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Good Morning,

In getting my newly acquired 750 GT back on the road in running condition, I have a carb question. I cleaned (ultrasonically), rebuilt, and hand synched the carbs. Bike has some type of electronic ignition although I can't figure out which one yet. I also replaced the exhaust gaskets. Bike starts right up nice and easy and idles well. Take off on a ride, and it runs great, good power through out the rpm range. Problem is once it gets warmed up good, when coming to a stop, the idle hangs up high and slowly comes back down. What could be causing this? I checked for air leaks around the carbs and no air leaks present. Should note, that I believe someone had upgraded the carbs on this bike at sometime in it's past to PHF32's as I believe the GT's came with PHF30's.

Thanks for the help.

Rob
 

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You can get air leaks from the boots on the throttle and choke/enriching cables. Also the o-ring under the throttle slide cap, if these items were not replaced during the carburetor service.
Also, when you say "hand synched" the carbs, does that mean you used a manometer?

Uli-
 

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It's either an air leak around one of the miserable hard gaskets that attach the intake runner to the head, a leak around the equally miserable nylon sealing ring on the carb to the intake runner or a carb imbalance with the sliders that's adjusted with the giant screw on the side. To make sure that the slides are lifting at the same time, look down the throat of one carb while feeling with one hand the slide on the other and then lightly twist the throttle to tell if the slides are lifting at the same time. Loosen the throttle cable screws at the top and then use the giant screw to adj this out, then tighten up the cables. There also should be no or 1/2 mm play on the throttle cables as these sit in the adj screws on top of the carb.

I believe that there is also an air metering jet (not fuel) which is the one having the holes in the side with the ball in it. The ball always gets stuck so get a new one. I presume also that you cleaned out the miserable pilot jet which clogs if you look at it the wrong way.
 

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adjusted the miserable mixture screws yet? bike fully up to temp, highest rpm, each miserable side? :wink2:
 

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adjusted the miserable mixture screws yet? bike fully up to temp, highest rpm, each miserable side? :wink2:
Ha Ha! Bigbadass2 gets it and who has undoubtedly been there!
 

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+ to everything that has been said, but also make sure that one of the cables is not kinked or hung up in any way. When you close the twistgrip, you should listen for a reassuring clunk from each carb as the slides stop against the throttle stop screws.
 

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+ to everything that has been said, but also make sure that one of the cables is not kinked or hung up in any way. When you close the twistgrip, you should listen for a reassuring clunk from each carb as the slides stop against the throttle stop screws.
Had my '74 750 GT since new and whenever I had this issue it was, indeed, a cable hanging up.
 

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Good Morning,

Bike starts right up nice and easy and idles well. Take off on a ride, and it runs great, good power through out the rpm range. Problem is once it gets warmed up good, when coming to a stop, the idle hangs up high and slowly comes back down.
Rob
OP reports problem only after warmup...a cable hanging would manifest itself constant, cold/hot, running or not running... no matter.

But should be checked and eliminated, regardless.
 

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Lean condition. All the suggestions above should be addressed. My favorite is a clogged or partially clogged pilot circuit. Make sure fuel sprays into the venturi equally on both carbs. You can clean the rest of the pilot circuit mechanically and with carb spray, but the tiny bleed hole into the venturi must be cleaned with something really small, like a needle, then verified with spray. Also remember the fuel now is diluted in many cases by ethanol, which can cause a lean condition in older engines which weren’t jetted for it. Also , many times you can make up for that with screwing the adjustment screws further out than recommended. If none of the suggestions work out you may need larger pilot jets.
 

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OP reports problem only after warmup...a cable hanging would manifest itself constant, cold/hot, running or not running... no matter.

But should be checked and eliminated, regardless.
Not quite -- by hang-up it could also be the carb piston. Which, in fact, may have been my issue (years ago guys) and was always after it was warmed up. Slackening up on the cable may have just freed the piston. FWIW.
 

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If the idle momentarily hangs, then slowly comes down, it’s lean. I know it may seem mechanical, and by all means make sure it isn’t. After you’ve verified it isn’t mechanical start screwing your mixture screws out 1/2 turn at a time until you’ve cured the condition, or change pilot jet up one size, no more than that.
 
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