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1981 CB750C Carb tuning

John Luke

CB750 Enthusiast
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I know y’all be mad at me. 😂 so I have the idle jets replaced with #38 and she now idles perfectly at 1100-1500 rpms secondary main is 75 and the main is 120. It idles and revs very response. But right around 6,000-9,000 rpms she boggs a little. But if I turn the choke on she pushes right through. I’m gonna ride her tomorrow to see how she performs. But I may go up higher on the main seeing as she runs better with the choke on. Anyway, what are y’all thoughts? I’m running pods and a 4 into 1 exhaust.
 
Still likely carbs clogged or down on compression, the engines do NOT rev cleanly with low compression. The choke on seeming to push right through may be choke removing part of your power to not show the sudden drop any more as it has been evened out by it ALL being too rich instead of just at 6-9. That jetting with an off engine SHOULD make the problem you describe, you are missing compression according to your earlier test results. Maybe the plugs are still messed up too, too rich like what you are doing will do the same as well, DOHC ignition is notoriously weak and must be perfect to not lay down and die. Your uneven idle is showing more of the same, valves leaking (compression) or plugs not good and clean to hit consistently are what make idle jump around a lot if there is no vacuum leak. The idle if right will be more like 1100-1200, nice and tight.

Every one I ever had a header on revved clean as a whistle to 10,000 rpm in the lower gears with 75-115 jetting. No bog no fuss no slow, in low gear you better be holding on, it does it pretty quick.

Pull plugs, if still too dark then change them or ride the bike more to let them color back correctly, sometimes all it takes is some time. Also, can't remember your initial valve settings before they got set, if you had any close it can be that the now looser settings then close up again to again be too close. It happens a lot and why DOHC guys know to run the crap out of the engine after setting valves that have never been set, they get close again sometimes in minutes showing they were WAAAYY off. That running the dog out of it is the 'Amish tuneup', it also helps scrub carbon off of the now hitting valveseats. When the carbon is gone the valve may again begin to leak.
 
So......... I have rebuilt the top end and now I have great compression. Jetting is 135 mains and 80 secondary jets. Pulls great all the way to 60 mph and then kinda Boggs a bit. Any ideas?

Thanks!


 
'Pulls great all the way to 60 mph and then kinda Boggs a bit. Any ideas?'

Get rid of that 135 jet, you are showing how it is too rich with what I just quoted you on. And great compression isn't spit without numbers, quote those too. If you are choking to run better with that kind of jet your carbs are NOT RIGHT.
 
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