your high idle problems suggest to me individual carburetor balancing, especially when it cuts out dead at about a thousand rev's, you can hear it lumpily trying to run as two or more of the carbs close completely. slacken off EVERY throttle connection and clamp on the balance rod so they are all at rest then individually set each carbs throttle stop to about three thou open, one of the carbs needs to be about a thou more than the rest for smooth opening progression to prevent flat spots and farting back when grabbing a fist full of throttle (usually an outside one - easier to adjust), tighten them all up again then set the tick over on the master throttle stop adjuster. hope this helps, I would also check all the neoprene manifold tubes in front of the carbs as any leak at all around them from bad fitment or a crack in the neoprene allowing it to suck neat air in will send it totally nuts....revs like a banshee but wont idle to save its life. I had much fun as an apprentice mechanic back in the day learning all this, all the local bike mad kids my age would rebuild their motors beautifully then get the carb placement totally out to cock - with exactly the same problems you are coming across - wouldnt idle under about two grand and ran like a bag of spanners.....while everytime I had any of my motor's out they would idle like sewing machine's at five or six hundred revs when I put them back in plus one quick stab of the starter button without touching the throttle at all was all it took to fire them up (I used to take mine out and strip it every other week in work - just because I could lol,) I made a fortune as a teenage apprentice setting their carbs up for them all. by the way, air filters are neutrally balanced they neither cause a vacuum or over pressure when clean, it shouldnt make ANY difference to the running of the motor if you use the individual air fliters or the one in the air box. putting that tape on them only had ONE effect.......to make it run WAYYYY rich when you open it up............that way be dragons! you would send engine temps skywards and end up seizing the pistons in the block. dont do it ever. also carburetor constant velocity diaphrams dont care about being stretched - what they care about is pinholes or cracks in them which interferes with the throttle barrel rising and has nothing to do with anything you are currently experiencing. If I lived closer I would come along and show you how to set carburation up easily.
Nat when the bike is warm, disconnect the throttle linkage and see if the idle goes down, just a suggestion as they might not be closing when connected.
Carbs are a bloody nightmare..I had a similar high idle issue on my old gs500 and that was an air leak..one of the vacuum hoses wasn’t sealing. Such a small thing causing big problems..just in case it helps lad, atb, Mark
The motorcycle MD put something up today about air mixture screws that might help. And watched a short one by some one who cut up an ice cream carton made a cone shape and fitted it into the cones if that helps. Good luck
I was just reading that these carbs at times had issues with the choke plungers leaking air through a seal. It explained how to test using a vacuum but i didnt understand it if im being honest. But im wondering if there was a way to be sure that no air was leaking past the plungers by adding some grease or something just to test them. No idea if im right or not
Dont know if i missed it but are all the air fuel screws even now? I still think the weakened diaphragms are causing issues as they will take little air to flutter about. New ones will be thicker. Worth a shot if you can find them. Love this build mate
It will run better with the air box cos you've jetted the carb to the manual ( which is for the air box fitted). Diaphragms are next to eliminate them ie renew. Throttle cables stretched or shrunk? Once running on idle, spray test on all the rubbers.... one step at a time.
Definitely better with the airbox on Nat it will be running a bit richer now I would imagine ,maybe there's an o ring leaking that keeps the carb bank together but your guess is as good as mine,the diaphrams are a bit suspect as well ,it would certainly piss me of if it was mine👍
Actually NO air filter made will cause the carbs to run rich unless they are partially blocked through being filthy allowing the suction of the inlet manifold to draw more fuel than the needle wants to provide with diminished air flow through the body and a negative vacuum.
When you balanced the carbs did you back the adjuster screws off and start from fresh? Could be a screw just holding a butterfly slightly open causing your high idle.
Check for vacuum leak before the carbs. Such as the rubber intake manifolds. Are there gaskets between them and the cylinder head ? Spray carb cleaner around the intakes. Best of luck.
I had this same problem on my 81 Yamaha xj550 I cleaned the both jets out and set them at 3 turns out and it idles at 900 rpm it’s straight piped and has aftermarket air filters on it so I would definitely try that
Carb variables eh! I know buggar all, so I'm very interested in the process of fault finding. No help to you, I'm afraid, but here's wishing you success!!!! Cheers, Andy.
IIRC from back in the day, to balance the carbs you needed the YICS blanking tool and remove the plug on the side of the head so you can insert the tool and blank the ports off. Just out of interest is the plug (bolt)missing? that would give you a big air leak.
You mentioned before that you think you have the wrong carbs and these are from the 750. Would it not be easier to try and pick up a second hand set of the right carbs as these will probably never work right whatever you do??
Hiya Nat,are you sure the throttle linkage is allowing the carbs to go back to throttle off?Try idling the bike and put a little pressure on the linkage to see if the revs drop.All the best.
Did you really say you have OCD 😂 every time I see your garage I think what a mess! Each to their own though! Vice looks 😎
I can see that bike joing the shiver
your high idle problems suggest to me individual carburetor balancing, especially when it cuts out dead at about a thousand rev's, you can hear it lumpily trying to run as two or more of the carbs close completely. slacken off EVERY throttle connection and clamp on the balance rod so they are all at rest then individually set each carbs throttle stop to about three thou open, one of the carbs needs to be about a thou more than the rest for smooth opening progression to prevent flat spots and farting back when grabbing a fist full of throttle (usually an outside one - easier to adjust), tighten them all up again then set the tick over on the master throttle stop adjuster. hope this helps,
I would also check all the neoprene manifold tubes in front of the carbs as any leak at all around them from bad fitment or a crack in the neoprene allowing it to suck neat air in will send it totally nuts....revs like a banshee but wont idle to save its life.
I had much fun as an apprentice mechanic back in the day learning all this,
all the local bike mad kids my age would rebuild their motors beautifully then get the carb placement totally out to cock - with exactly the same problems you are coming across - wouldnt idle under about two grand and ran like a bag of spanners.....while everytime I had any of my motor's out they would idle like sewing machine's at five or six hundred revs when I put them back in plus one quick stab of the starter button without touching the throttle at all was all it took to fire them up (I used to take mine out and strip it every other week in work - just because I could lol,) I made a fortune as a teenage apprentice setting their carbs up for them all. by the way, air filters are neutrally balanced they neither cause a vacuum or over pressure when clean, it shouldnt make ANY difference to the running of the motor if you use the individual air fliters or the one in the air box. putting that tape on them only had ONE effect.......to make it run WAYYYY rich when you open it up............that way be dragons! you would send engine temps skywards and end up seizing the pistons in the block. dont do it ever.
also carburetor constant velocity diaphrams dont care about being stretched - what they care about is pinholes or cracks in them which interferes with the throttle barrel rising and has nothing to do with anything you are currently experiencing. If I lived closer I would come along and show you how to set carburation up easily.
Nat when the bike is warm, disconnect the throttle linkage and see if the idle goes down, just a suggestion as they might not be closing when connected.
Carbs are a bloody nightmare..I had a similar high idle issue on my old gs500 and that was an air leak..one of the vacuum hoses wasn’t sealing. Such a small thing causing big problems..just in case it helps lad, atb, Mark
The green looked lovely on those pods.
The motorcycle MD put something up today about air mixture screws that might help. And watched a short one by some one who cut up an ice cream carton made a cone shape and fitted it into the cones if that helps. Good luck
I was just reading that these carbs at times had issues with the choke plungers leaking air through a seal. It explained how to test using a vacuum but i didnt understand it if im being honest.
But im wondering if there was a way to be sure that no air was leaking past the plungers by adding some grease or something just to test them.
No idea if im right or not
Also the bike does seem to smoke a lot are the rings worn and compression is low as this will also never run right
Balancing at idle is important, but more important is to balance at 4000-5000 rpm. The working erea of the engine.
When I get there mate!
Dont know if i missed it but are all the air fuel screws even now?
I still think the weakened diaphragms are causing issues as they will take little air to flutter about. New ones will be thicker.
Worth a shot if you can find them.
Love this build mate
It will run better with the air box cos you've jetted the carb to the manual ( which is for the air box fitted).
Diaphragms are next to eliminate them ie renew.
Throttle cables stretched or shrunk?
Once running on idle, spray test on all the rubbers.... one step at a time.
Definitely better with the airbox on Nat it will be running a bit richer now I would imagine ,maybe there's an o ring leaking that keeps the carb bank together but your guess is as good as mine,the diaphrams are a bit suspect as well ,it would certainly piss me of if it was mine👍
Actually NO air filter made will cause the carbs to run rich unless they are partially blocked through being filthy allowing the suction of the inlet manifold to draw more fuel than the needle wants to provide with diminished air flow through the body and a negative vacuum.
When you balanced the carbs did you back the adjuster screws off and start from fresh? Could be a screw just holding a butterfly slightly open causing your high idle.
Check for vacuum leak before the carbs. Such as the rubber intake manifolds. Are there gaskets between them and the cylinder head ? Spray carb cleaner around the intakes. Best of luck.
I had this same problem on my 81 Yamaha xj550 I cleaned the both jets out and set them at 3 turns out and it idles at 900 rpm it’s straight piped and has aftermarket air filters on it so I would definitely try that
Carb variables eh! I know buggar all, so I'm very interested in the process of fault finding. No help to you, I'm afraid, but here's wishing you success!!!! Cheers, Andy.
IIRC from back in the day, to balance the carbs you needed the YICS blanking tool and remove the plug on the side of the head so you can insert the tool and blank the ports off. Just out of interest is the plug (bolt)missing? that would give you a big air leak.
There is no YiCS on this mate
At least I don't think there is!
You mentioned before that you think you have the wrong carbs and these are from the 750. Would it not be easier to try and pick up a second hand set of the right carbs as these will probably never work right whatever you do??
See if the throttle cable is allowing it back to rest position ¬ to tight,also will your timing back plate adjust,could be to far advanced..Ian
Hiya Nat,are you sure the throttle linkage is allowing the carbs to go back to throttle off?Try idling the bike and put a little pressure on the linkage to see if the revs drop.All the best.
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Your channel is just like Dell boy
Well neither of us know what they are doing but at least I admit it!
You mean Del Bollox? Nat can not be compared to that bastard 😂. We are fans of the troll meister Hudson General
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