Red Hot Pipes and Carbs - Push Pull Cables

77 KZ750 B twin:

Had the carbs rebuilt. Reputable person/friend.

I put the carbs on, started the bike and  it RACED like CRAZY, up to 6k rpms. So I turned the idle down and nothing. Then the pipes got RED HOT so i had to turn it off. I dont think this is related to the push pull mechanism (could it be set wrong?) but something isnt right. Its my first time putting carbs on and Im pretty sure I did everything right. Could I have misadjusted the push/pull cables or do you think something in the carbs isnt right.

You more than likely have air leaks at the carburetor boots. Replacing the boots should fix the problem.
 

One of the boots, on the airbox side, had a big tear. I was told I could use crazy glue to repair it and did that. Maybe thats it?  Ive been reading a lot and have seen that throttle cable linkage could also be an issue, but when I roll the throttle at the handlebar it seems fine. When its idling high and i roll the throttle, it RPMS do go up higher, but only go back down to 5k. ive never seen my pipes get red hot so fast, but it happened today.

wrote:

One of the boots, on the airbox side, had a big tear. I was told I could use crazy glue to repair it and did that. Maybe thats it?  Ive been reading a lot and have seen that throttle cable linkage could also be an issue, but when I roll the throttle at the handlebar it seems fine. When its idling high and i roll the throttle, it RPMS do go up higher, but only go back down to 5k. ive never seen my pipes get red hot so fast, but it happened today.

Shoot the person who told you to crazy glue the ripped boots.  They go buy a new set.  Get the boots that go on the head too.

It was wookie who told me to superglue it. But since youre pretty much the go to guy here, couldnt this be a too-tight-throttle cable issue?

wrote:

That was wookie who told me to superglue it. But since youre pretty much the go to guy here, couldnt this be a too-tight-throttle cable issue?

There is one way to find out...loosen it and see what happens.  

Red pipes says to me there is a wickedly lean fuel mixture. A racing idle can be caused by major air leaks between the carbs and the engine, which makes for a ver lean mixture.
I would look first at the carb holders. Is the rubber hard and cracked? Are the carburetors fully seated in them? Is the vacuum port on the carb that is not connected to the fuel petcock properly capped? Is the vacuum hose going to the petcock hard and cracked?
Next I would check how the carbs are set up. Was the idle screw set near the middle of its range before doing the preliminary synchronization on the workbench before installing the carbs? Have you confirmed the correct fuel level in both carbs using the clear tube method.

wrote:

wrote:

One of the boots, on the airbox side, had a big tear. I was told I could use crazy glue to repair it and did that. Maybe thats it?  Ive been reading a lot and have seen that throttle cable linkage could also be an issue, but when I roll the throttle at the handlebar it seems fine. When its idling high and i roll the throttle, it RPMS do go up higher, but only go back down to 5k. ive never seen my pipes get red hot so fast, but it happened today.

Shoot the person who told you to crazy glue the ripped boots.  They go buy a new set.  Get the boots that go on the head too.

Nessism, glueing the split carb to air box boot is not causing this problem, the air leak would have to be on the head side of the carb. And to be fair the option he had was crazy expensive ($125 for one rubber) so was worth a go as "superglue" works well on rubber

wrote:

Red pipes says to me there is a wickedly lean fuel mixture. A racing idle can be caused by major air leaks between the carbs and the engine, which makes for a ver lean mixture.
I would look first at the carb holders. Is the rubber hard and cracked? Are the carburetors fully seated in them? Is the vacuum port on the carb that is not connected to the fuel petcock properly capped? Is the vacuum hose going to the petcock hard and cracked?
Next I would check how the carbs are set up. Was the idle screw set near the middle of its range before doing the preliminary synchronization on the workbench before installing the carbs? Have you confirmed the correct fuel level in both carbs using the clear tube method.

 

The rubber going from the airbox to one of the carbs was torn so I superglued it as per Wookie. He wasnt the only one who told me to do this. An oldtimer in the neighborhood who just went off for the holiday told me superglue works well on THAT side of the carbs.

Im pretty sure the carbs are fully inside the boots and the boots on the engine side are indeed soft, with no cracks.

Would the throttle cable being too tight mimic high idle/lean symptoms? Its the only thing I can think of that I may have done wrong because I had to fiddle with the throttle cables a LOT to get them in the push/pull thingy on the carbs.

I started the bike, it started RACING like crazy, and in about 1 minute the pipes got really red. So i turned it off. Oldtimer from the neighborhood said maybe the throttle cable is too tight and holding the Butterly (?) open and causing this?

I have to take another look at the vacuum port on the brake side carb to see if its capped off. Prior to the rebuild this NEVER happened!

When it doubt, check the factory service manual...

www.kzrider.com/modules/ServiceManuals/K...l%20KZ750%20Twin.pdf

Page 13 shows how to adjust the throttle cables.

BTW, air leaks on the airbox side carb boots will mess up the idle, but not cause out of control engine racing.  I personally would never superglue a carb boot, but that's just me, because it won't hold up long term.

I have 2 into 1 exhaust. The person who helped rebuild my carbs noticed the needle was notched at 4, which was fine for years. He said he notched it back to spec, and they are now clipped to notch # 3 and mixture screws are 1.5 turns out. What surprised me is, when the engine was racing, turning down the idle did NOTHING!

Nessim thanks for the manual. I printed it out and will adjust the cable tomorrow. I hope and pray thats what the high idle is all about.

wrote:

When it doubt, check the factory service manual...

www.kzrider.com/modules/ServiceManuals/K...l%20KZ750%20Twin.pdf

Page 13 shows how to adjust the throttle cables.

BTW, air leaks on the airbox side carb boots will mess up the idle, but not cause out of control engine racing.  I personally would never superglue a carb boot, but that's just me, because it won't hold up long term.

I don't disagree that it's not an ideal solution for a "forever repair" but if you make a decent job of it and are on a tight budget it will get you out of a hole.
Shanedude just out of interest did the repair hold when it was revving at 6000rpm ?