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78 CB750 SS Carb help

invaderb

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Hey fellow riders, I'm in need of some assistance. I have a 1978 Honda CB750F super sport and the carbs have been given me some trouble.

A little back ground I bought this bike from a gentleman on CL, bike has 13K and is in pretty good shape, pod filters with a 4-1 exhaust. He said that it needed a little bit of carb work to make it awesome, and the main jets have already been jetted up but it does start up and run good but when you drive it around under load it pops and lags above 4K rpms. I take the bike to a local bike mechanic here and had them clean, and sync the carbs and give the bike a look over to see if there is any underlaying major problems. I got the bike back drove it around and it was great no popping or lag all the way home. the day after next the problem comes back. It will start and run great but pops and major performance lag at full throttle above 4K rpms. Call the mechanic back and he was like probably need to jet the carbs. at this point I'm like blah I'll just do it my self I have cleaned and tuned carbs before I was just working on another project plus school and work and I just wanted to ride.

I order the jet kit, the main jets on the carbs were all set to 125 and the secondaries are 42, I'm right at sea level so I set carb 1 and 4 main jets to 127 and carb 2 and 3 I left at 125. put everything back together take it for a ride it felt better but still a little bit of lags and some pops here and there. So Friday I start adjusting the needles and couple hours later take the bike for a spin again for the rest of the day this time and absolutely no issues. take the bike out again Saturday night bam issues are back. So this afternoon I again spent a couple hours tweaking the needles but couldn't get it just right this time.

The bike idles pretty good but every once in a while the idles will fluctuate for a minute of two then it goes back to normal. the spark plugs look pretty close they are a medium charcoal grey color, pipes seem to be evenly hot, I'm sort of at a loss now.

Sorry for the novel any help would be greatly appreciated

I can post some pictures later
 
Have you check ignition timing point condition etc? Have you check for air leaks and or cracks in the intake boots?
 
Hey dirtdigger thanks for the reply!

I think it maybe the points, one was sparking more than the other so I'll clean those and adjust those to be the correct distance. the intake boots are in pretty good shape and on there pretty good so I don't think it's an air leak.
 
Hey Guys,

Bit of an update, still having issues. I cleaned and adjusted the the points to 0.016 as it says in the book, took it for a ride about 20 miles round trip and it was perfect again no popping or lag all through the rpms. however on my way home same issue comes back and now when ever I put it under load the bike dies. I adjusted the timing and it's pretty much spot on, I found that the 3rd plug wire was shorting out so fixed that but still having the same issue.

the other thing I'm noticing is when I rev the bike up it takes a while for it to come back down, like I'll have it idling at 2K rev and it will sit at 3k for about 30sec then come down to idle.
 
Hanging idle can result from out of sync carbs, air leaks, sticking ignition advance, weak ignition advance springs. Points should spark very little. If you are getting a lot of spark in the points then your condensors are more than likely bad.
 
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