Is my 81 CB750C DOHC performing at it's best?

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Is my 81 CB750C DOHC performing at it's best?

blankbrian
I don't post here much but I've been curious what all of you well experienced CB riders have to say.

I rebuild a 81 cb750c dohc last year and this year I finally got it out to do some real riding with it. Brace yourself, I have on pod filters (mostly because the guy I bought it from already installed them and the air box was missing pieces) and a carpy 4 into 1 exhaust. Yeah yeah typical cafe nerd blahblahblah....

Anyway, after riding it for awhile it just doesn't feel like I'm getting the performance that I could be getting out of it. I feel like every time I'm in 5th gear and going down a long straight road I ALWAYS try to go up another gear that isn't there (obviously). I get to around 55 mph and the thing is really reving high. Maybe I'm just dumb and the pipes are misleadingly loud because my tech is around the 6-7 mark. But i feel like the throttle at that point isn't too reactive and to get any faster would be kinda pushing it too hard.

I read about other riders hitting 100-135!! That seems so ridiculously on my bike.

A couple things I'm going to look into that I'm wondering if one could be the prob:
- Change the gear (any suggestions?)
- Check the vaccuum where the carbs meet the engine (I'm almost positive there fine because I put new boots on them)
- Tighten the Cam chain
- Also I've notice that my spark plugs are just barely brown, basically still white, on the tips. I'm told this is great but could it mean it's too lean?

Thanks for reading all this crap. Any comments would be GREATLY appreciated.
THANKS!
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Re: Is my 81 CB750C DOHC performing at it's best?

TOOLS1
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Your bike should be able to go 120 MPH without any problem. These engines are designed to run high RPMs. They are not V-twins or car engines. With ethanol gas, you will not get a very dark brown on the plugs. You could try a smaller countershaft sprocket. Maybe one tooth smaller. Also check your timing. Rev the engine and make sure it is going full advance.
However with stock CV carbs and pod filters, the bike will never perform as well as it did with the stock airbox.
TOOLS
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1976 CB 750-A X 2
1977 CB 750-A X 4
1977 CB 750-K
1976 CB 750 F
1981 CB 750
1966 Kawasaki SG 250
1981 KZ 750 LTD
1973 CB 350
1979 CM 185 Twinstar
1982 Honda XL 80
South of Eden (Kansas City MO)
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Re: Is my 81 CB750C DOHC performing at it's best?

blankbrian
The loudness of the pipes must be throwing me off then. I'll try to go down a sprocket and see if that helps too.

Thanks man!
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Re: Is my 81 CB750C DOHC performing at it's best?

shinyribs
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What gearing ( front and rear sprocket tooth count) are you running now?

In stock trim, my CBs ran about 6,000 rpm on the interstate. Like Tools said, they like to rev, and it don't hurt them a bit.

Concerning the pods/pipes/airbox,etc situation: if your bike won't freely rev to 8500-9000rpm then it's not performing as it should. My '81 CB750F had some work done to it ( lightweight valvetrain, head work...) and would rev past 10,000 rpm happily in the first three gears, which pushed the bike well over the 125mph mark. I'm not advocating ignoring Honda's redline, but these motors will rev just about as hard as the modern bikes if everything is in order. Actually, I remember ticking 12,000 a time or two. Man, what a glorious sound it was...
A free people ought not only be armed and disciplined, but they should have sufficient arms and ammunition to maintain a status of independence from any who might attempt to abuse them, which would include their own government.~George Washington



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