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1981 CB750C carb adjustments. Won’t idle lower than 1800 rpms

John Luke

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My 81 CB750C pulls great very responsive.

I have done the valves and the timing.
I have K&N pods and delkevic 4into1.
160 main jet and 75 secondary with 3 washers on needle. Nice chocolate on plugs.

So my question is how do I lower my rpms without it dying on me? I got it to around 1500 which I would be happy with but it only idled for like 5 seconds b4 dying.

Any help would be appreciated.
 
You are so far off in main jet that it will affect idle by being too rich before you ever get to idle, then dropping down to it will be having to clear the super rich you are running at there. Plugs are confirming it, chocolate of ANY amount is too rich, plugs should be more of a light tan color. And the 75 jet BETTER be the primary (front jet in carb) rather than secondary or you have messed up even more. You have zero need likely to mess with the needle and they are not settable anyway. The needle MUST flop loosely with the 3 washers on it and highly unlikely, if tight the needle will then not self center when slide drops and then the jet needle and needle jet will wear like lightning. Why the OEM needle on any carb flops around loose, it allows self centering to happen with no wear. That alone can be messing with your idle.

When you run an engine that rich the effect doesn't stop as soon as the engine drops off to fall to idle, the rich then affects the plugs to short down the sides to die when dropped to idle. The carbs being CV type also quit opening up and that rich will do it, I've seen them refuse to open fully with only 140 jets, it's way too rich.

Very responsive? If right the bike should rev almost instantly in low gear to 9500-10,000 rpm, and a speedshift into 2nd should yank the front wheel about 6-12 inches off the ground doing it.

Yours and do what you will but you should know.
 
You're not stupid, you just made a mistake is all, and I apologize if I seemed like I was saying you were. It can be a problem with me, I get into the details too much to then ignore the person.

Now you know why I made A++++s all through school but never had any friends, it continues today. We all live in self-created prisons.

Anyway, that rich at next to the idle would tend to kill engine every chance it got (the primary richness also feeds the pure idle feeds to a certain extent too).
 
U good bro! Just glad we have people like you to help me think things though and explore every possibility. You a boss bro.

When I get everything set properly I’ll let you know how it goes


 
Uggggg. I’m so freakin frustrated. I thought that it was the fact the I had the jets backwards. But after I turned them around. Same problems as b4. High idle amongst other things.

So what’s wrong? I feel like I’ve covered all my bases. So rn I have 115 mains and 75 secondary’s. No shims on the needle. Still giving me trouble.
 
Put stock airbox back on. At least you will eliminate one possibility. But you will probably need to put some new plugs in since you probably fouled the old ones badly from the very rich mixture.
 
Let’s scratch everything. I have 4 into 1 delkevic and K&N pods. What jetting should I use? I have 130 mains and 75 secondary. It gets up to 80mph but seems to be maxing out. Any ideas?

It isles much better but won’t hold a steady idle.
 
Lol it idles much better now around 1700rpms

But still has very random high idle I’m talking up to 7000+rpms
 
75-125 max and it will be slightly rich then. Using pods. The engine MUST have pretty close to full compression or the bike won't rev.

The 1700 rpm idle likely has the butterflies open enough that the air leaking under slide then gets enough volume to begin to float it and then you suddenly get the way too fast idle when one of the slides floats up.

Like said with the plugs, a few sometimes come back but it will take a week of repeat burning them back off to idle correctly, that too dark does not disappear instantly, it has to incinerate off. And if you are unlucky, some of it turns into glazing to short the plug out and changing it is the only way out unless you have access to a glassbeading machine.

I would try 75-115 myself if you have it.

If you can look at the carb bank off engine and into the front bore but not directly straight on, more like a 45 degree angle from up, if you can see that first small hole behind the butterfly like that the carb butterflies are open too much at idle.
 
I’ll try the 75 115. But it seems to run better and actually has a higher top speed with a bigger jet. But I’ll give it a shot
 
Have you ever run the compression on the engine? Being low is a major reason they won't rev, they don't pull fuel nearly so hard either and then the jetting has to go artificially high, that then prevents engine from pulling out fully too when the slides drop as a result. One of these DOHC in good shape should go through 80 mph so easy it cannot be stated enough. Watch your tach and listen to the header, a stock DOHC just begins to enter the powerband in 5th gear around 5500 rpm, and then it should rev up increasing past that harder and harder. You can tell in the header when the individual exhaust pulses suddenly merge together to begin the characteristic howl that headers are known for; it kicks in at the beginning of the power band. That is usually around 70 mph with OEM gearing.
 
I’ll be honest, I haven’t checked compression. Only thing I checked was to see if the valves had any leaks and they were air tight. Or at least I couldn’t hear or feel anything. I just bought a compression gauge kit and I’ll check that next week.


 
I’m beginning to think it’s compression. The bike ran great earlier with the crazy jet settings but idk why it’s giving me so much trouble. Right now one of the best ways to describe it is the fact that it won’t run unless you keep the choke on. And in fact, with the choke on she idles properly and revs to kingdom come. So is that the fact that the compressions low it’s not sucking enough fuel and thus the choke makes her run?

Thanks! Or is this something else?


 
Well, you know you can't run around with the choke on full. Likely you have freed up some trash to stop carbs back up again, common if the tank was rusted, the ethanol can do it in as little as a minute.

The engine can seem to run great at first with way too rich settings, then it settles in and the problems begin BTDT.

Your choke thing may be a different issue. If they use ethanol in local fuel it may drive you nuts. Or maybe same issue, if the super rich was because the carbs were already messing up.

Compression reading, to ensure not going crazy, and the carb aircuts can drive you nuts too if you haven't defeated them.
 
So, I took the carbs off my CB750 nighthawk and put them on my custom and it runs much better. It goes back to idle really well and revs okay still a few bogging spots. But that may be because I had 68 secondary 125 main jets in it. I got guitar string and could make it thought almost all the jets except the slow idle jet. The one under the plug. So I extracted them and guess what. Totally blocked can’t even see through them. So I need to buy new ones cuz I destroyed them when I removed them. Would anyone know where I can buy new one? While I’m at it I’ll buy 115 main jets. After I get all the parts I will put the 75 secondary and 115 main jets with the new idle jets and see how she run then.


 
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Here is my nighthawk and she runs really well.

Here is the photo of the bike I’m working on right now
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Oh and don’t forget my custom KZ750 that I built from scratch. And she purrs like a cat. And she pulls super hard. It’s just annoying that the CB750C is giving me such trouble.


 
Looks like you are looking for N424-6-XX pilots or slow jets for Keihin. Need like 35 or 38 there, 40 is a bit too rich. Looks like they are drying up too, way harder to find them than it used to be. I'm assuming the screw-in type, '81 was that type, '80 used press-in.
 
Already left ordered them. Size 38. [emoji1692] hoping this solves a lot of it and I also bought 105-110-115-120 main jets.




 
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