SO FRUSTRATED - Run away idle

1983 KZ750 H3, stock.

I've been through a whole bunch of drama with this turd. Bad wiring, tight valves, and then a sheered-off timing advancer pin, and a destroyed advancer.

I've now fixed everything that I've discovered wrong. The valves are adjusted, the advancer is new, the pin has been replaced, the cams re-timed and correct, the carbs cleaned for the 4th or 5th time, spark re-checked on every cylinder, checked float levels using clear tube, compression 120psi or better, checked boots carefully. In addition, I replaced a questionable main jet in the #1 carb.

The bike STILL isn't really running on all four cylinders. It seems to be running hot on 1 and 3, cool on 2, and not at all on 4 (still.)

The good news is it no longer back fires out the carbs, and no longer requires riding the starter to start. It fires up pretty much right away.

The bad news is it will not rev up (it used to) except when it is sitting there idling and all the sudden it runs away to redline and has to be shut down.

I'm about to part the bike out. Not because I don't think it can be fixed, but just out of sheer frustration!!!

I took the bowl off of the #1 carb just incase I didn't tighten the new jet down, and if any of the plugs were going to leak, it would be #1. I found no problems.

So I started it up and started spraying water and looking carefully, trying to find a air leak.

I was unable to identify any air leaks, but, it was smoking a bit like it was sucking the water in, and, oddly, after letting it run for a few minutes, all four pipes were hot. So the water must have plugged the leak enough for it to fire on all four?

I'm thinking it is the engine-side boots; one does look very questionable (happens to be the non running cylinder, #4)

Any other thoughts? I will probably clean, polish and check all the slides one last time before I order anything.

Do you have the stock ignition and coils? The sparkplug caps may be failing along with the coils when they get hot.

Have you thought about doing the "Wired George" relay modification to the ignition system? This bypasses the aged wirng in these older bikes. His website is www.wgcarbs.com it covers the parts and how to wire in the relay.

More than a few guys here have done this modification to their bikes.

WBPDX, troubleshooting old bikes is like pealing an onion. You shed a few tears with each new layer.

Sounds as if you have found the next layer. I changed my carb mounts and it solved one of my symptoms. Not the cheapest fix but what can you do.

You will not know how many layers you have until they are all solved.

Best of success.

If the carb holders haven't been replaced in recent memory I would start there. Reason is, even if that doesn't completely solve the problem it eliminates one more variable and it will need to be done eventually anyway. I replaced mine a couple years ago and it really improved the idle. Are you running the stock exhaust and airbox? Ed

I would check the intake manifolds(carb rubbers)Mine were in good shape but the bolts that hold them onto the block were loose.I would check them and you could probably put a little hi-temp sealer where they plug onto the motor.I was gettin' pissed at my back fire and a cool #3 pipe too.Take a break and come back with a cool head.You've already done the hard stuff-you're almost there.

Yes, stock airbox and exhaust.

I tried putting some gasket maker on the cracks in the carb holders, it didn't really make any difference.

I did discover two slightly sticky slides. I cleaned them and put a drop of 2 stroke oil on them...

I think what that did was let them not stick DOWN, as the bike now pretty much fires up and idles at 5-8k, if reved, it will go higher and stay there. Seems like a major vac leak, but where it is coming from is completely NOT obvious.

I think I am going to get new carb holders, and also replace the float needles for the heck of it. Maybe the little plugs that hide the idle-meter jets as well. If there is no improvement, I am going to part it out or sell it as a parts bike.

Also, I would consider rejetting for pods, just because it is SUCH a chore to get the carbs off! But I'd like to get a baseline where it actually runs first!

When you check for vacum leaks use carb cleaner or something flamable and spray suspected areas, if leaking the idle will rise. I dont think water is what you should use, but I have never tried it.

Kawickrice wrote:

When you check for vacum leaks use carb cleaner or something flamable and spray suspected areas, if leaking the idle will rise. I dont think water is what you should use, but I have never tried it.

Would concur about not using water spray.

Could try WD40 or propane gas sprayed about the carb holders and vacuum caps to help determine whether outside air is being sucked into the fuel mixture. Listen for changes in rpm and look for extra smoke from the exhaust, as indications that fuel mixture going into the combustion chamber is being affected by air drawn in through cracks or leaks in the carb holders or vacuum caps.

Good Luck!

Hold up a minute.If you have cracked carb boots and you know your slides aren't moving this is where your problems are.Is this a 650 or 750?Your post says '83 750 H3.I have a '83 650 H3. Typo? Anyway if it was me I'd Get some black electrical tape and give those carb rubbers a few wraps to possibly stop air leakage and get the idle down.Don't throw anymore money at it till we figure out it's problems.Airbox with filter in place?If you have an '83 you have CV carbs,right? It is possible that you may have diaphrams with holes or tears.Fixable so it's not the end of the world.Have you done a compression test? Stay cool and keep posting.If not and you are serious about parting it out post your location. these bikes piss everyone off at times but it takes patience and you've got alot of people here that can help.Stay with it.

Sorry I went and read you're first posts.You've got good compression the bike runs but the idle is way out,right? Air leaks or maladjusted linkage/cables.Try slowing down the air flow you've got the stock airbox and filter so take a rag and stuff it into the air filter opening under the seat.This is how I start my 650 when it's cold-it blocks alot of the air and richens up the mixture.It's worth a shot right? Pull the plugs and check the color.Super white means super lean.Keep us posted.

Here's another tip or something you can try.Find the carb synch screws. 3 of them right? The one that the idle screw pushes against is your base carb.Loosen the lock nut and turn the screw (clockwise?) one turn.This will help getting the linkage at a lower idle point.If you have to pull your carbs take the airbox rubbers out and soak them in brake fluid.Helps to revitalize them a bit and make them easier to install.You have tight clamps on the manifolds?