Hi guys! Need some advice here
My kz on start up starts pretty quick but I have noticed at times the idle is a bit all over the place, then I also noticed the header from cyl 4 isn’t getting hot at all when this happens. I checked compression and I’m on 130 on cyl1 and the rest on 120. I also have spark on all of them. I have figured that with some good revs or just putting the bike on choke again it will after a few seconds jump from let’s say 2500 to 4000 (just a figure) and by then the header 4 is heating up and when choked off the bike idles pretty good. Can someone with exp help me solve this? I think the main jet might be clogged but idk much abt bikes, just have done car mechanical stuff. Thanks
Sounds like your carbs need to be cleaned. Best way to do this is by unganging them and soaking the hard parts in carb dip then reassemble with new O-ring seals. I've done scads of those carbs and recommend staying away from aftermarket "carb kits" since your originals should be usable after cleaning. If after close inspection the float needles look bad then get some from Keyster. All the other aftermarket needles I've seen have weak springs and don't seal as well. OEM needles would be good too but they are crazy expensive.
Oh, and if you look in the archives you'll find several threads posted with the O-ring sizes needed.
If you've never done carb work before, it's really not that hard. The same thing is done 4 times on most stuff & most parts only go back one way, so it's kind of hard to mess up. use good fitting tools. I prefer to keep the little parts for each carb separated from the others so that they go back to the same carb from whence they came (whence, hee). I bought 4 plastic divided lunch containers to keep everything organized. some guys use egg cartons.
The problem is probably carb related & I know you said there was good spark, but while the fuel tank is off, I would redo the spark plug wire connections at all points >> unscrew the ribbed compression boots at the ignition coils & untwist the plug caps from the wires. At the coils the wires have o-rings & washers n stuff, so be sure to reinstall to ensure a watertight connection. If the plug wires have the gray/white oxidation, I would nip off a little of the wire. or go ahead & get new. I thought I had good spark, but doing these things solved an intermittent firing problem on #1 & #2. If you get new, you might be able to reuse the outer sheathing from the old wires (credit to Nessism). some new spark plug wires may not need the covering. I would use LW silicone spray lubricant to aid in the removal of the covering.
Before ripping the carbs apart there a few simple tests you can do to ensure your issue is carb related. These engines run on a wasted spark system so if no 1 is sparking no 4 should also be sparking at the same time but on a differnt stroke.
Therefore swop plugs one and four and see if the issue moves to no 1. If it does its the plug. If not then swop the ht leads for one and four and see if the issue moves with the ht lead. If it does its the ht lead or plug cap or ht lead connections.
Another thing to try is when its running on three cylinders just ease the ht cap slightly off the cylinder that is not running chances are it will start to run on that cylinder also. Never really got my head around why easing the plug cap off slightly gets a plug firing but it does work. Got to be something to do with resistance. Once its running on that cylinder push the plug cap back on.
If none of the above changes anything chances are its carb related but as you say once its running on all four its fine I suspect its not carbs. At least not carbs pilot or main jets. It could be the choke plunger o rings on number 4 is worn and not sealing thus not creating an airtight seal to suck the fuel up. Have you checked if the plug is wet with fuel when its not firing.?
I apreciare all this info from everyone, I will give those a try, and see if the issue will be traced that way. I know basic mechanics but when it come to carbs I don’t know much. I really appreciate all the advice given to try to solve this mystery. Btw, when I checked them 1 and 2 had a bit of oil, I guess the valve seats but I’ll check again after it runs for a while then open it up to do another compression test and check the plugs condition.
In case you need the manual. 750turbo.com >>> Downloads >>> Full manual (128mb PDF)
80-85 750-4 chain drive models. Yellow-gold cover w color wiring diagrams. 1980-84 KZ750-4 & KZ700. 1983-85 ZX750.
Starting on pg 150 there's a detailed description of the operation of the carburetors.
Wiring diagrams.mbsween. bike-night. KZ750L3.
Scroll down & click "The Bible" link.
The site has a poorly copied black-&-white 750-4 pdf manual, so be sure to get the good version at the 750Turbo site. not that it matters here, but some of the diagrams are mislabeled in the links: the 1981 750H is an H2, the 82 is an H3, & the 83 is an H4.