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Won't start on all cylinders...???

JakeGrafton

CB750 Enthusiast
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Need some help...

Just spoke to a guy selling a '81 CB 750K with some 46K on the meter. According to seller the bike usually only fires up two cylinders when starting, then the third and finally the fourth cyl once the engine gets a bit warm. But last time he tried the fourth cyl wouldn't start at all...

He think's it's a carb problem. What's your suggestion? And would you buy it? (Prize asked is some $2000+) Otherwise looking pretty good on pictures.

TIA
 
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I've heard of leaky valve seals causing something similar to that on other bikes. After it sits, oil leaks into the cylinders and causes them to not run well until the oil burns off and the bike warms up. That may not be the problem with that bike, but something to ask about. Have him fire it up after sitting for a while and only run it for like 15 seconds. Pull the plugs and see how they look. Then since he'll have the plugs out, check the compression on each cylinder.
 
At least the top end if that's the problem, but I really don't know. Cost would depend on if you can do the work yourself and how extensive the issue is. I'd still want compression numbers. If it were a carburetor problem it would be the idle circuits and or possibly a synchronization issue, but I would think they would only pick up with an increase in rpm. Could be ignition, but that's usually the other way around where you drop cylinders when the coils get hot. There are others on here with better diagnostic abilities and hopefully someone else will chime in.
 
I would concentrate on the plugs. There are subtle differences between black plugs from running too rich (fluffy black) to black plugs from oil getting in the cylinders. That will tell you whether it is a carb issue ($) or head issue ($$).
 
My Clymer manual had pictures and included descriptions of the source of the problem. I am betting that a Google search would bring up something similar or better.
 
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