80 KZ750 LTD racing idle

I'm new to both motorcycles and carbed engines but I've read about this a bit.

I've got an issue with my bike where it will start with no choke and idle for a few minutes. After that, the idle will increase to gradually to 4000. I can get it to return to 1100-1200 by putting the choke on and then taking it off. The idle will increase shortly after the choke is taken off until it gets back up to 4000. The bike is mostly stock, but has pod filters installed in place of the stock airbox.

edit: I went out and inspected the petcock vacuum line. The line is in good condition. The material is pliable w/ no cracks or other damage to it. The petcock pulls vacuum off of two carbs. 1 and 3. #2 has a line on it but its clamped off with a bolt in the end. I guess the PO did this when he installed pod filters.

I've read about this sort of issue on the forum and tried most of the diagnostic steps.

1-The throttle snaps back and the racing idle doesn't go away no matter the position of the handlebars. I don't think the issue is cable binding.
2- Vacuum leak at the carb boots. I've sprayed WD-40 around the boots and detected no drop in RPM.
3-Advancer. I disassembled the advancer and checked that the weighted arms move freely. They do. I lubed the arms and reinstalled. Started bike and issue remained.
4-I recently replaced the petcock diaphragm because it was weeping fuel from the side when on prime. The oil also smelled like fuel when I changed it. I attached the vacuum and fuel lines to the correct spot as near as I can tell. Big line to big fitting and small line to small fitting. When i was checking the carb boots, I also sprayed WD40 on the vacuum line fitting but no change in RPM occurred. The clamp looks like its just bent heavy gauge wire so that could be the issue. Could it also be a torn diaphragm or a bad connection?
5-I have not cleaned the carbs but the PO used it as a daily ride until he upgraded to a big yamaha(v max something or other). In another thread, someone mentioned that racing idle is related to a dirty pilot circuit. Because there was oil in the fuel, I also suspect that the float needle was not seating itself properly. Could that be the cause of my issue? I tried the 'put the bike on prime and tap on the float bowl housing' thing a few times and that hopefully flushed the debris out.

I'm hoping that its 4 or related to it. If the lines were hooked up backwards, the bike wouldn't start, correct? The vacuum line only comes off of one carb if I understand what I've read. But, a bad connection, either at the carb or the petcock OR a torn diaphragm would lead to a racing idle condition if I understand what I've read.

for what its worth: there is only about a gallon of gas in the tank.

Sounds like a vacuum leak along with improperly jetted carbs. Spraying WD-40 on the carbs/boots often doesn't find vacuum leaks.

Sounds like 2 problems to me.

1. Vacuum leak. See below for testing the right way.
2. Hoses are hosed up. It makes no sense that there would be hoses from #1 and #3 carbs to the petcock. If you can post a picture showing the carbs and related hoses someone will be able to help.

CARB HOLDER LEAK TEST
You should check the carb holders for leaks. They will eventually develop tiny cracks in the mating surfaces between the holders and the cylinder head that can cause them to leak at idle. These cracks cannot be seen with the carb holders in place. I suggest you try the following. Get the engine warmed up and temporarily set the idle as low as possible without killing the engine (lower than 900 rpm if possible). The lower the better because with the idle very low the slides are nearly shut and the vacuum inside the carb holders is at its highest. Then spray carb cleaner around the carb holders, especially where they mate with the cylinder head and where the carbs attach to them. Carb cleaner works best for this test as it doesn't leave an oily mess with WD40 or evaporate so quickly that it is not drawn into the leaks as can happen with propane or starting fluid (ether). If the engine dies or reacts to the carb cleaner, you need to replace the carb holders. Carb holder leak related problems are noticeable at idle but generally don't have much impact when riding because the throttle is open. If you find you need to replace the carb holders soak the screws with Kroil for a couple days before attempting to remove them, and use JIS standard, NOT Phillips, screwdriver bits, and this will greatly improve the odds that you will not break or strip a carb holder fastener as so many have done. Let me know if you need info on JIS bits, I have a good link for making your own at no cost. Ed

from the petcock: one line tees off into 2 and connects to the vacuum nipple on carb 1 and 3. Carb 2(I think the stock carb for pulling vacuum for the petcock) has a line that is capped by a bolt. This leaks when I blow into it. Carb 4 has a vinyl cap.

I bought new vacuum lines, clamps, and vinyl caps for the vacuum ports and will replace them. Should I return it to a stock configuration to the petcock?

Dumb question: the throttle cable isn't stock(at least compared to the clymer manual). It has two nuts on either side of the arm that holds the cable sheath to the carb rack. I can't get a wrench on the bottom nut to loosen it. Do I need to buy a wrench and cut it down or can I take the carbs off and then remove the cable?

What carbs does the bike have?

If they are Mikuni BS32 or BS34 carbs take a look this link to see the proper hose connection photo and description:
www.wgcarbs.com/index.php/using-joomla/e...gories/86-bs-connect

If they are Keihin CV34 carbs take a look this link to see the proper hose connection photo and description:
www.wgcarbs.com/index.php/using-joomla/e...gories/91-cv-connect

Ed

Keihin

I was wrong, The 1 and 4 carbs are attached to a tee and then they route to the petcock. The #2 carb has a line with a bolt clamped in the end. The #3 carb has a cap on it. The picture mentions a vacuum switch that 1 and 4 should route to but I'm going to guess that it has long since been removed.

So...to do this correctly I need to:
1-Put caps on 1, 3 and 4.
2-Run vacuum hose from the #2 carb to the petcock. Alternatively: replace the petcock with a non-vacuum operated type and cap off all vacuum ports.

Thanks for the help!

So half my problem went away.

I capped of 1, 3 and 4 and ran 2 to the petcock. New lines and new clamps. Racing idle went away. Now it just hangs a little bit. If I rev up above 3000, it'll hang around 2500. Maybe slowly drop below 2000 but never return to 1100 to 1200.

Turned the idle down to about 900 and hit the carb boots with cleaner. the boot around the 1 carb would kill the motor if I hit it with carb cleaner. So...that's where the second leak is.

I should just buy 4 new boots and clean the carbs out at this point then.

Yup

After I fixed the vacuum lines, I also had to get at the countershaft seal and replace that. While I was working, every few minutes the #1 carb would dump an ounce or two of fuel out of the overflow. This is a symptom of a float needle not seating itself, right?

It could be just a bit of dirt on the float valve; this usually can be cleaned with a Q-tip and carb cleaner. Don't use anything abrasive. It also could be a crack in the overflow tube attached to the carb bowl, or it could be that the floats aren't set properly. You can check for a cracked overflow tube by swapping carb bowls between carbs to see if the problem moves with the bowl. Float levels can be easily checked using the "clear tube" method. Ed

I took the carbs off the bike, disassembled them and this is what I observed:
-3 secondary jets and a pilot were all mangled from the previous owner. This might have made my idle a little rougher than it should have been.
-Polished the float needle seats. I guess I should probably drain the gas out of the tank and refill to avoid the leakage issue again.
-Fairly clean overall. LIttle varnish in the bowls. I poked around all the holes I could find with a piece of steel brush that I yanked out.
-After reassembling the top half of the carbs(waiting on new jets), I noticed that one of the slides doesn't seat fully. It hangs about 1mm higher than the rest of them. When I install it, I can push the slide fully closed. Once I get the cover on and push against the slide to open it, it doesn't return to a fully seated position. Both the body of the slide and its passageway in the carb body are smooth. Can I just shim the spring with a washer for the time being?

I got the carbs cleaned and reinstalled but now It won't idle, with or without choke. Where do I start?

I can get it to start if I apply some throttle but it'll die without throttle.

edit: I'm thinking that I probably didn't get the float heights set properly. Is that a remote possibility?