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Variable performance / exhaust noise on my 79 CB 750K

CDN750K

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Greetings. First post new member.

I was gifted a 79 750K with around 35k km's on it. Its in factory configuration aside from a 4 into 2 exhaust system that is ok at best. The exhaust has no bypass.

The bike starts perfectly warm or cold and after a decent warm-up period, runs and drives nicely with smooth linear power. However at around 4000 rpm the exhaust note changes from smooth to a gnarly growl that sounds like its coming from 12 feet of plastic tubing. There's a significant power increase that's not so linear anymore and the engine becomes somewhat rough. At 4500 rpm it settles down and feels better but I think it should be smoother.

Question: Is this normal for this bike or does this sound like a jetting issue. Carbs were cleaned and synchronized a year ago but could likely use another sync. job.
Cheers.
 
Hi...if not done yet, check the aircleaner/box...things sometimes live in there in storage(e.g. mice)...plus if its the original a/cleaner you may find its deteriorated with age.
What do the s/plugs look like??

Re sound..of the fitted pipe....dunno...mine now has a Delkovic s/s 4 into 1....higher revs it does sorta "scream"...but a lot of inline fours do in the higher revs.
Your profile pic seems to indicate aftermarket pipes....these originally had 4 exit pipes as you know/mention in your post.

I'd leave the carbs alone until you eliminate other things then just put some good gauges on....

Have you ever checked the valve clearance or know when it was last done?
 
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Hi...if not done yet, check the aircleaner/box...things sometimes live in there in storage(e.g. mice)...plus if its the original a/cleaner you may find its deteriorated with age.
What do the s/plugs look like??

Re sound..of the fitted pipe....dunno...mine now has a Delkovic s/s 4 into 1....higher revs it does sorta "scream"...but a lot of inline fours do in the higher revs.
Your profile pic seems to indicate aftermarket pipes....these originally had 4 exit pipes as you know/mention in your post.

I'd leave the carbs alone until you eliminate other things then just put some good gauges on....

Have you ever checked the valve clearance or know when it was last done?
Yeah, I think a clearance job would be a good idea. I never considered that as a possible contributor.
 
Yeah, I think a clearance job would be a good idea. I never considered that as a possible contributor.
The valve clearance decreases with use....
So if the bike runs better hot...it mighten be the issue...
However if the valves are poorly adjusted that can be a contributor to roughness...

If you have carb gauges by all means check the balance..
Look at your plugs, maybe even use a Gunson colortune...😁 ..... b4 changing the jetting tho......these carbs can be finiky....

Good luck...
 
I'm still trying to figure out what's wrong with a "gnarly growl"... 35,000 miles.. I would definitely take a look at the top end.
 
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