years. The results were horrible. I pulled the main tank and cleaned it but the second tank was a pure mystery to me. I used a good bit of Sea Foam in it and that helped but the slides in the carburators were stuck and everything was gummed up. I pulled the carbs - a major pain in the butt - and cleaned the float chambers and the slides and their passages but it wasn't enough. So I took it to the dealer who did a complete rebuild and had to replace a number of parts, including the slides (I knew one of them had a ripped diaphram so it was no suprise). Hundreds of bucks later it ran well, but it didn't last, as the rear cylinder problems started a couple of weeks later. Constantly changin plus has helped.

On to plan 2.