1979 10th anniversary cb750 hanging idle......

hey guys, I just bought a 79 cb750 off of my friends uncle for 500$. I may be able to post pictures later on to show the amazing condition it was in when I bought it. The guy had it worked on probably a year and a half to two years ago and his mechanic rebuilt the carbs (lucky me), got new spark plugs, and put new tires on it (lucky me again). He had it running but out of sync and it was backfiring really bad from what I've been told. I tore into it and cleaned the carbs back out, wire brushed the spark plugs and started electrolysis on the tank because it was super rusty.... I now have the bike running after two days of ownership

but....... I had to bench sync the carbs to get it to run and not backfire, and I've now run into the problem of it either starving for fuel or a bad vacuum leak.... I cleaned the carbs out completely, I haven't gotten them soaked yet because I don't want to do that myself, and I semi checked for vacuum leaks using zep around the intake boots... to no avail...
It'll run, but it idles down super slowly and it will stall out under 1800-1500 rpms...... I'm stuck. Before I get pummeled with the "you're a moron" comments, keep in mind I'm only 18 and have already rebuilt a 74 cb360 and an 85 magna... I also don't have the money or time to buy the carb soak stuff and take the carbs all the way apart to soak them... Someone please help me diagnose whether it is an air leak or lack of fuel... I've had too many varying comments from people I know on the matter to be sure... My need to know comes from the fact intake boots are around 60-90 bucks from what I've researched, and soaking the carbs with someone I know would probably be around 40-50$ so I don't exactly have the money to do both and then have the possibility of neither of those fixing my problem... HELP please haha

Sounds like you have a unique problem. Sounds like you have both an air leak, and a fuel starvation situation. Take your tank off, drain it, clean it, get any rust out. Remove the petcock, do the same to it, soak it, clean it, etc. Replace the fuel lines. Take the float bowls off and check the float needles for damage and/or trash. Clean the needle seats (scotch brite).

The high idle/slow drop sounds like an air leak. Somewhere, somehow you're getting extra air into your cylinders. If you don't have the skills to do it, ask for help. Even a tiny pinhole leak will cause a lean situation. These CV carbs also have an air/acceleration pump, which is a bitch to deal with, and often gets stuck either fully open, or closed. It sits on the #2 carb (from shifter side), it's the only carb that looks different than the others. Again, get someone knowledgeable if you don't know how to do it, or can't follow the diagrams/exploded view/maintenance manual explanations. Knowledge learned goes a long way and is invaluable.

I appreciate it, that's what I was thinking as well I just needed someone more experienced to give an opinion to make sure. I went through the carb and cleaned everything except the acceleration pump, I have researched it a little, and I looked in it, cleaned it to the best of my knowledge, however, I've never dealt with one before and I wouldn't know the proper function of one. If the diaphragm on the acceleration pump needle has any pin sized hole could that cause either of my problems? I've thought about going ahead and buying a new pump because I found some on ebay relatively cheap. Also I have been using an auxiliary fuel cell due to my tank being completely trashed haha. I shocked it and now I'll be putting gas and pebbles in it to clean it out some more and then probably shock it again.

But, correct me if I'm wrong, with me already cleaning out the carbs except that pump, I could safely assume I should just go ahead and order new intake boots and an acceleration pump, right? Do you think that could solve my problems? I also found a guy that will soak my carbs if need be as well, but I'm really not sure about taking my carbs completely down (mainly worrying about the mixture screw...) because I'm not sure I could put everything together and have it tuned right as it was....

if it was a lean fuel condition likely it would be farting,popping and spitting
check the advancer for smooth operation, cleaning and lubing it lightly would be the ideal thing
and use ether(starting fluid) to check for air leaks just be sure and keep your face back away,use only short spurts and do it outside with a far 'sting-wisher handy

It's running smoothly minus the idling problem, no misses, no backfires, no anomalies. when it dies under 1500 rpms it just quits, it doesn't sputter or spit or anything like that... and I've checked for air leaks using zep and found them... they were on the jug side of the intake boots. I'm ordering intake boots asap. I'll post pics in a bit.

good news !! what is zep that you used ?

Could be your ignition as well. Just a thought. I had a slight problem with my idle hanging after extended sustained highway speed runs. Turns out my advance wasn't returning when the rpm's came back down. It would idle up around that 1500 range for a while until it eventually worked itself back down. These things run crazy advance. something like 30 or 40 degrees at highway rpms. If it's sticking there when you come down to idle, that could very well be something else to look at. Good luck.

Jewbacca said:

Could be your ignition as well. Just a thought. I had a slight problem with my idle hanging after extended sustained highway speed runs. Turns out my advance wasn't returning when the rpm's came back down. It would idle up around that 1500 range for a while until it eventually worked itself back down. These things run crazy advance. something like 30 or 40 degrees at highway rpms. If it's sticking there when you come down to idle, that could very well be something else to look at. Good luck.

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+1 that was the point i was trying to make but you say it mo better

xb33bsa said:

+1 that was the point i was trying to make but you say it mo better

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Ha, I figured that was what you were talking about.