QuestionI have a Yamaha Virago XV750 from 1982 with 15,000 miles on it, which I bought used a few months ago. It ran great when I first bought it. Decided to take it out for a ride a few days ago since it was the first nice day in awhile - 58 degrees out. Running great, but then 1/2 hour into the ride, in 3rd gear with about 2,500 rpm, i'm riding on pretty level ground at about 40mph, and haven't done a single thing when suddenly the engine just loses power and the rpms gradually (but smoothly) dives down and down, and 4 seconds later its I pull to the side of the road, it dies. I think to myself, "hmm, gas?" and then switch it to prime for a sec and then reserve. I try to restart it in neutral, and it starts up but then very quickly climbs up to 4 or 5,000 rpms, without my touching the throttle.
I check to make sure the throttle cable wasn't stuck, and the choke cable isn't either.
I check the plugs, and the rear one looks a little white - not too bad though. The front looks a bit sooty, but again, not bad. The rear cyclinder is very hot of course, but I assume that's normal for a V-Twin. I decide to let it cool off for a half hour, and try starting it again. Same thing. Then I check the levels in the fuel bowls with the clear tubing I carry around in my tool kit - looks good, just a hair under the fuel bowl split.
I call my friend to pick me up, and take the carbs off while waiting, put the cover on it, and bring the carbs home. I drained the bowls and looked at everything, and to my eye it all looks in good working shape. Just a few weeks ago I had mailed them off to a guy who is very highly reputed for cleaning Virago carbs, and he did an ultrasonic cleanse on them and checked them out.
The strange thing is that the bike was running just perfect for 30 minutes, and then not at all. Very sudden, with no apparent cause. Must have hit a bump or something I guess....
My prime suspicion is an air leak, but I was extremely careful to avoid this, and since I've take the carbs on and off about 7 or 8 times now, I feel like I've got the hang of it. I checked for a vacuum leak by taking off tubes and checking each one by covering one end and sucking on the other. All tight.
So I went back out to the bike the next day and put the carbs back on carefully, actually using some Permatex semi-hardening gasket sealant on the carb intake holders to get a really nice seal on those (between the engine body and the carb intake holder). I also put another gallon in the gas tank, since it probably only had a gallon left in it. And just to be sure, I reset the idle mixture pilot screws on each carb to 2.5 screws out - which is rich I'm sure, but shouldn't give me these kind of problems, right?
When I started it up, it did the same sorta thing - idled really rough with the choke in, between 1-2,000 rpm at first, then when it warmed up it started climbing, and then when I rolled the throttle a bit, it climbed to 4,200 and just stayed there, even after letting off the throttle. Only stopped when I turned the key to the "off" position.
I started it up again, and it went up to 4,200 again, and then I sprayed starter fluid from all angles on the intake manifolds and carb intake holders, but no sign of any reaction from the engine. So I don't think its an air leak now.
These incidents both occurred on 50-60 degree days out here, so I don't think weather is a factor. I have NOT synched the carbs yet since getting them back from the cleaners, because I don't have a carb synch tool. But a friend said I could borrow one - could this be the problem? I didn't think carb synch problems could be this bad, and that doesn't really explain the intermittent nature of the problem.
I am fairly new to motorcycles, but a pretty mechanically-inclined person. Any help you can offer would be appreciated.
- Paul in RI
AnswerHi Paul, sounds like you have the majority of your issue covered, but with the same result, thanks for all the background.
Viragos are known for having issues with their "fuel evaporative system" located under the left chrome "a/c" cover and sometimes become stuck. This will cause a strange idle/vacuum/fuel type senario. next time its idleing high, try tapping on this valve (with the chrome cover off) see what happens. I think this could very well be your issue. Unfortunately there is no real solution to checking, and to add insult to injury, the part is not cheap, If memory serves, around $85.00
Hope this helps!
~Mark
