More Idle Pain, Please Help!

Hi all, I've posted on this before but I've come a little further and I'm still lost. Hoping someone out there has any idea.

1.) Bike ('76 KZ750B, dual vacuum carbs) won't start with the choke on. I've tried different combos of mixture screws (all the way in, up to 2 full turns out), different idle screw settings, different levels of having the choke on. It won't start with it, and it dies if I put it on when it's running.

2.) After warming up, (without touching it at all) the bike's idle will rev way up.

Sealed the boots to eliminate air leaks. Checked for holes in the diaphragm. The carbs had new jets put in them and were cleaned very well 2 years ago with hardly any riding since. The bike wasn't exhibiting this behavior before.

One person told me this:

"i have encounted this same issue several times. it's a rather tedious repair but it can be done. it involves rebuilding the carbs, floats, jets . what's happened is 99% sure the bike sat for an extended time with the gas in it and now it turned to a urithian type mixture and is keeping the bike from runnig as it should."

I don't know what a urithian mixture is, nor can I find any evidence of anything like it. Does anyone have anything else I can try short of replacing the carbs altogether? The rest of the bike is just fine.

If you are sure the carbs are clean, then it sounds like a vac leak. They can be the hardest things in the world to find.

The carburetors need to be cleaned and rebuilt. The idle and enrichener circuits are the issue for the starting problem. The idle jumping up is an air leak that could be cause by worn pads on the choke plungers, orings bad, etc. OR your carb holders. I suspect you didn't use new carb holders which don't require sealing as they are still soft and pliable.

Ah, he has clarified that he mean urethane (glue consistency) and that it could have built up in the carbs. Hm, does this idea have any credit? Think I'll just do a total clean-out of them to be sure.

Also, the Z1 site shows these under my bike's category:

www.z1enterprises.com/detail.aspx?ID=444

... but I believe I don't have any vacuum ports because I don't have a vacuum petcock. Is that correct? I don't see anything like this on my carbs.

Also asking Z1 about the rubber carb holders for intake, which yes, I need to replace, but Z1 doesn't seem to carry them.

Have you tried adjusting the master idle screw?

I met a guy with a KZ650 the other day and when he started his bike up the revs went right to 4500 and stayed there with the choke on.

I asked him if this was "normal" and he said yes.

Then I told him to back off the master idle screw and the bike clamed down to 1100 - 1500 rpm's or so. He was amazed that this solved the problem of the smoke puffing of dark smoke coming out of his exhaust after start up.

Also, check to make sure there are no holes in you exhaust, the other day i found a huge gaping hole on the left side pipe and patched it, after which my bike ran a lot smoother at idle and increased power throughout the RPM range.

I heard that another trick that you can try is: spraying WD40 around the carb holder, and air box boots to see if the engine revs are affected. This trick ma help you isolate where the vaccume leak is.

Just a couple of ideas you can try to see if it will affect the bikes performance.

Run 87 octanne gas as well as your bike like to drink the light stuff and not the 89 or 91 or whatever as mentioned =on other posts listed here.

Oh yeah, make sure that the battery is charged as well. My bike runs way better after I put it on the trickle charger overnight, much stronger at idle.

Hey KOOL,

Yeah, I've played around with the idle screw. The engine will rev up high if it's above a certain point. Maybe once I figure out the choke thing, I'll be able to start the bike ok with the idle screw down lower instead of having to keep it up a little higher. It just doesn't seem to behave right. If I tweak the idle screw up past a certain point, the idle takes off toward the 4000-5000 range, not proportional to how much I'm turning it, which I assume is how it should work.

I think my exhaust is pretty solid. I'm going to replace the copper gaskets again for good measure. I had a hole once too and you're right. Amazing amount of power gain.

Gonna buy a trickle charger for sure, but the battery's only a month old and was trickle charged (appropriately) right when I installed it. Had the problem before and after the new battery.

Interesting about the 87 octane. Guess I'll stick with that. I didn't know these bikes preferred that.

And WiredGeorge, in the choke plunger, are the O-rings what is pictured in the attached picture? The plungers seem to be in pretty good condition.

I've already pulled the carbs and they'll be cleaned thoroughly. All the jets are relatively new so I'll just hit them with carb cleaner and air.

Pretty sure you should have these on the connectors of the carb to engine. They point down and are where you sync the carbs using a manometer. Make sure the caps are good and on there you do not want an air leak there.

So what I've got are 2 connectors of the carbs to the engine (dual carb). There is a stem pointing down under each of them, but on my bike, there is just a tube running from one to the other. So I guess I don't need those caps on my particular model.

What I've just learned from Rob at Z1 is that the rubber engine connectors for the carbs on my bike are discontinued. :dry: Guess I'll have to start searching around. Anyone know where I can find those?

So tell me if this makes sense. I rebuilt my carbs, which seems to allow me to start the bike with the choke on. I had to keep bumping the idle screw up while I kick-started it to get it to start.

When I finally got it to start with the choke on, it ran ok for a while and I let it warm up. Then as I eased the choke down, the speed of the engine shot way up, similar to what it was doing before.

Shouldn't the idle speed go down as I close the choke?

*** utter frustration with this unrideable bike ***

You say that the vacuum pipes on both carb to engine mounts are connected to each other by one hose? :huh:

That just doesn’t sound right to me. Try disconnecting the hose and plugging both vacuum pipes. For temporary vacuum plugs just use two short hoses with a bolt stuck into the open end. If I'm guessing right, this should solve the out of control engine revs.

Best of success.

PS
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Hrm, unfortunately as the Kawasaki schematic shows below, there is a "Balancer Tube" in place there (#25).

It doesn't specifically show them connected, but it's correct based on the use of the word 'balancer'.

Just in case, if not already doing ---

When first cranking cold engine with choke fully on, do not touch the throttle, meaning leave the throttle fully closed while attempting to start a cold engine with choke fully on.

This is because the choke is actually a separate enrichener circuit inside the carb which is designed to function with the throttle slides in the full down position. Cracking open the throttle while attempting to crank a cold engine prevents the choke from properly functioning.


Another just in case, if not already done -- would assure that the advancer unit is not stuck in the advance position. Best to check and verify correct ignition timing by using a strobe type timing light, and watch the advancer unit move back and forth as the rpm's are varied between 1000 and 4000 rpm.

An advancer stuck in the forward position may cause or aggravate the difficulty in cold starting.


Good Luck!