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Discussion Starter · #1 ·
I'm at a loss folks, really appreciate any input on this one.

So here's the deal, after picking up this bike for a steal. i learned that the main problem of it not running correctly is that the diaphragms were completely blown out. i replaced them with some off of jbmindustries and it made the bike fire up and run pretty good. i also cleaned the carbs(took out all the jets, ran guitar string through all the holes i could and verified that carb cleaner ran nicely through every hole i could find). I've also checked the throttle shaft end caps, the left one was still in place and the right one must have fallen out somewhere in the bikes life so i replaced it. i checked the boots from engine to carb for leaks(there isn't any) same for the boots from the carbs to the air filters(which are stock and new). the air screws in the carbs have both been seated then turned out 1 and 3/4 turns. as for other things, I've set the valve clearances, set cam chain tension, set points gaps, and ignition timing, and I've changed the plugs, after doing that i meticulously balanced the carbs with gauges where i set them as perfectly as i could.

And after all that work i still have this very specific problem that i can't get my head around. the bike will idle fine, it will accelerate like a bat out of hell with no complaint. BUT when you try to keep the throttle constant at anything above 2000rpm it spitters and sputters and runs overall really rough and choppy(but it doesn't backfire, or want to die) this makes cruising basically impossible, i put the vacuum gauges on the carbs to see if the readings would differ at the constant higher rpm and they stay perfect relative to each other, however the vacuum read at the higher constant rpm(like cruising) is significantly lower than the vacuum read at a 1200rpm idle. also at idle the right side cylinder will give off a small back fire ever so often. I know these bikes are notorious for running rich but is that what is doing this? i live at 4,200ft of elevation and i highly doubt this bike has ever been re-jetted. i haven't checked the compression of the cylinders but i assume it must be good due to how easily it fires up and how strong it runs otherwise.

Like i said i'm really at a loss on this one, if anyone has had this problem or has any idea what it could be please let me know, Thanks.
 

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You're at 4500' altitude where the air density is much leaner; I suspect your fuel flow is too rich in the cruise, though your idle jetting may be perfect for altitude? You might need to experiment with several plug chops to determine what your engine is doing; you can do that inexpensively with a Gunsons colourtune sparkplug which allows you to see into the engine whilst it's running. It may just be that running the guitar string through the main jet has widened it too much; though that is clutching at straws...
Once your engine has warmed up go for a run and once you encounter the spitting and popping and banging; pull in the clutch and at the same time hit the kill switch. Stop; spin out your plugs and look at the electrodes to see what colour they are. That should indicate what's going on in your engine; black and sooty = too rich; white: too lean; a tan colour is what you're aiming for.
Good luck
Nigel
 

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Discussion Starter · #3 ·
Thanks Gixxer-18 Thats mostly what i've been thinking about, the bike smokes like a chimney with black smoke and the plugs are always black and prone to fouling, my assumption was that if it was okay under acceleration that it should be okay under cruise. i'm not going to cheap out this time and pick up a couple of rebuild kits with new needles and leaner jets and see what happens, Wish me luck!
 
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