81 Kz750LT (n2) runs very erratic on cold start.

So, My 750 is running horrible when it's cold. I start it on full choke and wait for the RPM's to climb (shouldn't it be the other way around) before I drop the choke about 1/4 and repeat until I can turn it all the way off. than I have to wait another 5 minutes before I should ride. once its warm it runs at about 1300RPMS. and anything below that is acts like its going to die.

If I try to ride to soon it sputters and backfires and runs like a dying horse. Once it warms up though it runs great without choke except for a flux in RPM's at idle.

anyone have some suggestions?

Check the carb holders for leaks. Ed

After correcting any Vacuum leaks, and still having issues. The carbs are more than likely dirty in the Pilot circuit, not plugged but somewhat restricted. Might just be in the Mixture Screw area. If it still has caps over the screws, they will need removing. Then you can pull them out, and using some compressed air, blow out the circuit. Then re-install the screws and adjust them, your manual will have the starting point for turns out from lightly seated. Or before removing you can turn them in, and then count how many turns they were set at. Otherwise a full removal/ cleaning would be required.

I would do what 650ed and Motor Head said.

I'm not totally sur of the differences/similarities between your 750 and my 1980 750/4 E1.
I start on full choke and after maybe 20 seconds warm up the bike will seem to stumble. at this point I reduce the choke a tad and it smooths and sound better. Put on helmet and zip up jacket and reduce choke a bit more. I have been told it's wrong but I dont wait for the bike to totally warm, I just go with the choke at about a 1/4. At this point the last part of choke travel is just a high idle and the actual choke butterflies are fully horizontal. At the first stop light I'll usually turnt he choke off altogether. When fully warm I have the idle set at maybe 1200 rpm. I like it like that and I think the manual says to have ith in this area anyway. The first part of your post sounds perfectly normal. Your second paragraph is definitely not normal. A good examination of the carbs or a carb clean or synch may help.
One last thought - do you have that anti smog tubing from head to airbox that was fitted to US bikes?

JR wrote: One last thought - do you have that anti smog tubing from head to airbox that was fitted to US bikes?

Not sure what this is. I have a tube for the crankcase vent and another tube that runs up to the frame under the gas tank. I assumed it was part of the vacuume system. should I remove that?
JR I can tell you the 1980 and the 1981 engines are actually different. the 1981 kz750N engine is redesigned based off of the 650. I can't remember what exactly the differences were though.

Ok, I took compressed air and blew out all of the circuits when I got the bike. I will try removing the idle screws and blowing them out from there.

UPDATE: I am deffinately thinking it is running lean as today on my way to work the bike lost power on acceleration intermitantly for a few minutes. It now idles irratically as well. I am going to hope I make it home with out any issues. no backfires though.




If I find a cracked boot are there any suggestions on temp fixes until I get a new one in?

also, when I got the bike the petcock vacuume feeder was sticking and it was flooding the carbs until it was used a bit but It seems to work fine now. could it be sticking and not feeding enough fuel into the carbs? only getting enough fuel ro run after a little sit time.

We never had that ani smog tubing in Canada but I have seen pics of it. It is basically an exhaust recycling system with a whole mess of tubing connecting right and left sides of the valve covers with the carbs and the airbox. I have read here that it can cause running problems and air leaks and many people have run a single tube from one side of the valve cover to the other. got rid of the rest, plugged the opening in the airbox and capped the carbs where the tubing connected. Some people have used bloick off plates on the valve covers.
Patton has posted a nice schematic. See if you can find it

edit> found an example. Click here kzrider.com/forum/3-carburetor/531216-79...ve-carby-info#531826

and scroll down

Ah yes, the dreaded clean air injection system. :lol:








Good Fortune!

Wow that's interesting. So I assume this system is just like the smog systems on cars. It recycles some of the air out of the exhaust to reburn.

Anyway. I think my problem may indeed be electrical. whats the best way to check for intermittent spark? I think the bike is intermittently misfiring. I am gong to go get some spark plugs later.

ironslave wrote: UPDATE: I am deffinately thinking it is running lean as today on my way to work the bike lost power on acceleration intermitantly for a few minutes. It now idles irratically as well.

Sounds like what happens when rusty junk from the gas tank gets into a main jet.

Has the tank and fuel system been cleaned?

Inline filters installed?

ironslave wrote:
Anyway. I think my problem may indeed be electrical. whats the best way to check for intermittent spark? I think the bike is intermittently misfiring. I am gong to go get some spark plugs later.

Original coils? Old coils will often misfire after warm up. Sometimes they just misfire all the time.

Coils, thats what I was thinking too. It has a stock coil and an aftermarket coil. the aftermarket coil is much stronger.

no inline filter, tank is dirty but the petcock filter looks great.. I will flush the tank tomorrow.

I thought about how this all works out, I decided to simulate riding and rock the bike on its center stand while it was running. It decided it that idle should raise almost 1000RPM's if I rocked it for a few seconds. I would snap the throttle and it would drop 4-500 rpm and then after I turn it off and start it idle returned to normal.

The idle is stable after it warms up. it ran for like 30 minutes just sitting there at idle with no problems.

ironslave wrote: Coils, thats what I was thinking too. It has a stock coil and an aftermarket coil. the aftermarket coil is much stronger.

no inline filter, tank is dirty but the petcock filter looks great.. I will flush the tank tomorrow.

I thought about how this all works out, I decided to simulate riding and rock the bike on its center stand while it was running. It decided it that idle should raise almost 1000RPM's if I rocked it for a few seconds. I would snap the throttle and it would drop 4-500 rpm and then after I turn it off and start it idle returned to normal.

The idle is stable after it warms up. it ran for like 30 minutes just sitting there at idle with no problems.

I hope you had a fan blowing on it. These bikes have air cooled engines and need air flowing over the cooling fins or they will over heat. Letting a KZ sit and idle for 30 minutes is not a good thing to do. Ed