Motorcycle Club » The Yamaha Shop Talk

1979 750 triple carb prob.

27 posts from 6 voices
About This Topic
  1. Vaz1
    #

    Ok.,.,I'v taken the carbs apart 3 times and cleaned them and it is spotless,..,I have new petcocks.,.,installed new plug wires and caps as well as new spark plugs.,.,ok so here's the thing.,.,.when the bike is cold I have to use the choke to get it running.,.,.as it's warming up the left and center pipes get hot but not the right one.,.,with the choke off the two outter pipes are hot and the center cools down.,.,I can hold my hand on it.,.,but the other two are freaking hot~!
    I am trying to sync the carbs.,.,but it seems impossable as soon as I get it close and let it run the vaccum changes and my meter goes nutz~!
    Can one sync the carbs without the center one working.,.,this whole thing is freaking me out as I have tried everything.,.,as in.,.,.swapping plugs.,.,.swapping wires.,.,(from one coil to the other).,.,I have tested the spark with a new plug and all seem to have very nice blue spark.,.
    any Ideas~??
    Thanks.
    Randy.

  2. MigejCojode
    #

    Check for a vacuum leak on the intake runners. Spray some carb cleaner around the runners, if the engine revs up you have a vacuum leak.

    If you don't then check the jets again. The pilot jet (idle) has such tiny holes in it that it's easy to miss a clog, the opposite is just as bad as corrosion can make the holes too large. I would consider changing both the main and pilot jets as part of a rebuild on a bike this old. Also check the throttle shaft for wear, it shouldn't have side to side play. Last check the vacuum sliders. If this has the old aluminum sliders in an aluminum bore they wear out, if it's not a vacuum leak I would guess that this is the problem.

  3. LiddleBen
    #

    OK, let us assume that you have assembled the carbs correctly. Take the idle mixture screws and turn them in until they gently seat. Then turn them out one and threequarter turns. Choke the bike and start it. starting with the middle carb gently turn the screw in or out about 1/4 turn til bike improves. then use the same procedure on the two out board carbs. tweak these until the bike runs relatively smooth. Now rev it up and see if it hesitates or spits back through the carbs. do these adjustments with choke off. when you have the bike running fairly good go to the vacum gages and fine tune the carbs. If you can't get close readings. check for vacum leaks around the intakes with carb cleaner.. First make sure that your intake flanges have no cracks and that the clamps are tightened properly.Also, Have you done a compression check on all cylinders? They should be within 10 pounds or so of one another. Thats what little I know hope it helps. Good Luck. ,LiddleBen

  4. Vaz1
    #

    I have got my hands on a compression tester.,.,and will test it in the AM.,.,
    If it has good compression.,.,then I'll order a compleate carb kit and rubber carb intake boots.,.,I have noticed mine are in poor shape.,.,they are original.,.,
    I don't think the compression is the problem.,.,the bike has only 8,000 miles on it.,.,.it has been sitting in a garage for the last 9 years.,.,I was able to get it for $600.00.,
    Since obtaining it I have changed the oil and filter.,(it smelled of gas).,so I ordered new petcocks from Mikes xs.,.,.changed the spark plug.,.,changed the spark plug wires and caps.,.,and have ordered new XS Pods.,
    OK I just returned from testing the compression.,.,it was 75 lbs on all cylinders.,.,so thats Good~???

  5. MigejCojode
    #

    That compression is quite low but it's good they are the same. Did you do it with the throttles open or closed? How many times did you crank it?

    75 psi / 14.7 psi atmospheric pressure = 5:1 compression .... Pretty sure they had a higher compression ratio than that.

    I usually do it with the throttle's open as it takes carb restrictions out of the equation. I also crank it until the needle stops moving, usually 3-5 or more revolutions, that also takes any other restrictions out of the equation.

    8000 miles then 9 years sitting can reek havoc on the piston rings and cylinder walls.

  6. Vaz1
    #

    OK .,.,I goofed up on the test.,.,I didn't let it crank.,(just one stroke).,.,
    So i retested it .,.,with throttle closed I got 126 on all cylinders.,
    I'll go out and test with the throttle wide open.,.,and post in a few.,.,

  7. MigejCojode
    #

    126 sounds pretty good to me, again even better that they are all the same. Likely be higher with WOT.

  8. Vaz1
    #

    Ok then.,.,everyone had a compression of 140 psi WTO.,.thats good huh.,.,ok so now do I order the carb kit and carb boots~?.,.,anything you can think of why when I choke it I get the (when facing the bike) right side and middle cylinders pipes get hot and the left doesn't.,.,.but after it warms up a bit and idles without the choke .,.,then the left and right pipes get hot and the middle one cools down and i have a hard time sync ing the carbs.,(I have made a homemade sync tool outta tube.,., a yard stick .,.,and 2 cycle oil).,
    So do ya think I might have a vaccum leak~?

  9. Suzhipiger
    #

    Eeesh. I'm putting an '82 750 back together with a co-worker. This one has 4 carbs. Your post was informative for what I'm getting myself into.

    It hasn't sat for "years and years", just "years" so I'm not sure how bad it'll be. He rode it for a few seasons with no problems, just that he had to keep cleaning the varnish out of the carbs after it sat for a while. We noticed that the brass "plungers" in the carbs were gummy and we're soaking all the jets and pins in a gallon can of carb dip. I'd love to soak the main housing but I didn't want to ruin the rubber diaphrams inside. I'm hosing the brass cylinders and the needle shafts with Gumout at regular intervals. It seems to be working.

    You may have some things sticking in your carb assemblies.

    What's the low-down on fuel storage additives? Or should you just drain the tank and run the carbs dry? Or what?

  10. MigejCojode
    #

    I have made a homemade sync tool outta tube.,., a yard stick .,.,and 2 cycle oil).,So do ya think I might have a vaccum leak~?
    My guess is a vacuum leak. If you are unsure spray the backside (boots, intake runners whatever you want to call it) of the carbs on a warm engine with WD40 or carb cleaner, ensuring not to get any in the carb throat by using the straw. Either one won't hurt the carbs but if you get it in the carbs you will get a false positive for a leak. If it revs up you have a leak.

    That homemade manometer is more accurate than mercury manometers or vacuum guages.

  11. Vaz1
    #

    Well today I tore apart the carbs.,.,AGAIN~!.,.,and I set the floats.,.,and cleaned all the jets.,.,again.,.,then when I was trying to start it.,.,I had gas flowing out of the #1 carb.,.,I have a stuck float or needle and seat.,.,I tried to start it after I shut off my petcocks.,.,just cause.,.,and it backfired out the exhaust on the #1 cylinder.,.,freaked me out it had never done that before.,.,I haven't messed with the timing.,.,just the carb.,.,well I'm going to give up and buy a carb kit and new rubber intake things.,.,Thanks for your help.,.,. I'll post more when I get the kits in.,.,
    Randy

  12. JezzeFrasier
    #

    I am in the middle of an expensive carb clean and sync job. I was having all the same problems that you are facing. I cleaned them and cleaned them. Finally I reassembled themand in a stupid move I forgot to check to see if they were leaking. The bike backfired when I tried to start the bike. suddenly flames shot up my arm from the carbs and air box. Five minutes later Ihad the fire out but the wiring harness and a host other parts along with the darbs were ruined. So my advise is don't even attempt to start the bike untill you are sure there is no fuel leaking anywhere.

  13. Vaz1
    #

    Well I hope you have better luck with your bike than me.,.,
    The PO only had it for a few weeks before selling it to me.,.,.so I have no Idea of it's history.,.,so I realy have to start from scratch.,.,
    I'd drain the float bowls if you'r going to store it for the winter.,.,
    Good luck~!
    Randy

  14. Vaz1
    #

    I am in the middle of an expensive carb clean and sync job. I was having all the same problems that you are facing. I cleaned them and cleaned them. Finally I reassembled themand in a stupid move I forgot to check to see if they were leaking. The bike backfired when I tried to start the bike. suddenly flames shot up my arm from the carbs and air box. Five minutes later Ihad the fire out but the wiring harness and a host other parts along with the darbs were ruined. So my advise is don't even attempt to start the bike untill you are sure there is no fuel leaking anywhere.
    Woah~! thats freaky,..,.,.,and that realy sucks.,.,I have had the fuel lines come off and dump gas all over the floor.,.,,.but I shut down the bike in time as not to have any excitement.,.,so then I went inside and asked my wife.,.,."honey.,.,where's the fire ext.??" .,.she said OMG what happened??.,.,.nutton.,.,I need it just in case.,.,,.so now it is sitting right there.,.,by the bike.,.,in case.,.,
    Randy

  15. cjvelchyr
    #

    Ok then.,.,everyone had a compression of 140 psi WTO.,.thats good huh.,.,ok so now do I order the carb kit and carb boots~?.,.,anything you can think of why when I choke it I get the (when facing the bike) right side and middle cylinders pipes get hot and the left doesn't.,.,.but after it warms up a bit and idles without the choke .,.,then the left and right pipes get hot and the middle one cools down and i have a hard time sync ing the carbs.,(I have made a homemade sync tool outta tube.,., a yard stick .,.,and 2 cycle oil).,So do ya think I might have a vaccum leak~?
    That all sounds like the mixture is off. Make sure to follow the steps set out in the earlier reply as that is the right steps to take. The only thing I can add is to make sure that you turn each one for max RPM using the mixture screw that should be on top of the carb just behind the rubber boot attaching the carb to the engine.

    Also if things are running right the middle cylinder should run cooler than the outer ones as it's done that way to protect the middle since it has less cooling available to it. This is the same as on the 4 cylinder xs1100 only it's the middle two on them.

    My guess on the choke part is that the cool cylinder is not enriching or is too rich with the choke on, more likely it's got a clogged enrichment circuit on that carb. Also cooler tends to mean richer mixture and hotter generally means leaner mixture, and middle cylinders tend to be jetted richer to keep them cooler.

  16. Vaz1
    #

    Ok heres the new scoop.,.,My carb kits came yesterday.,.,Installed them last night.,
    Today I installed my new XS pods and the carb on the bike.,.,it fired up and ran with out the chokes on.,.,I set the idle screws (2.5 turns out)
    I put the manometer on and synked all carbs.,
    Took it out for a test run.,.,(freaking cold~!.,30's).,.,when I got back i pulled the plugs.,.,.and # 1 was black.,.,#2 was new looking.,.,.,no signs of anything.,.,#3 looked normal brownish with no deposits.,.,
    So I pulled all plug wires except for the middle one.,(#2).,and tried to start it.,.,nothing.,.not even a fart.,.just spun.,.,tried T-W-O and choke.,.,nothing.,.,pulled plug .,.it was dry.,.,tested it by grounding it to motor.,and it had a nice strong blue spark.,
    So then I thought I'd swap the jet slides from #1 to #2.,then installed just the #2 wire and tried to start it again.,.,nothing.,.,.installed #1 wire.,.it ran.,.,installed #3 wire and it ran too.
    I pulled the plug out of the float bowl on #2 carb and it was full.,
    I have sent for new carb intake rubber boots.,but I have not gotten them yet.,.,(but I don't think that would be the problem as it would send some fule air mix to the cylinder).,.it wouldn't be a total failure on just one cylinder if it leaked vaccum.,.,am I right~???
    So now I'm at a stand still.,., any Ideas~??
    Thanks.
    Randy

  17. Suzhipiger
    #

    Ok, not yet being a bike mechanic I can only apply my general mechanical knowledge from rebuilding a DeLorean, 3 vintage aircooled VW's and a triple-carb Chrysler outboard boat engine:

    1. The cylinder with the black, sooty plug seems way rich.
    2. The cylinder with the "nothing" plug may not be getting any fuel at all. You verified that it had spark so that's not the problem.
    3. The brown cylinder seems the most normal.

    If I understand correctly, swapping fuel slides seemed to verify that. You're confused because the float bowl is full of fuel, yet that cylinder acts as if it's not getting any fuel, right?

    You're correct, even a cylinder with a torn intake boot should fire, it would just run poorly...very lean as it gets extra, unmetered air.

    Is there any way to observe that the #2 carb is actually putting out any fuel?

  18. Vaz1
    #

    Well I pulled the #2 plug out and sprayed carb cleaner in it and it did fire but would not stay running.,.other than that every time I pulled that plug it is very dry~!.,.,still lost.,.,
    The fed ex man just delivered my new carb intake boots.,.,.,must install them.,.,in the mean time any new ideas~??

  19. Suzhipiger
    #

    Ok then, it sounds like #2 is not putting out any fuel -at all-.

    You have some options....directly pursue troubleshooting #2 carb, or look for replacement carbs.

    I have (will soon have) a 4-carb bike. I can only imagine the fun I'm going to have. I think that once the float bowls are full, you should shut it all down, and carefully remove the #2 carb (or the carb bank if they're all stuck together in one assembly) and operate the linkage to see what comes squirting out. Try swapping jets. Try buying replacement jets. Try blowing compressed air through the jets to see what you get.

    There's probably a piece of solid matter (perhaps something metallic) stuck in a jet. Even that caustic "carb dip" won't eat a speck of rusty metal that gets sucked into a jet from the fuel tank.

  20. JezzeFrasier
    #

    I took my carbs apartand cleaned them. But due to the fire could not get them to work. So I sent off my spare set as an exchange for rebuilt set. Ok now back to your set. If you take yours off as suggested by our freind. To see if there is any fuel squirting when opperating the butterflies you will find that this style of carb has no accelerator pump and thereforeno fuel will squirt. These are cv style carbs and depend on vacume to pull air through the carb, like a siphon style paint gun. Thereforeit does not take much of a blockage in one of the venturies to severly affect fuel flow. I decided to exchange mine because I don't have one of those gallon cans of carb dip to let mine sit in. Good luck and safe riding to ya

12

Reply »

You must log in to post.