79 cb750L idle problems

[mgp214]

So I bought my '79 cb750 limited from some guy who put pods on and changed the jets (up from 68/102 to 75/110) but never got it running well. I've been tinkering but have been having trouble getting the engine over 5k-6k so I figured I'd throw the airbox back on and go back to the stock jet sizes.

The good news is that the rpm goes all the way up (except it stutters around 8k), but the bad news is I can't seem to get it to idle now. I've tried having the idle screws tightened all the way, 1.5 turns out, 2.5 turns out, and 3.5 turns out -- whenever I adjust the idle rpm thumb screw so that the rpm falls below 2k, the engine dies. It seems like the idle rpm is bouncing around a lot. Also, even when fully warmed up I can't get it to start without full choke.

Is there something I'm missing?

[TOOLS1]

You have a vacuum leak. Check the boots between the carbs and head.
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[mgp214]

The thing is—I've pretty generously sprayed starter fluid all over the carb boots, throttle shafts, and carb--airbox joints. I had leaky boots in the past and replaced them, and also recently put in new throttle shaft felt seals (soaked in white lithium grease) and the rpm doesn't jump at all when I spray these areas.

I've never ran it with this airbox before -- is it possible I'm not seating the filter correctly?

[shinyribs]

I would guess plugged pilot jets/ idle circuit since your vacuum leak tests showed no leaks.

[mgp214]

I'll pull the rack off and clean the pilot jets again tomorrow, but I've cleaned them pretty thoroughly only a few weeks ago.

Is there any common problems associated with the airbox itself? I didn't have this idle problem when I had pods installed. I was able to set the idle at a steady 1000 rpm, then I installed the airbox to fix high end problems and now I can't set the idle below 2k or it'll die.

[TOOLS1]

High idle means too much air. This could be a mechanical problem, like something keeping the throttle from closing. However since you can idle it down until it dies, you can rule this out. The other could be something missing in the carbs that would cause them to let too much air through. However again not likely since it was running/idling before. Last would be a vacuum leak. What you describe as the bike dieing when you idle it down, and having the carbs off, leads me to believe this is your problem. Sometimes spraying carb cleaner, or starting fluid does not show a vacuum leak. I use acetylene, and sometimes it does not even show a vacuum leak.
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[mgp214]

That makes sense—when I block part of the airbox intake with my hand (to test its suction) the rpm jumps way up.

I'm definitely open to any tests I can do to identify where the leak is, I just don't really know what to do next. Should I grease up everything and see if that's a temporary fix? use propane?

I'll try to reseat the carbs and boots tomorrow, but again I'm flying blind without being able to narrow down the leaking area.

[TOOLS1]

Propane will work. However I am betting that the carbs are just not sealing in the boots.
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[mgp214]

So I carefully re-seated the boots on the head, the carbs in the boots, and the airbox on the carbs again today, and it didn't seem to make any difference. I did notice that the airbox is in pretty bad shape though, and I suspect that what's causing the leak considering I didn't have these problems with the K&N pods I had on previously.

I also noticed that I still had problems revving the engine into the high end band which is the reason I tried to set up the airbox in the first place. So I switched back to the 75/110 jets and the taped-off pods (which solved my vacuum leak and idle issues) and decided to take a look at other reasons for high rpm problems. I replaced the spark plugs and it's running significantly better all around; I'll be taking a look at timing and the mechanical advancer soon.

Thanks for the advice, as always!