1980 CB 750k sputter at higher RPM

[Gator]

New to the site, love the information on here, Awesome.
I have question about my cycle. It has 19,000 miles and has been setting for a couple years. This summer I had the carbs rebuilt by a guy in Califorina that a friend with a VMax recommended. I just went through and adjusted all the valve clearances after I read a post on this site. All are at .005 or close to it.
The problem I’m having is when I am cruising at 4k to 5k rpm there is a sputtering or miss in the engine. It does it in 3rd 4th or 5th gear.
I synced the carbs after the rebuild and so far nothing has made a difference. I was wondering if fuel mixture could be the problem. Other than that it runs like a striped ape. If I accelerate past 4 to 5k it’s fine. Only does it when I try to hold it at that RPM or higher.
I know someone here has an answer, PLEASE!
Thanks for the wealth of info.

 

[brettp]

I don't have a 1990, but trouble at that throttle range seems like it might be a needle adjustment. If it idles fine, runs up to 4k RPM fine, then misbehaves through 5k RPM, and is fine again to WOT, I'd say your needle needs to be raised a tick. Perhaps the guy that did your carbs did not put them back where they were. Could be an honest mistake.
The valves would show issues at 4k then most likely misbehave all the way through the RPMs (if it could even get past 4k). The fuel mixture is only going to effect idle. I can't think of anything else.

 

[Gator]

Thanks for the reply,
So would it be just one carb or more that need adjustment?

 

[Gator]

Thanks for the reply,
So would it be just one carb or more that need adjustment?

I just realized that the year is wrong. It should be 1980. Good Grief!!

 

[Gator]

Sorry not sure what needle needs adjusted other than the fuel mixture.

 

[brettp]

Ok, 1980 it is. I have an 81 and my needle is not adjustable. Are they stock carbs?
Oh, I forgot about timing. Have you checked that?

 

[Gator]

Yes they are stock carbs, and I have not checked timing. It starts and idles good , thought about it but didn’t check it. I’ll check it.
Thanks again for your help.

 

[Gator]

Well I checked the timing and it was on the money. Not sure what do now. I did notice a coating of oil on everything when I took the cover off the CDI side of the engine. Is this normal?

 

[brettp]

Let me ask, does it feel like a miss or does it feel like the bike is starved for fuel?

 

[Gator]

Let me ask, does it feel like a miss or does it feel like the bike is starved for fuel?

More like a miss. Kind of a Studder

 

[Gator]

More like a miss. Kind of a Studder

When I turn the throttle past that point it’s fine.

 

[TheAlt]

If you add some choke does it smooth out?
Alt

 

[Gator]

If you add some choke does it smooth out?
Alt

Not sure! Didn’t think about that. I need to try that. If it helps what does it mean?

 

[TheAlt]

Well if it's fuel / air related. Altering this would indicate either an airleak (less likely since I would expect it across all) or something the carb. (jet size etc)
Pulling the carb will get more fuel vs air into the system, so you were not getting enough if it runs better.
If it runs worse, you are getting too much fuel to air.
The long needle into the carb can be changed on most. Usually positions so if that's not adjusted I would begin there before jet changing.
but otherwise.
Runs better with choke? a larger jet may help. (more rich)
Runs worse, A smaller jet may help. (more lean)

My 400 would only run well on Choke at highway speeds. A carb rebuild fixed it,
Personally factory OEM jets only. The knockoffs are for knocking off the table into the trash IMHO .