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Discussion Starter #1
I have a 1977 honda cb 750k that I just got back together after installing new rings and carburetor kits. It seems to run ok at higher rpms and will idle around 1000 rpm, but if I jerk on the throttle it will die. It does it at low rpm all the time, and higher rpm it will act sluggish and then catch up. I have adjusted the air/fuel mixture screw like the manual says. Not sure if my problem is carburetor related or if it could be electrical.

While running the bike at 3000 rpm I lifted the contact points on 1/4 cylinder and the engine slowed to about 1200. Then did the same with 2/3 cylinder and the engine slowly died. Could something be wrong with my coil to the 2/3 cylinders? Is there a way to test it.

I have great compression, new carb insulators, and carb to air box insulators and have tested for vacuum leaks and have none. Contact breaker gap is correct. Timming is correct with a timing light.
 

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It's pretty simple points ignition you have, so as long as it's tuned up like you say it is and your battery is good it's something fuel related IMO. It sounds like too little fuel/too much air.

Is it stock intake and exhaust?

Great bike, post some pictures sometime!
 

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Discussion Starter #3
I tried adjusting the mixture screws. I turned them all the way out and it didn't seem to make much of a change. Everything is stock on the bike.
 

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Wow, great deal!

Idle mixture screws will only effect the mixture at idle. Anything above that they are meaningless.
 

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You can adjust the idle mixture, float level, and I think on your bike you can adjust the needle position. That said though, you have a stock bike, so keep everything stock and fix the problem.

Float level is a big deal - make sure that is perfect! Also, if your carb kits came with jets make sure they actually correctly sized - some cheaper kits are inconsistent.

Maybe someone else will chime in with another idea, before you take everything apart again.
 

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Pale Rider
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I'd suggest the Nabble CB750 forum, Link, for expert advice. Tools, and the gang, are quite helpful, as well as very knowledgeable. Cheers!
:coffee:
 

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Discussion Starter #12
Yeah, it has an accelerator pump. I checked it while I had the carbs apart and had no cracks or holes in the diaphragm and it sprayed gas when I pumped on it. Then I checked the gap on the rod when it was back together and it all seems to be working correctly.

When the carbs were off I used a drill bit method to sync the carbs. I was waiting to get it back together before using a syncpro carb tuner. But I have not used it since it wasn't running well. Should I try using the snycpro now while its running like it is?

Thanks, I will post something on that forum as well.
 

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When you have the engine revved to, say 3000-4000 rpm, and release the throttle, does it go straight to idle rpm, or does it drop below, then rise? Or, does it drop to idle over a few seconds? First condition is too rich at idle, second it too lean.

Also, the drill bit method only gets you close, as there are more things that affect flow than just the gap at the throttle plate, so, yes, use the sync tool. Then, try evaluating the idle mixture, since adjusting it has little effect on carb sync, but carb sync has a big effect on idle mixture.
 

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Discussion Starter #15
I checked the ignition coils and they tested to be OK. I then used the sync pro tool and synchronized the carbs. Any ideas of what to check next?
 

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Okay, I'm going to make a suggestion that might be wrong but it's something I would try in trying to work my way through a problem like this. So I'll add, don't do this unless one of the more knowledgeable say sure, it would say something one way or the other. Shade tree mechanic that I am isn't the best but have you tried with the air cleaner cover off, revving until it gets sluggish then squirt a shot of start fluid in the breather? I'm thinking if it's fuel starved you should see it go on past that point. This is assuming you don't have a clogged filter or a mouse nest in the muffler. Okay guys, is that good or very very bad? It is something I would do, right or wrong in the case. It just sounds like it's fuel starved to me. That's my big fat GUESS and a way to prove it wrong.
 

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If he's having a really lean condition, a squirt of starter fluid would briefly let it rev, assuming it doesn't require the engine be under load. Also, four carbs, so, which one? Hard to hit all at once, without some special rig.
 

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Discussion Starter #19
Right now I have the carbs off and planning to pull the pressed in pilot jets. Clean them out and see if they were plugged and might help it run better.
 

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Yup. I can't for the life of me figure out why Honda (or anyone) would press in a pilot jet.
 
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