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Author Topic: When pulling up to a red light, rev at 3-4k  (Read 1153 times)

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Offline narcoticrex

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When pulling up to a red light, rev at 3-4k
« on: April 24, 2009, 04:25:32 PM »
Hi SOHCers!  Ive got a confession - this is a 79 CB750L, which means its a DOHC.  But similar problems appear on SOHCs and I was wondering if anyone had any suggestions, becuase im running out of ideas.

When I pull up to a red light, i idle at 3-4k.  Everything else about the bike is fine.  No cables are binding; I just replaced all carb insulators.  The carbs appear to be quite clean, and most parts have been replaced.  The bike has been given a complete tune up - valves adjusted, timing is on, carbs are relatively synced (this problem was happening on test rides before these were fixed too)>  It almost seems like an air leak in that it runs fine starting up and idling - but when I start riding and try to stop, she dont wanna stop.

im going to check the float heights right now. any other ideas?
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fuzzybutt

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Re: When pulling up to a red light, rev at 3-4k
« Reply #1 on: April 24, 2009, 04:28:24 PM »
did you replace the diaphrams in the tops of the carbs? sounds like you have an airleak someplace
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Offline Jerry Rxman Griffin aka MuthaF'er

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Re: When pulling up to a red light, rev at 3-4k
« Reply #2 on: April 24, 2009, 04:45:35 PM »
Sounds like you have charging system issues with a bad battery and you are keeping the voltage up ?!  ;)  ;D
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As of today 3/13/2012 my original owner 75 CB750F has made it through 3 wives, er EX-wives. Free at last.  ;-)

Offline Spanner 1

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Re: When pulling up to a red light, rev at 3-4k
« Reply #3 on: April 24, 2009, 04:47:54 PM »
Engine vacuum lifts those slides..sounds like the slides are not dropping down to their idle position...get a SingleOHC they are only 1/2 the trouble of a DoubleOHC....!!!
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If your sure it's a carb problem; it's ignition,
If your sure it's an ignition problem; it's carbs....

Offline TwoTired

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Re: When pulling up to a red light, rev at 3-4k
« Reply #4 on: April 24, 2009, 07:08:37 PM »
Those are CV carbs.  Very few SOHC4s have those.

If they have air cut-off valves, check them for integrity and function.
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Lloyd... (SOHC4 #11 Original Mail List)
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Offline dummkauf

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Re: When pulling up to a red light, rev at 3-4k
« Reply #5 on: April 24, 2009, 11:22:46 PM »
if you feather the clutch at all does that drop the idle?  Also, does it only do it after you've been driving, meaning that if you were to just start it and let it warm up a bit it idles fine but the high idle starts after driving it.

If the previous questions are a yes then this sounds like the same problems that my 82' CB900C(also a DOHC) was having when I just picked it up and the solution was  a can of SeaFoam.  I was about to pull the carbs before someone on another forum suggested the seafoam so I figured I'd give it a shot.  Half the can went in the gas tank, the other half in the crank case.  Took about 30 miles or so and my idle was dropping back down like it should, which I am assuming means that the seafoam did its job and unstuck whatever was sticking.

Or it could be the carbs.  You said they were rebuilt, might be something put back together incorrectly too.  I'll probably be rebuilding the carbs on my 900 next winter regardless, but the seafoam saved my the hassle for now.  Might be worth a shot, worst case scenario you're out $10
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