Ok, it's time I asked for help

I've never been able to get the idle on my CB750 quite right and now it's just getting me confused. Here's where I'm at.

The idle adjuster screw is turned all the way out, but the idle is still quite high. What's more concerning is that the left pipe is popping a LOT and now there's actually fire coming out. When sitting on the bike you can hear the right side firing smoothly, but the left side isn't really making any noise til you give it a little throttle. Off idle the bike runs great. Sometimes there's a little stumble while easing off from a start, but once you're on the gas it pulls hard and runs smooth. There's just a lot of popping on decel, and when standing at idle. Pretty much any time the throttle is closed.

Up til now I had attributed this to the fact that the carbs had never been sync'd. Well tonight I finally hooked up a sync tool and got em all dialed in. They were all over the place, but now they are pretty good. The bike seems to idle better, and doesn't try to randomly die anymore, but the popping is just as bad if not worse. Also now the idle is higher, and since the adjuster is out all the way I'm not sure how to get it to go down.

What has me really confused is that it only seems to be the left 2 cylinders that are causing problems, and only at idle. the right side seems to be running quite smooth, even though all 4 carbs are adjusted the same and are now sync'd. I'm now unsure if it's a carburetion thing, or if maybe the left coil is starting to go and is just weak at idle.

Here's a video of the bike at idle right after my test ride this evening. I'm open to suggestions. Thanks!
http://youtu.be/VLLtjmlTyGs

EDIT: Guess I should add the bike specs. It's a 1980 CB750F Supersport, DOHC. Recently got new rings and shimmed the valves. Internals are otherwise original. Pod filters, and straight through mufflers on the stock 4 into 2 exhaust. The left and right exhaust pairs are not linked together.

Sounds and looks like a mixture problem.

even though your synced them

it sounds like you might have a vacuum leak at one of the isolators try spraying some carb and choke cleaner

around the isloators to see if you have aleak fix that then re sync the carbs

Call me dumb, but what are these isolators you speak of?

And on the mixture front, should I be adding or taking away fuel?

By isolators, he means the carb manifolds (or boots). the rubber devices that hold the carburetors to the engine. Often, the rubber seal under them can crack, or the manifolds themselves can be dry with age and letting air in.
Had a similar problem with my Tempter. Ran great with the choke on and under load, but idle would run away and would sometimes even get super hot or not work at all on the left. But I'm lucky, it being a twin makes the culprit easier to find. Spray some intake cleaner around the base of your manifolds or, if access is not so great, try wiggling the carburetors while the engine's running.

Good luck!

StinkBug said:

And on the mixture front, should I be adding or taking away fuel?

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if the problem is a cracked manifold boot, you shouldnt need any new jetting adjustments. Just fixing the leak will effectively be adding fuel. Although having sync'd them with an air leak, you'll have to re-sync the carbs once your air leak is repaired.

Ok I just took a close look at all the boots, and they are all in very good shape with no cracks or dry rot or anything like that. I also tried the carb cleaner trick with the bike hot after riding home from work and there was no change in idle no matter where I sprayed (with the exception of into the filters of course)

The idle is still crazy high, high enough to actually push the bike a bit under braking, which is annoying. Definitely takes a whole different riding style from my Ducati that has TONS of engine braking.

so I tried playing with the carbs a bit more last night and nothing seems to make a difference. I tried raising the needles as well as adjusting the idle screw and didn't get any real change either way. I'm starting to wonder though could it be as simple as a clogged slow jet or just needing a bigger slow jet ? does anyone know the size of the stock slow jets in this bike ?

Forget about needles and main jets if the problem is the idle. That's all about air leaks and pilot jets and air screws. Carb synch is important for a smooth idle too.

Umm who said anything about main jets I was asking about the slow jet . I also just mentioned that I had already ruled out air leaks and changes to the air screw don't seem to make a difference

Interesting response. OK, let's start again . What year and model of 750. What modifications have been made and what carbs are fitted. Stock airbox or pods?

High idle can be anything from a hanging cable not allowing the slides to fully drop/butterflies to close, to an air leak or clogged jet.

Have the carbs been cleaned and cleaned again? Are the jets OEM originals or a jet kit?

Are the carbs fully closed or is a cable causing them to hang? Are all four carbs moving together?

That left side missfire could be a weak coil or break in a wire on that side of a fouled plug. Typically 4s have two coils 1-4 and 2-3. Are the matching cylinders also missing? If so inspect wiring and the points or pickup for the bad pair.

And check your timing with a strobe light.

I'm sorry if I'm coming off as a jerk but I get a little bit frustrated when people give general answers for a specific question without reading . I already posted the year and modifications to the bike in the very first post, as well as what I've done recently like syncing . Carbs are stock, jets are 75/110, brand new cables, nothing hanging or sticking. That's why I asked about the coil and clogged or too small slow jets.

ok maybe a backtrack is in order

have you put new plugs in recently?

have you swapped the coils from left to right to see if the problem moves with the coil?

have the carbs been cleaned if it is not electrical it may be that the pressed in slow jets are not clean

i only have success with cleaning those carbs by removing the slow jest removing the emulsion tubes and cleaning

everything in my ultrasonic cleaner then passing a cleaner thru every jet hole blowing them out

and re assembling the carbs

mal adjusted throttle cables or cables that are binding could though so check your cables are free routed correctly and

returning to the stop position and not holding the carbs open at all

look at the idle stop make sure the linkage is resting against it in the throttle closed position

i personally suspect its the carbs as a bad coil would not make it idle really high

but doing a coil swap around as a test wont take to long

I put in fresh plugs within the last couple thousand miles.

I haven't swapped the coils yet, but it is on the list of possibles. Wanted to see if that was even a possibility first though. Seems like it would idle low rather than high if that were the case though.

I cleaned the carbs back when I built the bike and re-jetted them, but I know the slow jets were not removed. The bike was running when I bought it, and when I pulled the carbs they were already quite clean inside so I didn't do a complete piece by piece teardown. BTW, I believe the ones in these carbs thread in.

The cables are absolutely not the problem. I loosened them up and can visually see the slack in them when I operate the throttle. They are brand new and have absolutely no drag.

I have a hard time seeing where the actual throttle stop is, but I know right now when the throttle is closed it's not resting on the adjuster knob thingie. Yes, I said thingie. It's a technical term.

I'll ask again, could it be possible that it needs bigger slow jets? And does anyone know what size comes stock? Pulling those out and cleaning them is next on the to-do list.

Re: Re: Ok, it's time I asked for help

StinkBug said:

I cleaned the carbs back when I built the bike and re-jetted them, but I know the slow jets were not removed. The bike was running when I bought it, and when I pulled the carbs they were already quite clean inside so I didn't do a complete piece by piece teardown.

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So clean your carbs, for real this time. You'll have no idea what the problem really is until you eliminate that. What have you done that makes you think you would need larger slow jets?

what year is your cb750 when i get to the shop in the morning i can look up the stock jet sizes for you

but if 2 cylinders are running right and 2 not something is going on other than jets

unless you reversed the 2 jets in 2 of the carbs

the smaller number screw right into the carb and the larger number into the brass holder not to hard to mix up

Since people seem to be having trouble finding this in the first post...

StinkBug said:

It's a 1980 CB750F Supersport, DOHC. Recently got new rings and shimmed the valves. Internals are otherwise original. Pod filters, and straight through mufflers on the stock 4 into 2 exhaust. The left and right exhaust pairs are not linked together.

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Re: Re: Ok, it's time I asked for help

Rich Ard said:

So clean your carbs, for real this time. You'll have no idea what the problem really is until you eliminate that. What have you done that makes you think you would need larger slow jets?

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It seems to be running very lean at idle, so I'm asking a question. If I knew the answer I wouldn't be asking. Figured the people on this site seem to be pretty good with this stuff and could simply give me a "yeah you might wanna try that" or a "no that's probably not your problem" or a "sounds like this is the issue, here's what to do". Seems that asking for advice from people who know more than I do is an exercise in frustration more than anything though since a few of you seem to want to talk down to me rather than share your wealth of knowledge.

I know carb tuning comes up a lot here, which is exactly why I'm asking here. If it pains you too much to just help someone out maybe you should save the arrogant responses and just move on instead.