Virago misfire/stuttery/sudden power/backfire/carb farts etc

Well, here is the story so far, brace yourself.

I owned my Yamaha Virago XV700 for around 3 months, during which time I rode a hell of a lot. Around early November I noticed the bike starting to stutter a lot, being very jerky with intermittent power, so I put some red X into the fuel to see if I could clear the crap out which I thought might be causing the problem.

Anyway, it got worse and worse and started to heavily misfire, a lot of backfiring, carbs popping and pretty much running on one cylinder, with the other kicking in suddenly giving me the odd burst of power.

I managed to get it to a local mechanic and after 2 solid months with it (messing me about NO END), he had sent the Carbs off to NRP to be cleaned and rebuilt AND replaced an ignition coil which had gone and proclaimed it was done!

On the way home I knew something wasn't right, and the next day when i took it for a longer ride it was just the same as it had been when I took it to this guy.

I called "The Hogfather" In Southport, a cruiser specialist and asked them to take a look at it. After talking me through what had been going on, they told me that all they thought it may need was a good service, and it seemed to work.......seemed to. Just before calling me to confirm to go and pick it up, they test rode it once more just to check, and all the problems were still happening.

So.......I am fresh out of ideas right now. A few things have popped up when chatting to people; The CDI box, fuses, petcock, faulty wiring, wrongly balanced carbs and on and on and on yet around 3 months since the problems first began it is still in the same shape as it was.

I have been told that if these guys at Hogfather can't sort it, they will have to call in a Yamaha specialist, this looks like it won't even be worth the money because of what the bike is worth in total. SO this is a cry for help hah.

 

were abouts are you

 

I had a very similiar problem with my Yamaha TRX - carbs on carbs, off re-jetted - new spark plugs, evertything tried. Turned out it was a hairline crack in one of the carb float bowls, are you using more fuel than you would nornally as this was one of the symptoms too?

 

I am in Preston.

No actually. I noticed I used a lot less fuel. This seemed to me because the fuel might not even be getting through to fire the other cylinder, but I don't know.

The MPG didn't seem to be that good in the first few months I had it before this all happened, but I worked it out and it seemed to be average for a larger cruiser.

Hogfather have said they are slowly working through the issues, and this has really had their mechanics stumped. The engine is running a lot better apparently but the cost of this looks like it will be unbelievable.

 

I'm no carb expert but from what little I've gleaned over the last 18 months since buying my (carbed) bike is that it sounds like a vacuum leak. That said, it should be the first place a decent mechanic would look so...*shrugs*

How old's the bike? If might be that the rubber bits between the airbox and carb or carb and head have perished or one (or more) of the vacuum caps have gone walkies.

How does the bike idle? Is it even or do the revs go all over the place?

To test for a leak, you can either visually inspect all the rubber bits for cracks or, with the engine running, blast the rubber bits with carb cleaner. If the idle speed rises when you do this, it's a vacuum leak. Do it in a logical manner and you should be able to isolate the offending item.

Like I said, I'm not a carb expert but it'd be the first place I'd look.

Let us know what happens.

[image]http://inlinethumb01.webshots.com/10176/2496112910001097251S500x500Q85.jpg[/image]

This is a pic of a Keihin CVK carb but it gives you an idea of what you're looking for. There are 2 vacuum caps visible (one stuck straight up, the other at 45°)-you need to check that they are present and aren't perished and also check the rubber bits held on (in this pic) by the jubilee clips. Carb boots, I think they're called.

The problem may also be a vapour lock. Run the bike without the fuel filler cap; if it runs OK, you've found the problem.