KZ750 Idling at 7000RPM

Hi, I recently bought a 1980 KZ750H LTD and have been doing some work on it. I rebuilt the carbs (new gaskets, cleaned jets, etc.) and put them back in the bike. I fired it up, but its idling at 7000RPM. I think its the throttle being stuck open, however the slide seems to work fine, and the throttle play is adjusted properly. Any ideas on what it could be? Thanks

That's not idling! Try disconnecting the throttle cables at the carbs and turning the throttle stop screw counter clockwise to see if it will idle at a normal rate. If it does you need to find the problem with the cables or throttle grip. If it still runs at a high rpm you'll need to dig into the carbs. Ed

650ed wrote: That's not idling! Try disconnecting the throttle cables at the carbs and turning the throttle stop screw counter clockwise to see if it will idle at a normal rate. If it does you need to find the problem with the cables or throttle grip. If it still runs at a high rpm you'll need to dig into the carbs. Ed

Air leak around carb ? I put carbs back on once and forgot to tighten the hose clamps on the carb to engine rubbers and idle went to the sky.

I took it apart today and realized when I bench synched the carbs, I opened the idle screw far enough to stick a needle in as a gauge and never closed it. So the butterfly valves were way too far open. Fixed it and it idles right around 1200.

It still leaks a little fuel out of the third carb. I'm out of ideas on what it can be. The float heights are all identical, I just replaced all the float needles, replaced the gaskets, and gave everything a full cleaning. Any ideas on what could be causing the float to stick or leak from the overflow tube? Thanks

astark wrote: The float heights are all identical, I just replaced all the float needles, replaced the gaskets, and gave everything a full cleaning. Any ideas on what could be causing the float to stick or leak from the overflow tube? Thanks

...Did you replace the seats WITH the needles? If not, that could be your problem.

astark wrote: I took it apart today and realized when I bench synched the carbs, I opened the idle screw far enough to stick a needle in as a gauge and never closed it. So the butterfly valves were way too far open. Fixed it and it idles right around 1200.

It still leaks a little fuel out of the third carb. I'm out of ideas on what it can be. The float heights are all identical, I just replaced all the float needles, replaced the gaskets, and gave everything a full cleaning. Any ideas on what could be causing the float to stick or leak from the overflow tube? Thanks

Fuel coming out of the overflow hose (the little hose on the bottom of the carb) is the common symptom of the 3 possible problems described below. Remember – NO SMOKING or other activities that could ignite the fuel while working on carbs!

The brass overflow tube inside the carb bowl is damaged. When you remove the carb bowl you will see a brass tube attached inside and rising toward the top of the bowl. This is the overflow tube and it leads directly to the overflow nipple on the bottom of the carb. Normally, the fuel level within the bowl rises somewhat close to the top of this tube. If the tube becomes cracked or separates from the bottom of the carb bowl fuel will flow through it and out through the overflow hose. Checking the condition of the tube is very easy after you have removed the carb bowl from the carb. Simply hold the bowl level and fill it up near the top of the tube with water of alcohol and see if it leaks. If it does the best remedy is to replace it although some folks have found creative ways to repair them.

It sounds like you have already checked the next two...

The float valve is not sealing. This may be (and probably is) just a matter of some minor dirt in the float valve and is very simple to remedy and can be done with the carbs still on the bike. Turn off fuel; drain carb by loosening big brass screw near bottom; and remove the 4 little screws on the bottom of the carb bowl. This will enable you to remove the carb bowl. You will then see the float. If you carefully remove the float you will see a stubby little needle that mates with a brass orifice; together, these two pieces are the float valve. Assuming there is no obvious damage to the needle or seat, use a Q-tip and some carb cleaner to clean the seat of the orifice and the needle. Be careful not to bend the tang that is attached to the float. This tang is the piece that the bottom of the float valve needle rests upon, and it determines the fuel level in the bowl, so bending it will change the fuel level. With the float valve cleaned it should no longer leak. To prevent a reoccurrence a quality inline fuel filter should be installed between the fuel tank and carbs. If there was damage the needle and seat should be replaced as a set; they are available.

The fuel level is set too high. This condition occurs when someone has bent the float tang as mentioned above. This condition can be checked using what is referred to as the “clear tube test.” The test involves attaching one end of a clear piece of flexible tubing to the carb drain hole and holding the other end of the tube above the bowl/carb joint. When the fuel is turned on it should rise in the tube to a level 2.5 – 4.5 mm below the bowl/carb joint. If the fuel rises higher than that level it may run into the cylinders or over the upper end of the brass overflow inside the carb bowl (mentioned above) in which case it will run out the overflow hose. This condition is corrected by removing the float and gently bending the tang to raise the float valve needle position. Doing this may take several tries before achieving the desired fuel level.

Correcting each of these three conditions involves removing the carb bowl. This task can be made easier if a short screwdriver bit is used, and depending on which carb you are fixing a mirror may help you locate the screws on the bottom of the carb bowl. Also, it is not a bad idea to have a new carb bowl gasket available in case the old gasket is damaged while removing the carb bowl. Ed

KraZyRIDER wrote:

astark wrote: The float heights are all identical, I just replaced all the float needles, replaced the gaskets, and gave everything a full cleaning. Any ideas on what could be causing the float to stick or leak from the overflow tube? Thanks

...Did you replace the seats WITH the needles? If not, that could be your problem.

If your carbs are the same as mine - Keihin CV34 - then the seats are not replaceable but they can be cleaned better with some liquid Brasso on a Q tip followed by carb cleaner. I've had to use the Brasso.

I've also had cracked brass overflow tubes like Ed described

Thanks for the advice, I've been working with them this afternoon, and still can't get it to stop leaking. They are the CV34s. I've tried swapping the overflow valve screws with the non-leaking carbs, and it still leaks. I've checked that the needle is seated properly and seals, and it does. I'll test for the cracked overflow tube tomorrow, if it is can I just JB Weld it? I guess I'll do a little more research on it and get back to yall tomorrow.

JB Weld will work . Just let it cure for 24 hours. I have used it. The other approach is to get some slightly oversize brass tubing at a hobby/model shop and slip it over the existing tube and solder at the bottom.

A slight crack in the brass overflow tube can be difficult to find. You can connect a rubber tube to the overflow nipple, fill the bowl with water, put finger over the open end of the brass overflow tube and blow in the rubber end and look for bubbles.

Good luck

JR wrote: JB Weld will work . Just let it cure for 24 hours. I have used it. The other approach is to get some slightly oversize brass tubing at a hobby/model shop and slip it over the existing tube and solder at the bottom.

A slight crack in the brass overflow tube can be difficult to find. You can connect a rubber tube to the overflow nipple, fill the bowl with water, put finger over the open end of the brass overflow tube and blow in the rubber end and look for bubbles.

Good luck

That is Brilliant! I'm going to plagiarize that advice into my overflow page. Ed

Thanks guys, I found the crack! it was actually pretty sizable, running down the whole brass tube. I covered the whole tube with heat shrink, and JB Welded it at the base (the fast curing kind) and it works great, no leaking at all!