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carb problem? 1982 Yamaha virago xj750

Discussion in 'Mechanical & Technical' started by rowlex, Oct 28, 2006.

  1. my dads bike has been having a lot of trouble recently. bike has about 12K on it, it sat for a long time.

    during mild/hard acceleration around town or from a stop the bike backfires and misses a good deal. There doesn't seem to be any carb adjustment screws. We found one adjustment screw for the pilot jet we adjusted it some but it didnt seem to help much.

    Anyone familiar with these bikes/problems?

    also, sometimes when idling the tach....goes "tachy" as in in flutters some when the engine stays at a normal idle.

    The bike seems to run much better when it is cold/choke on.
  2. james1300

    james1300 Track School Dazed

    For the Tach, Id make sure the batterys charged, and ALL the electrical connections are clean and in good condition.
    The carbs are probubly stale from leaving gas in them.
    Your gonna need a repair manual. And may have to take the carbs apart to clean them.
    Stabil Gas stabilizer works great to keep gas from going bad. But its useless now.

  3. yea it has a brand new battery in it. He has gone over most of the wiring connectors. Sorry I didnt mention it, he just purchased the bike has had it for probably 8-9 months. Previous owners let it sit.

    He has also ran more then enough carb cleaner and techron through the carbs.
  4. james1300

    james1300 Track School Dazed

    The carbs are Varnished, That meabs they need to be disassembeled and the brass jets soaked in carb cleaner, and the internal passages of the carb bodies sprayed with something nastey! I like to use' Berrymans B-12 Chem-tool'.
    Dont get it in or on anything you like, like eyes, paint clothes, use gloves and eye protection
  5. cool thanks for the tip...would a re-build kit be neccessary??

    I think I have used the chem-tool stuff before that stuff REALLY is nasty.
  6. james1300

    james1300 Track School Dazed

    No, not unless you damage or destroy a gasket. But, some Yamahas are EZ to get bits and pieces for. You should invest in a manual, for Dad.
    'They make a cool 'Christmas Gift'! and cost about $25.00
  7. Oh he has one I picked it up on ebay shortly after he got the bike
  8. carbs need to be boiled to remove varnish.
    And if you use B-12 to clean the slides, the diaphragms are not removable and B-12 will eat a hole really quick, so use care to clean them.
    Also check under the needle and seat, there is a filter screen that might be clogged with crap.
    Hard to get parts for those carbs so be careful with the parts you have.
  9. There's a very good owners group for early 80's Yamaha XJ's. Used to be on Micapeak. They have tons of information, and a CD with complete carb overhaul procedures- those bikes are known for needing rebuilds as they age. Getting one dialed in and running right, especially when cold is not easy. Getting the float levels correct is critical.

    A google search should turn them up. "XJ OWNERS" and/or "DUANE VERHEY" (I think).

    Overhaul kits are available, but they do not have the same float needles as the stock kit, some people say not to use the aftermarket kit.
  10. StageRulz

    StageRulz Doom Crew Inc.

    Theres a product called "Sea-Foam". It can be bought at Shucks that you add about a 1/2 a can to each tank of fuel that works Great!
  11. alright guys thanks for the previous help here is our situation with the bike now....

    carb's were rebuilt with a rebuild kit from the dealer.

    we balanced the carbs but the bike is backfiring even worse now (usually only at low rpm) and at idle it'll backfire so hard it blows out the spark. There seems to only be two adjustments on the carbs 1. the pilot jet/screw and 2. the balancing screw.

    Any thoughts? We've also tried replacing the ECM.

    edit: PS. The bike runs GREAT with choke on.
    edit#2: so after reading a bit it seems that the bike may be running lean with the choke off...I was watching the "backfires" and I could see flames/burn coming out of the vacuum vent on the top of the carb on the front cylinder.

    EDIT#3!!!*******Sorry this bike is a virago XV750 (the v-twin)****
  12. james1300

    james1300 Track School Dazed

    Sounds like you may not have got all the crud out of the carbs. Air filters clean, right?
    No obstructions in the intakes? Critters love to build nets and hide stuff.
    Running with the choke on indicates the jets or the passages in the carbs are still pluged.
    Running with the choke on, chokes out the air and enriches the amount of fuel going into the engine.
    OR you have a bad vacume leak from where the carbs are fit into the engine.
    Those manifolds are rubber and get old and brittle.
    Heres a test. start the bike and warm it up.
    With it at abit of high idle, squirt WD-40 around the intake boots . Where the carb slips into the head.
    your listening for an R.P.M. increase, or decrease.
    If that occures, then you know you have a vacume leak. the offending boot will need to be REPLACED.
    Id replace them all, if ones acting up, the others arnt far behind.
    K&L supply sells aftermarket boots cheaper than O.E.M. Yamaha, most of the time.
    Id look at the low speed jets and circuits. As well as the needles and emulsifer tubes the needles sit in.
    The tach being twitchy, can be caused by a, worn, broken, loose, or dry tach Cable.
    Some 80's Yamahas used Mechanical tach CABLES. Driven off a cam gear.
  13. Are they CV carbs? If so, the "pilot air screw" you are refering to is not really what you think it is. The pilot screw on a non-CV carb richens the pilot mixture as you turn it in (by restricting the airflow more). On a CV carb, it is the opposite; the "mixture screw", as it is called, richens the mixture the more you turn it OUT. Sounds like you have a very lean mixture. Check all your pilot jets visually and see if the orifice is perfectly clean. Also check your float level; this can have a dramatic effect on the mixture.

    Please read THIS from Factory Pro for more Low-RPM carb tuning procedures.
  14. The Virago carbs use a pilot-screw that meters fuel, not air. You can *usually* tell the difference between the two based on location relative to the venturi. If the screw is on the motor-side of the venturi, then 9 times outta 10 it meters fuel. If the screw is on the air-filter side of the venturi, it's usually for metering the air.

    Viragos (like most v-twins) are *tremendously* sensitive to the synch of the throttle-blades in the carbs. The screw for adjusting the synch is between the two carb-bodies, you hafta remove the tank and have a long, skinny flat-blade screwdriver to reach it - it's a real pain. ...and of course, you will need vacuum-gauges or carb-stix or similar method to actually measure the vacuum so you can balance the two cylinders. On the virago it is a two-step process as the stop for the idle has to be done first(if it is off - this adjustment usually doesn't change), then the carbs need to be opened barely off the stop and then synchronized between themselves.

    ...or... you may just still have crud in the tank that is getting into the carbs. Typical pilot fuel-jet hole is tiny, tiny, tiny and takes virtually nothing to plug it up).
  15. RedVFR

    RedVFR Fast, Easy, and Old Fashioned

    Don't forget to do a very careful inpection of the slide diaphragms. I had one develop a crack that was nearly invisble on an '83 Shadow 750 that I owned. The cost on those is a hard pill to swallow. I think I paid $120 for one!
  16. Take them apart and run them through the dish washer, hot water only, twice..I use the dish washer to clean my SUs for the race car...
    I know it sounds funny....but it works....And don't forget to run the dish washer afterwords with just soap and bleach before you do your dishes..
  17. awesome guys thanks for the help! We opened up the carbs again last night and found some more stuff to clean. brand new air filter and nothing in the intake track (we've run it w and w/o the intake tubes going to the filter).

    we are still messing with that pilot adjustment screw. dad says they are the CV carbs, we were adjusting it as if it was non-CV. However the adjustments we're making are not making much of an improvement(the further we screw them out the less it backfires but the more it seems to "choke" when giving it gas). We are gonna re-balance the carbs and try again I guess.
  18. james1300

    james1300 Track School Dazed

    Dont forget to check the rubber diaphrams (on the carb slides) for cracks and tears (Hold them up to the light), as mentioned above.^^^
  19. yep we did that they looked good but if we pull it apart again i will take a closer look
  20. It sounds like the pilot circuit is still plugged up. Did you set the float-heights? Did you remove the pilot fuel-jet and blow air through the passage in the carb-body and clean out the tiny hole in the pilot jet?

    I'm not trying to be the know-it-all here, but there is a lot more to cleaning a set of carburetors than pulling the float-bowls off and spraying a can of carb-cleaner at it...

    The viragos also had a weird vacuum-operated valve thingie that bleeds air into the diaphragm portion of hte carb-body when the vacuum rises, and I've never seen a virago run properly without that system, or if the vacuum-operated valve thingie - it's mounted to the frame, under the tank, just behind and to the left-side of the steering-stem, and it has two hoses that connect to the inlet-manifold for the carb of the front cylinder. There are also tubes between the carbs and the opposite carb's rubber-connector to the frame/airbox section... all those hoses have to be connected properly and leak-free or it will never run right.
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