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Still need help with tuning CB750C carbs

John Luke

CB750 Enthusiast
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I guess my question is which jet controls WOT?


I have totally rebuilt my 1981 CB750C. I have done the timing and the valve clearances. I have 4into1 delkevic exhaust and individual K&N pod filters. The current jetting I have is a 135 secondary main and a 80 primary main. With 3 turns out on the a/f mixture screw. She idles great and roars to 12000 + rpms. But when I give her full throttle on the highway she seems to be bogging. She’ll get to like 90mph and won’t go any faster. Im tempted to put tape on the filters to see if that helps. Hopefully that is a good explanation.

Any ideas or help would be appreciated.


 
[emoji23] I'm ordering some air boots. Gonna throw her back together with the airbox on and see if that solves my problem.


 
Still a bit frustrating because it seems you said that you've done multiple bikes with like say a 75/115 set up and had absolutely no problem.


 
Not making that up (did it on 4 myself), the bikes run great other than the last bit of pullout is not there in high gear ONLY, who needs the 125 mph there, 110 is fine. Others with a solid motor have gotten there too after I told them the setup but the engine MUST be in prime shape, most aren't.

I'd bet the valves are leaking a bit too, I have never pulled one apart that wasn't if it had 20K miles or more. The exhausts go through hell and the rev range then dies up high. No way you will have a 40 year old bike run fine up there and the pods make that worse and why everybody goes crazy. The only way to get away from that is valve job and changing the exhausts, they cannot be ground, only lapped and doing that too much makes it worse too.

The engines were made in a era of low lead fuel but even low lead gave massive advantages to valve life as compared to no lead now and ethanol only makes it worse. The valves are only thin coated with nitride and it goes away with lapping or fairly low wear and then the super soft steel underneath then burns like lightning. Add to that needing the OEM airbox way more than most engines due to the type of CV they use (labyrinth instead of rubber diaphragm, it leaks more) and the carbs with aircuts and double sided idle circuits that clog to defy even the best carb rebuilding guys and you see why the engine type is so expensive to get back to running right as compared to most other fours. Why the parts network never developed like for the SOHC.
 
Thanks for returning to this website AMC49. Listen to this guy. I did my best to follow his advise and rebuilt my cb750. First tried the PODS. While it ran nicely, I soon noticed that I could not go over like 8500 RPM. Everything else was great. Ordered 4 replacement boots and a new filter, cleaned up the air box and WHAM! The bike nearly jumped out of my hands the first WOT pull with stock air box.....

Also, you said it revs to 12K+. Do you mean while idling? Under load the truth comes out. My rebuilt 750 will climb to 11K under load and it still seems like its climbing
 
Careful, you'll pop a rod. The engines can be made to run higher rpm but it really produces no more power and all they do is pop rods and timing chains. Why we tend to advance cams to keep the power in the mid range where the engine likes it, then it still pulls out max rpm.

The head is the problem, straighten the kinks out like a few have and they then make power up high too. Until then the intake port curves tend to make the engine act more like one intake valve cuts off the higher you go, they choke.
 
OK, I'll back off this season. She pulls so hard and strong. It's just so tempting to Rev it high because she goes up there so strong and smooth, the bike really seems to like it. No stumbling what so ever, just smooth rpm climbs. The 4into1 with a SuperTrapp muffler sounds wicked! But I'll keep an eye on the shim clearance and tensioner slack, routine oil changes and stay out of those high RPM's.
 
Not saying don't enjoy it, just watch any going over 10K rpm. They do it too easy.

It's a shame so many others never get to experience how well they can run. I know the feeling once you hit that 7000-10000 rpm solid powerband, it's a real head rush, almost like riding a hard running two stroke.
 
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The stock air box is a pain to install but it's worth the effort. The hard part is ensuring the boots seal 360 degrees around along the air box face. I should make that write up on installing and removing the air box. Use new rubber and after a few installs and removals for cleans/adjustments you'll develop a repeatable method.....or sell the bike out of frustration. Have patience because it's a nice reward.
 
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