Help with High Idle at Start CB750K 1981 DOHC

luisazmz

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I took the carbs off the bike to bench sync them, put them back on and now the bike is starting up at 5,000 rpm + and it won’t go down, choke is off. I was afraid of letting it run cold on such high rev, max time was 30 seconds and it won't go done. When I killed the engine some type of fume/smoke started coming out of piston 1 (furthest left).

I've checked for air leaks, stuck throttle/choke cable and it’s not that. What can be causing this? It was running well before the bench sync. I also attached the vacuum sync and all the carbs are perfectly in sync, which they weren't before.
Unknown.jpeg
 

MickMick

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What process did you follow to bench sync? What happens if you back off the idle?, force throttle off? You may have too much air somewhere. Leak or one of the butterflies may be letting too much air in the venturi to such up too much fuel.
 

luisazmz

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What process did you follow to bench sync? What happens if you back off the idle?, force throttle off? You may have too much air somewhere. Leak or one of the butterflies may be letting too much air in the venturi to such up too much fuel.
I used two bits of the same size, one placed on carb #2 and then synced 1,3 & 4 so they would be opened the same amount. Hopefully that makes sense. I was sure that they all covered the furthest hole of the two holes in the middle of the carb. I can’t get them to close more. The photos show how much light is going through when fully closed.

It seems carbs 1 & 2 are the ones giving the issue because if I unplug those two spark plugs the bikes idles much lower, on the contrary if I unplug spark plugs 3 & 4 the engines idles at 5k+
 

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MickMick

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It does seem like to it me as well, but that’s as for as they'll close. Any suggestion as to why they’re not closing fully?
I assume that the idle screw on number 2 is fully off?. Another thought is that they may be catching due to being misaligned. Try loosening the butterfly screws now they are bolted together to see if they self align and shut. Don't for get the thread lock.
 

user8086

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Take your bench sink down on number two until it will not go any further find a reference point (second hole).. do the same for the other three they must match number 2 that's what all your reference comes off of..that's why you will find there is no adjustment on that carburetor when you are doing a running synchronization we still use the old school Mercury gauge. Take your main idle control loosen it until you visually see it not touch back it back so it barely makes contact... Double check your plugs also one Dead plug can also give you one hell of a headache.. and you can also use a micrometer for butterfly measurements
 

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