'96 Virago 750 running problem

Status
Not open for further replies.
Joined
Oct 2, 2008
Messages
1,007
Location
Dallas, TX
Hopefully someone here can help me with some bad running I'm experiencing.

Upon cold starting, the bike fires quickly and seems to run really well with the choke about halfway on. But slowing it down, or turning off the choke completely even after a couple of minutes yields a very slow, lumpy idle and the sound of a 'miss'...not on every stoke, but on every few. I'm getting a 'sputtering' from the exaust. Once the bike is fully warmed up from a long ride, it either idles at a normal level, or a little high, so adjusting the idle up isn't a good remedy for cold idle. I tried it...it only exacerbates the following problem:

While riding, whether hot or not warmed up yet, it likes to hold onto revs, which makes shifting awkward since the engine is still revving by the time you're ready to release the clutch for the next gear.

I've also noticed that when sitting still with the choke off, even when the bike is fully warmed up, revving between 2000 and 3000 rpms yeilds a random and repeated backfire from the right carb, which causes a very noticable miss. I don't notice it when riding because it passes by that engine speed while acclerating. For the most part, it's got good power and responds well.

I've pretty much ruled out a vacuum leak of any kind, although I haven't tried spraying starting fluid to see if engine RPMs increase. The bike only has 8600 miles on it and I had it apart yesterday for a valve adjustment. All the hoses are in pristine condition, connected, and all the clamps are tight. The only conclusion I can come to is that the carb is dirty from sitting so much. I just didn't expect this type of running from a varnished carb. Plugs are new, gas is fresh. Tried a tank with some Seafoam and that made no difference.

Anyone have insight on this? Do I just need to disassemble the carb and clean everything, or is this caused by something I'm overlooking? Soak carb parts in Berrymans, hand clean and re-assemble?

I greatly appreciate any input.
 
It almost sounds like something is partially plugged in the idle circuit. If they are mikuni carbs I would bet on it. I would bet that the float arm pintle rod is made of steel rather than stainless and there was a fine silt in the bottom of the carb. Rust from the steel pintle. This ultra fine silt will get sucked into the idle circuit metering orifice and slighly plug the microsreen causing a lean condition and a creeping or high idle warm and a bad idle cold. Pull the diaphram and needle assembly out and clean the micro screen on the idle circuit port and it should be fine. Do not adjust the sync screw on the side of the carb unless you want to resync the carbs to the engine.

Hope this helps.
 
Hey, thanks for the input. Can I trouble you for one more bit of info? I'm not sure where this metering orifice is. Is it a jet?

Viragocarb.jpg
 
The pilot jet is generally part of the idle circuit.
A good carb guy can pull the cap on the adjusting screw and do a good adjustment on the idle mix.
You might try some SeaFoam or MMO in the gas for a couple of tanks worth.
Turn the idle up a bit in the mean time.
My 2¢
 
Thanks. Seafoam has had no effect thus far. I have idled it up to keep it running long enough to get out of my garage with it, just to end up idling it down a few stoplights later when it's idling at 1200.

I think I've got a cleaning job ahead of me.
 
Well, guys I pulled the carbs last night and removed everything I could and cleaned it. Both main jets and the pilot jet, on both carbs. All diaphragms are in perfect condition. Nothing has changed. I've still got the 2000 - 3000 rpm backfire through the carb, yet it cruises smoothly down the highway this morning at 70mph and has strong acceleration.

Could this be a cam timing or ignition timing issue? Seems like the bike would run badly at all speeds if it was that, but I'm at a loss. The carb wasn't bad at all. 03CVPI was right about the fine dust at the bottom of the float bowls, but the varnish and crud I was expecting to find just didn't exist.

Anyone?
 
The high hanging idle is a sign of lean mixture or a vacuum leak in my experience. You cleaned the carb and have not changed the jetting so I would do the old spray something around the carb and vacuum lines and see if there is a rise in idle from a leak. Valve lash out of spec (to tight) can also cause similar problems. Hope you get it sorted out.
 
Thanks. I'm gonna do the spray test tonight. Valves checked last weekend. I almost hoped that was part of the problem, but they were good.
 
At the bottom of part 14 in the exploded, it should look like a ultra fine mesh screen. I have had numerous bikes with an idle problem and it always seems to be the micro screen on the idle circuit. Especially if there is silt present in the bottom of the carb bowl. CVEN is also right, cracked carb to head boots, loose screws at the test port or on the boots themselves, or bad gaskets on carb could produce the same result, very lean mix or vac leak. Also is the fuel controlled by a vacumm diaphram. I have had a few bikes where the fuel will only flow to the carbs if engine vacuum is present at the fuel petcock diaphram. If the diaphram cracks, it can begin to leak a small amount of fuel into one of the carbs through the vacuum line causing one carb to appear to run lean while the other carb runs rich.
 
I did a carb synch first with the valve clearance to see if that was the problem.

03cvpi, I'm unsure how to get #14 out of the carb. Does it come out through the bottom after removing the main jet?

I did a check of the vacuum petcock a few nights ago by unhooking the vacuum line, setting the petcock on "PRIME" and plugging the vacuum line. It made no difference with the backfiring / rough running.
 
Just had a thought: What about a valve problem? Would an intake valve that's not sealing against the seat cause backfire through the carb? Or would that cause other bad running problems as well? The bike starts very easily on 1/2 choke and runs / accelerates great.


Thoughts?
 
Ok guys, just wanted to thank you for the ideas. Here's the saga as it has played out this afternoon.

On my ride home from work, I noticed that it was no longer holding onto RPMs during shifts and was acting very normally in that respect. I guess I did SOMETHING right last night during the carb cleaning. But, the carb backfiring was still present.

I picked up some carb cleaner and went home. As I sprayed various areas of the carb, sometimes the engine would speed up marginally, and twice it slowed to nearly dying. But what happened next was an awesome surprise. The backfiring at 2-3000 rpms stopped...completely. It idled smoothly. I couldn't get a carb backfire no matter what I did. So, I shut it down, washed all the carb cleaner goo off, and went for a ride. About 8 miles later, I returned home and the carb backfiring had returned, although not as pronounced as before.

This is a pretty clear sign that I have a vacuum leak, correct? I focused a majority of the spraying on the intake boots, so should I plan on ordering a new pair of those?

Tell me what you think.

Thanks
 
It seems you have found your problem. I would do it again to make sure that is it so you don't throw money away on new boots, there not cheap most of the time! They are a semi-common problem after many heat cycles they dry, shrink and start leaking . I'd check everything else for leaks FIRST. You may be able to reuse them with some sealant (yamaha-bond #4 is good @#$% ,its the grey stuff)if they are leaking at the flange. If that seems like the problem be careful taking the screws out that hold them on they may be seized in the head and can be a $%#& to get out. Again please check other things first so you don't throw some money away.
 
In my excitement, I immediately ordered new boots. I'm pretty sure that's the problem. I'll be picking up some Yamaha-bond. That must be what the dry crusty stuff was on the engine side of the old boots.

The bolts on the boots came out easily the other night, once I was able to get a wrench on them. Wow..what a tight spot. I had to bend my box-end wrench to get the right angle on the bolts. I ordered new bolts with the boots since they were cheap, and the old ones took some abuse on the first go-round.

I inspected the rest of the vacuum lines and they all look and feel like they're brand new. Even the clamps are in great condition. I sprayed the throttle rod and all around the carb and got no response. All the signs point at the boots. They can be very deceptive, as they look and feel like they're in great condition. Pliable, no visible cracks. But clearly they're not sealing to the heads...or at least the right one isn't.

Thanks for the advice guys...I'll update you after the new boots are on. Cross your fingers.
 
So, how did it go after you put the new boots on?

Im interested because I have the exact same bike with the exact same symptoms that you've described and Ive basically done all the things you've tried to solve the problems, yet its still not right!

I cleaned out the carbs, checked compression & valves, checked for vacum leaks and cracked hoses, put on new intake boots and cleaned the exhaust and inspected for holes/cracks etc...and although its running has improved somewhat, it still holds onto revs and occasionally revs higher when hot.
Last night I couldnt go any faster than 125km/h on a cold night!

So Im thinkin its that little mesh thing in the carburretor where brown silt was found in the float bowl. At least i hope it is cause I dont really want to pull it apart anymore its just getting annoying now
 
Status
Not open for further replies.
Back
Top