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'78 GS750 Carb Issues/Question

  • Thread starter Thread starter Anonymous
  • Start date Start date
A

Anonymous

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I had my carbs apart lastnight to clean them. I got it all back together and it runs better than it did before. The problem is, it's idling high. The idle screw is backed all the way out. (not touching the throttle...body...thingie where the throttle cables connect to the carbs) I've tried adjusting the air flow into the carbs in order to lower the idle. I turned the screws all the way in, on all four carbs, and it still ran fine. Idle only droped a little. Shouldn't it have stalled out? What could the problem be?


Another Question........

I hold my hands down to the end of the exhaust pipes, with the engine running, and one pipe has more pressure comming out than the other. What's up with that? Thanks a lot.
 
you want your idle screws out some, it will still get air from the airbox if they are shut all the way, just not as much.

to fix your high idle, check to make sure your throttle cable is not binding and sticking. also, there is a screw on the carb body that adjusts the throttle stop, and sets the idle. turn this screw to the right some to lower the idle.
 
It is possible you pinched the throttle cable(s), but they would have to be REALLY pinched to do what you described. Check that first, but it sounds like you did a poor job of manually sincing the carbs. Their "fully closed position" is important. If you had the carb slides up too far after re-assembly, the bike will idle without any contact with the idle adjuster knob.
Do you have a manual to tell you how to make this adjustment?
 
CHeck the choke linkage If it hangs up how ever slight you will never be able to set the idle
 
Thanks for the replies.

The cables are fine. The throttle moves freely and snaps back to closed without a problem. I don't have a manual YET. That's on the short list of things to buy. Is it an easy fix?

I don't think the choke is binding. I don't know if this is any indication, but the bike won't start when it's cold without the choke. If I put the choke on, even just a touch, the bike will fire right up. So, I don't think the choke is the problem.

Thanks again. I'll check the slide height tomorrow.
 
Most GS bikes will not start with out the choke when cold I would still recheck the choke linkage
 
Will do. Thanks.

Does anybody have any idea about the exhaust? It almost seems like one side has more compression than the other. Rings maybe?
 
Its most likely a weak mixture. The exhaust will feel weak and relatively cool. Since two headers each exit into one muffler, feeling the exhaust only narrows the problem down to either 1 and 2 or 3 and 4. When you first start the bike for the day and the header pipes are cold, once you have it idling, put your hand on the pipes just below the exhaust ports. The coldest pipe will be the cylinder that is the problem. About 9 times out of 10, the problem will be a cracked and leaking intake "O" ring. (the "O" ring is between the intake boot and the cylinder head. Its a rubber "O" ring about 1 inch in diameter. You will need to remove the two screws that attach the intake boot to the cylinder head to access it) After replacing the"O" ring, if the problem still persists, then the next most probable cause is the internal fuel passageway for the fuel pilot in the carb.

If you have an ignition matched pair of cold header pipes (1 & 4 or 2 & 3, then I would check electrical (coils, wiring and timing) before looking into induction and carbs.

Earl


Mark C said:
Will do. Thanks.

Does anybody have any idea about the exhaust? It almost seems like one side has more compression than the other. Rings maybe?
Click to expand...
 
Hi. Here's how to manually sinc the carbs. First, while you have the carbs off, inspect the o-rings in the rubber manifolds and the manifolds themselves. ANY air leaks will give you a higher idle once warmed up, sometimes a LOT higher. A 2-300 rpm rise can be considered normal. Don't use silicone sealer to repair manifolds/o-rings. Replace with new parts.
To manually sinc the carbs: Fully closed position first. Turn the idle knob to make a good clearance between the tip and the throttle pulley. Take the tops off the carbs and have the ENGINE side of carbs facing you. Above the carb slides is a slotted screw held by a nut. Loosen all 4 nuts. Some say you should start with #3 carb and make the others match it, that's fine. Turn the slotted screw to fully close the slide. Don't tighten it hard, just enough to seat it, if you see it stop dropping, stop turning. HOLDING THE SLOTTED SCREW STILL,tighten the nut about 3 lb/ft. Do the same thing to the others. When done the slides should be fairly uniform to the eye.
Now the fully open position: There is a slotted screw under spring tension mounted on the carbs bracket.When you open the throttle the pulley will stop when it hits this screw. Turn the carbs so FILTER side is facing you. Raise the slides fully by pushing up the pulley. Look up and note the bottom of each slide. The bottom of the slide should be between 0.5 and 1 mm above the intake chamber. Turn the throttle stop screw in to decrease this distance and out to increase. Now turn up the idle knob enough to allow the bike to idle.
Replace o-rings(greased) and manifolds if needed and torque 8-10 lb/ft, replace phillips with allens if possible. Make sure the carbs "pop" into manifolds, HOLD them still and tighten the clamps. Slip on filters. Make sure ignition timing is correct and sinc with your tool.
If done properly,you will have carbs sinc'd and no intake leaks.
 
Thanks for the sync step-by-step. I'll take a look at it as soon as I can.

I'll also check the exhaust manifolds the next time I start up. Thanks for all the help. You're making my life a lot easier!! 8)
 
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