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'82 XV750 Carb Issues

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'82 XV750 Carb Issues

PostAuthor: otyler14 » Mon Jul 08, 2019 6:12 pm

Skip to bottom for main questions.

Backstory: I have a newish to me '82 XV750 that I got running pretty well so I decided to perform a carb sync (using a DIY tool, see pics below) and noticed that they were way off, so much so that the fluid was being sucked up into the front cylinder reservoir on the carb sync tool. I decided to take the carbs off and inspect them since I knew the bike had been sitting for a while. Upon inspection I noticed the screen on the needle seat bolt had fallen off and was blocking the passage below. So I ordered a rebuild kit that had all the same jet and pilot screw sizes and rebuilt the carbs and bench synced them so I at least knew they were close before putting back on the bike. Once on, the bike will only run on full choke and is spitting white maybe grey smoke? out the exhaust. I've tried adjusting the pilot screws but nothing seems to help. With not choke the bike will idle but any throttle input will kill it.

So questions:
a) what could be causing the bike to not want to idle?
b) why would the carb sync tool show them being so far off? I did switch the lines on the tool to make sure it wasn't the tool itself and the problem followed so I don't think it is the tool.

Any input would be greatly appreciated!

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Re: '82 XV750 Carb Issues

PostAuthor: Flyingdog » Tue Jul 09, 2019 4:25 am

Long distance diagnosing is always fun. And something like this is probly going to take patience and a lotta trial and error. So be patient and with the help of others, hopefully something will turn up.

otyler14 wrote: what could be causing the bike to not want to idle? [/img]


First thing comes to mind on "not" idle'n, would be the pilot circuit. By saying it only runs with choke on points to this. As the chokes on these things are not like conventional chokes. IE: Pulling choke lever, you're actually opening a passage that dumps "more" fuel into the carbs, instead of cutting off air, as in conventional chokes. Why the technical term for the Virago choke is: Starting Circuit. Sounds like a more thorough cleaning is needed. Your statement about "just inspecting" the carbs is suspicious. Ya just can't throw new parts in the mix and expect "new" conditions. Nothing about what or how you cleaned "all" the passages. Making sure where you may have squirted carb clean in one passage, it also has another opening where the spray "should" come out. Also making sure the float levels are set correctly. As in you can set them on the bench, but also doing a fuel level check by the "clear tube" method, is needed. But, the bike has to be running/idle'n fairly decent. You technically can do the clear tube method on the bench, but sometimes not quite as accurate. This is where the back and forth comes into play. Frustrating..yes..but necessary. Far as setting the pilot screw is concerned, gotta make sure the pilot screw stack (spring/washer/ (new) 0-ring is in good shape. I'd set them at 2 1/2 turns out from a soft seat and leave them alone, for now, until you get the bike running. Basically, the pilot circuit (pilot screw) will be used to get ya the best "idle".

otyler14 wrote: why would the carb sync tool show them being so far off? I did switch the lines on the tool to make sure it wasn't the tool itself and the problem followed so I don't think it is the tool.


I would suspect this is cuz..they probly "Still Are" outta sync. When on the bench how did you accomplish this and at what height of the butterfly did you use? As you probly noticed, the #2 carb is set with the idle adjuster, so basically it is not synced, but set by the idle adjuster. Then when on the bench, you have to loosen the little adjust screw/lock-nut on the #1 carb, then you'll actually be setting sync (by way of sync screw) the #1 carb "TO" the #2 carb. Just leave that screw (#1carb) loose until you get the carb sync'd on the bike (running) All this is just ball park settings. The actual sync'n is done on the bike..running. Then, that little screw is turn up so it's "just barely" touching the lever and lock it in place.
Far as the greyish smoke, could be clogged air filter, carbs running too rich, venting hoses (2) on top of carbs could be clogged. Possibly over filled crank case oil (coarse this would be more bluish) and you'd smell it. But, still very critical.
So as you can see, it's going to take some back & forth, up & down fiddl'n to get a good baseline, then on to fine tuning.
Kinda tough, to give out an "exact" starting point.
Bottom line, get a manual and bone up on the fuel section.
And..don't forget the electrical portion is just as critical. IC: coils, primary and secondary wiring, plugs and plug caps etc etc all have to be in tip top shape.
Air..Fuel..Spark
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Re: '82 XV750 Carb Issues

PostAuthor: otyler14 » Tue Jul 09, 2019 6:55 am

Oh yes, long distance diagnostics and tuning is always fun....

To address some of your questions/concerns:

Flyingdog wrote:Your statement about "just inspecting" the carbs is suspicious. Ya just can't throw new parts in the mix and expect "new" conditions. Nothing about what or how you cleaned "all" the passages. Making sure where you may have squirted carb clean in one passage, it also has another opening where the spray "should" come out.


I used carb clean to clean all passages and components and made sure what went in, came out of the correct place.

Flyingdog wrote:When on the bench how did you accomplish this and at what height of the butterfly did you use?


I backed off the #1 lock nut and bolt, set the height of the #2 butterfly using the thumb screw so that a very small needle could just barely fit under the butterfly. Then used the screw on the linkage between the two carbs to adjust the #1 butterfly so the needle fit the same as it did under the #2. Once on the bike I use the #2 thumb screw to set idle and then set the #1 bolt and nut to just touch the arm that comes off the butterfly.

Flyingdog wrote:could be clogged air filter, carbs running too rich, venting hoses (2) on top of carbs could be clogged. Possibly over filled crank case oil (coarse this would be more bluish) and you'd smell it.


Not air filter or oil because there was no smoke when the bike ran before taking the carbs off, I do think it is running rich though because the smoke does have a gas smell to it but if the "choke" is adding more fuel, and the bike will only run with the choke on, makes me think its actually lean? Might have to play around with it tonight and see if doing 3/4, 1/2,1/4 etc. choke eliminates the smoke and keeps the bike running

I am starting to think maybe something was in the gas/gas lines from the remote reservoir and blocked a circuit in the carb....plus i pulled a dumb dumb and forgot a fuel filter :bonk: so its very possible. Might be worth pulling the carbs off again and cleaning and checking float height again. Thanks alot for the help Flyingdog that was filled with a lot of good information and gave me something to think about
:thanks:

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Re: '82 XV750 Carb Issues

PostAuthor: Flyingdog » Tue Jul 09, 2019 8:52 am

Your welcome.. never entirely sure if what works or not, or how another goes about it. Nor hopefully doesn't get someone to far into the weeds. Kinda like I see something blue, you see it as aqua and that other person sees green. But like you mentioned, it's a start and who knows..you may find that one "Oh Crap" item along the way, that'll clear things up. But does sound like ya got a decent grip on the situation. And yea..only running with choke sure points to the pilot circuit and/or fuel not flow'n right, atm. Could be worth it to unscrew that choke plunger (2) and clean it all out. Besure to check your vacuum piston/diaphragms. And the needles have a loose(ish) feel within the pistons and these pistons move smoothly in the bore. You can kinda check this by using your finger and pushing the pistons..up..should be smooth and, gives out a swoosh sound, with a very slight resistance and fall fairly slow/controlled back into place. Can also gently blow air into one of those black pipes (with the little black bulbs). This will also cause the pistons will rise. Kinda a make shift test for the diaphragms. Cuz if bad/torn/holes, there will be very little to no resistance. Just don't give them a huge blast. Speaking of which..so you know..those black bulbs are air filters for the piston diaphragms area. They contain just foam as a filter and can/will break down and get into the carbs.
Lotta good people in here, with various ideas. Something should pop up.
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Re: '82 XV750 Carb Issues

PostAuthor: chrismalm » Tue Jul 09, 2019 7:53 pm

Hey otyler14,
Is this the same bike with the burnt cam in the #1 cylinder? Did you get that sorted out?
What are your compression test results?
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Re: '82 XV750 Carb Issues

PostAuthor: Flyingdog » Wed Jul 10, 2019 2:55 am

Ah Yes..always comes to "Devil in the Details".
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Re: '82 XV750 Carb Issues

PostAuthor: otyler14 » Thu Jul 11, 2019 10:07 am

chrismalm wrote:Hey otyler14,
Is this the same bike with the burnt cam in the #1 cylinder? Did you get that sorted out?
What are your compression test results?


Yes this is the same bike. Rockers on #1 and #2 were almost complete gone. Installed new rockers as well as good used cams. Compression was 150 if I remember right. I had the bike running really well and even drove it around for a bit so decided to sync carbs and do the fine tuning and that’s when all this started.

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Re: '82 XV750 Carb Issues

PostAuthor: mark75 » Thu Jul 11, 2019 1:48 pm

otyler14 said, "... Once on, the bike will only run on full choke and is spitting white maybe grey smoke? out the exhaust. ..."

that makes me think you got some fuel contamination from somewhere? - water?
[oil = blueish, xs-fuel = black, water = whitish(steam) ...] :huh?: :ider:
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