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engine misfiring, probably carb problem

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engine misfiring, probably carb problem

PostAuthor: Rodinal » Wed Jul 04, 2018 12:44 pm

Hi there. I´m new here but I´d like to ask you for an advise. It has been a few months when I bought Virago XV1100, 95´. It was customized by previous owner. There is a open air filter, open exhaust and I also I can´t find the pressure sensor and vacuum lines. The engine was running poorly since beginning. That means it was misfiring at higher rpm until it eventually stopped, I could barely reached 70 mph and soon after i got slower and slower until the engine stopped completely. I´ve got a repair manual and went through some troubleshooting but kinda at the end of my invention.

The spar plugs seem to be firing ok. One ignition coil seem to be within spec, second has around 43 kohms so that I´m gonna replace it anyway. But I found both spark plugs very white so I assume very lean mixture. I took carbs out, cleaned them, replaced fuel valve which was quite dirty. Otherwise both carbs seemed to be clean and alright including diaphragm. However when I put carbs back, there was no change, spark plugs still white. But the carbs are full of fuel, fuel pump seems to work ok.

Now I´m getting to the point. When i removed the rubber elbows, i noticed the sliding valve with the jet needle is raising just a few mm when i open throttle fully although it should be lifted by the suction all the way up like in this video (42:16). That could explain the lean mixture because the engine starts fine and keep neutral rpm but starting to miss when you open the throttle...

I guess i will have to inspect carbs again, but do you have any suggestion where to focus? I mean how to make the diaphragm and sliding valve working? Thanks
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Re: engine misfiring, probably carb problem

PostAuthor: Flyingdog » Wed Jul 04, 2018 1:23 pm

:VTF:
Yep..those plugs are pretty lean..white insulator (iffy) cuz the electrode only has a hint of white, but the white ground strap..sure sign.
My first suggestions would be to use either spray carb cleaner or propane burner (unlit of coarse), start checking around those rubber carb joints. The joints could be cracked bad enough to let it idle..but no revs. I think that's your major problem. That's what usually happens when they go south on ya. Or the joints' 0-rings seal is broke/cracked/or missing a piece. Sounds like you got some major leaking going on, not letting your slides move. Try moving them with your finger before testing for leaks, to be sure they move within the bore smoothly and with just a tad of resistance. To confirm they are seating in the carb caps correctly. And not folded or pinched. You'll hear a slight "phoofff" when you push em up. When you (if you did) put the slides back in the carbs, are they OEM? Meaning, did the diaphragms have the little location tab? If not, make sure the two holes in the bottom of slide faces toward it's respective cylinder. Having this open air cleaner, what's that mean? You using velocity stacks? You may also have to re-jet (up) to get the A/F mixture back on target. Make sure those pipes w/the black bulbs are clear. They're also essential for the slides to work right. They are to equalize the air pressure above the slides diaphragms when they move. The are to be pipe to calm air...usually to one of the bug-eyes. You having Mikuni carbs..believe there's three of em. I think, have to check. Not having the MCV/AIS/pressure switch is no problem. Just make sure all the brass nipples on the joints are capped off. Hope of few of these items help ya get started tracking down the problem. Oh yea..the propane torch? If ya really get pissed..light it. :bg: (kidding of coarse)
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Re: engine misfiring, probably carb problem

PostAuthor: bstig60 » Thu Jul 05, 2018 8:28 am

The symptoms you describe would suggest you are running out of fuel at higher Rpm. Check your fuel levels using the clear tube method with the bike on the centerstand. They should be 1-2mm below the gasket line. The Clymers manual shows you the procedure. Also, I would question the fuel pump, check it to make sure there is good fuel flow, the pressure should be about 2 psi. Check your fuel filter to make sure it isn't restricting the fuel flow....
Bill
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Re: engine misfiring, probably carb problem

PostAuthor: Rodinal » Thu Jul 05, 2018 12:21 pm

Thanks for tips. I took a carb out to clean it again with air pump. So far I didn´t find anything unusual. The membranes have location tab and were positioned correctly. I guess it is this small hole next to jet needle which creates a vacuum and lifts the slides, am I right? (see a pic).

Re joints, O-rings were missing, but I used a high temp silicon and it seemed to be sealed fine, but is there any way how to figure out properly? (btw. i wanted to buy O-rings in local shop but these bastards sell only the whole kit of two milions rings...)

Re fuel, when I drained it, it was really a lot and I think it is fine, also fuel pump was checked. There was a broken wire, but fixed and seems to be fine.
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Re: engine misfiring, probably carb problem

PostAuthor: Flyingdog » Thu Jul 05, 2018 2:34 pm

Rodinal--So sorry, sometimes when I need to get somewhere, I try typing out an answer/suggestion too frikk'n fast and don't catch all the pertinent facts. Just like this one...you got Mikuni carbs..One of those major brain hiccups, always thinking Hitachi first. duh :doh: In answer to your question, yes. In conjunction with the equalization tubes (my phrase), on top of carbs. (tubes w/black bulbs)
As I said in my first post...carb cleaner or unlit propane torch around all joints, vaccuum hoses etc etc etc. If the idle changes..you found one of the leaks (if more than one). Some use WD..messy. You can still get just the 0-rings at all kinds of sites---> #14--- >> https://www.yamahapartshouse.com/oempar ... 907/intake But you might as well hold off until you find if the whole joint is bad...then just get both units for each cylinder. K & L is probly the top of the list..but you can get Chinese knockoffs for $10.
As mentioned earlier..don't forget to check fuel level in yer float bowls.
Here's the Hitachi slider holes I was referring too. My Bad!
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Re: engine misfiring, probably carb problem

PostAuthor: Rodinal » Thu Jul 12, 2018 11:53 am

guys, i´m getting a bit mad. No progress so far. I cleaned carbs again with air pump, checked the diaphragms again, they both seem totally fine, carefully assembled whole carb, put back and the slide throttle still doesn´t lift up when you rev up. With finger it goes easily. So I checked the vacuum leaks in the manifold with the starting fluid and didn´t really seemed to be leaking. I´m gonna replace them anyway, just ordered new on ebay. Anyway, could there be any other problem where i should look? Thanks I wanna ride finelly!

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Re: engine misfiring, probably carb problem

PostAuthor: Flyingdog » Thu Jul 12, 2018 2:08 pm

While your waiting on the carb joints, try blowing your compressor air (gently) into those tubes with the black bulbs. Watch the slides. They should rise. If not, the bulbs are probly blocked. They contain foam like filters material and breaks down. When you were cleaning the carbs, did you run a wire cleaner or guitar wire thru the opening. Did you shoot carb cleaner thru all the openings? If you shoot it in one hole..its' got to come out another. But if everything checks as you claim..those slides should work. Only thing that stops them (since you say you can move em with your finger) is an vacuum leak. My 750 was doing the exact same thing. No leak what so ever. But still wouldn't rev over 3k. Until I took the manifolds off and gave them a good squeeze. There is was..tiny cracks open like the grand canyon. Which in this case, the vacuum leak would only show up under the pressure of revving. Idling, it showed "NO" signs of leaking..spraying carb cleaner, propane, stethoscope..nothing showed it..."At Idle". I would almost wager if it's not those tubes..it's the joints just by the way you're describing. Do you still have the MCV/AIS system? Those are also prone to massive leaks. But usually not to effect the slide/needle thingie. Also, not 100% sure what that third pipe/tube with the bulb go to. Just not all that familiar with Mikuni..someone should help with more info.

#24--? In this diagram: https://www.yamahapartshouse.com/oempar ... 907/intake

#31-? Not sure where is pipe goes to: https://www.yamahapartshouse.com/oempar ... carburetor
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