On the older larger 4's the left coil goes to # 1 and # 4 and the right coil goes to # 2 and # 3. As you sit on the bike, the sparkplugs are numbered left to right 1,2,3& 4 (not sure if the same setup is used on the GPZ 750).

 

have you had a compression test or leak down test on that engine? how many miles does it have? just asking as i have my '84 gpz in the shop right now due to similar problems, and i'm having the heads rebuilt as well as the carbs....

 

Try fresh sparkplugs and check for a nice fat spark. Check also the voltage at the coils and the battery. If there is 1 volt or more difference. You would benefit from installing Wired Georges coil mod. If the PO wasn't lying and it was in fact running well until just recently. It could be a bad fuse or broken wire or dirty or loose ground connection.

 

I have the exact same problem at the moment but can't seem to find the answer, i had to take it to the workshop and it refused to start even though it was fine the day before, tank has been cleaned out,carbs rebuilt,firing order checked, the top end was rebuilt and all is fine there but the mechs couldn't find the problem and they were charging me £30 an hour just to tell me they couldn't fix it but at least i know the cam timing is fine according to them,i get a good spark at the plugs and fuel in the carbs so i reckon it must be an ignition problem,i also got another igniter and tried that but no difference, it just turns over but doesn't quite fire so at the moment i am going through all the wiring, will let you know if i pin it down to anything......

 

I've decided to go through all my wiring also. I put new plug wires on and have very good spark at all of the plugs, as well as good fuel delivery. It must be a wiring problem somewhere, its a tad overwhelming at the moment, I need to decide where to start and what direction to take for testing; the Haynes manual is providing good info but it would be nice if I had a few more pictures to work with; the previous owner did made some modifications to the bike like flush-mount turn signals and an LED brake light, so I'm thinking perhaps a wire came loose or theres a bad ground somewhere.

 

Don't replace anything else till you check the following, even if I am repeating other suggestions.

Check compression with a gauge, all spark plugs removed, and full throttle.
You need to know these things before continuing.

The owner said that it had been running great until the Friday before I saw it; he had been riding it and it started to sputter and miss and finally just died.

Sounds a little fishy to me. More reason to check compression. Engines don't just do this.

Is it the turbo model? Not water-cooled is it?

You know if the coils are backwards, then just changing the positive wires from one coil to another would be like swapping the coils. I believe that model has an electronic advance, not the mechanical advance like on the '82?

What jetting was in the carbs? If there are K&N Pod Filters on the carbs, you'll need to up the jetting or it will run lean, when it runs.

Let us know whatyou find out in your search. Good luck.

 

A good multimeter or a self powered test light, the wiring diagram, a note pad to keep straight what has been tested, peace and quiete and a well lit area to work will make trouble shooting much easier.

When I'm doing wiring, I turn off the radio as I find it very distracting. I get all my supplies at hand, wiring diagram,soldering station and solder supplies if needed,tie wraps,wire,heatshrink tubing selection,solderless crimp splices,crimpers, wire cutters, wire of different gauges and colors, heat gun with the proper nozzles and wire strippers.

 

It does say in the Haynes manual there are three cutout switches on the bike, on the sidestand,Clutch and gearchange and you can bypass those three by running a wire from the negative terminal on the battery to the Black and white wire on the ignitor (position three) this didnt solve my problem but it might help you though, good luck

 

Well, I finally got around to doing a compression test, and the results are not good...I'm consistently getting 80-90 psi across all cylinders However, it just so happens that my neighbor also has an '84 GPZ 750 with only 16k on it. It's been sitting for years but he said the motor was strong and in very good shape when he parked it, and the bike is stock. He let me have it for $50, so here's hoping the motor in that machine will be alright.

EDIT: Forgot to have the motor at WOT during the testing, so I'll go back, try it again and report back any improvements or changes i find.

 

Before you even think of firing that neighbor's bike up, if it's been sitting that long, change the oil and filter first. The add a little oil in the cylinders, and turn the engine over by hand. The you can fire it up. Most damage is done from an engine sitting, and then started with the cylinder walls dry, and old oil them getting pumped all over.

Keep the current GPz with the low compression, and do a ring and hone job on it. That will freshen it up. You could go "Krazy" and install a 810cc Bore Kit.

 

For $50 bux you should ****** up that other one. Heck scratch that, where are you at? I'll go get it.

 

How long has it been since someone checked the valve clearence ? It may need a simple adjustment & not a engine change. But get the 50 dollar bike !

 

Success! After properly doing the compression test I'm getting 135-140 psi across the board. I'm still going to buy the other bike anyway, the more the merrier

 

Dry hot, and wet hot(not a porno, but add a little oil in the spark plug hole), both full throttle is the way to check compression. All spark plugs removed, and battery completely charged.

 

I've decided to go through all my wiring also. I put new plug wires on and have very good spark at all of the plugs, as well as good fuel delivery. It must be a wiring problem somewhere, its a tad overwhelming at the moment, I need to decide where to start and what direction to take for testing; the Haynes manual is providing good info but it would be nice if I had a few more pictures to work with; the previous owner did made some modifications to the bike like flush-mount turn signals and an LED brake light, so I'm thinking perhaps a wire came loose or theres a bad ground somewhere.

It sounds like it could be a ground issue because of the way it stumbled before dying. You're getting power to the lights and gauge block? Check the harness for burnt insulation near the motor/exhaust, and wear points at the harness retainer clips (if your bike has any) if the harness has been cut look for burnt insulation downstream.

Good luck, and score!! on the backup bike!

 

Has this bike been sitting for an extended period of time? I have been saying this over and over, this sounds a lot like "E10" fuel debris within the carbs and or internal fuel tank rust sediment. I kinda' dabble as a hobby working on bikes for friends as a hobby, and over the past few years, the hands down #1 issue with respect to erratic running engines is simply dirty carbs, (specifically clogged pilot jet passages). Keep in mind the pilot jets are only about .012" in diameter, and it doesn't take much to clog them with *****.

Always check the simple things first, and I think as others suggested a compression test will rule out or isolate internal engine issues..

 

Has this bike been sitting for an extended period of time? I have been saying this over and over, this sounds a lot like "E10" fuel debris within the carbs and or internal fuel tank rust sediment. I kinda' dabble as a hobby working on bikes for friends as a hobby, and over the past few years, the hands down #1 issue with respect to erratic running engines is simply dirty carbs, (specifically clogged pilot jet passages). Keep in mind the pilot jets are only about .012" in diameter, and it doesn't take much to clog them with *****.

Always check the simple things first, and I think as others suggested a compression test will rule out or isolate internal engine issues..

Yeah the bike had almost a full tank of fresh gas when I bought it, although after he had filled it the bike had quit running a little while after, so the gas is a couple weeks old now.; reckon I should drain it out and put some fresh gas in later when I'm ready to try starting it again. Since I have the tank off now I've been thinking about taking the carbs back off and cleaning them once more, and take not of the main jet size this time around. I remember there was a tiny bit of brown gunk in a couple of the bowels the last time I cleaned the carbs out, so it's quite possbile that the bike has sat for a while at one point in its life. Thankfully the compression is good (135-140psi across all cylinders resulting from a dry test.) I have the day off on Wed, so I'll go grab the other bike from under my neighbors house (where it's been parked for quite some time) and start scoping it out.

 

Thought I'd stop by for an update. After sorting out the firing order and putting in a couple of new main relays, the bike started up and stayed running. And not only was it running, it was running **** good! It idled smoothly, revved crisply and doesn't smoke, leak or make any odd noises. It sounded awesome! I had it running until it started running out of gas, so I added half a tank of fresh 87 octane last night and let it run for about 20 minutes, revving it every now any then because the burble and tone coming out the Yoshimura exhaust is a treat to the ears. It continued to run excellent until I called it quits and shut it down for the night. Now, only one day later, I started it up, and it again idled just fine, but it started developing issues. It started to become more and more reluctant to being revved; it just bogged and threatened to stall unless i let off the gas. It got to the point where it would only run if I let it idle, until finally it started sputtering while idling and stalled. I haven't been able to get it started since then. Could something have clogged the fuel system somehow? Where should I start investigating?

 

If you have FI, I'd look at the fuel filter, If carbs, look at fuel flow problems like a diaphram gone bad in the petcock, or a clogged tank vent.

 

Fuel, air, and spark is all that's needed.
-Sounds like a fuel issue to me. Open the drains on the carb bowls, and work your way back to the petcock.
-Maybe the fuel cap is sealing too well? Open it up.
-Carb bowl vents capped off, instead of open to filtered air?
-Got fuel in the tank? You said it ran for 20 minutes, could have run it out of fuel?

Let us know what you find.

 

So far I've pulled all four plugs, and here's what I've found so far: cylinders 1, 3 and 4 were black and sooty, so I cleaned them off with a wire brush and reinstalled them. the plug from cylinder #2 still looked brand new, nothing on it at all. It also seemed a bit hotter feeling than the other four when I pulled it out, although this could've been my imagination. I put about half a tank of fresh gas in last night, after the old gas pretty much ran out. Just seems like once the bike warms up, it doesn't want to run anymore. A friend and co-worker of mine, who has experience with motorcycles, was saying something about how the plug for cylinder #2 can tell me a lot, since it has something to do with the petcock, or something like that. Cracked open the bowl screws on the carbs and fuel is coming out of all of them. I'll try getting the bike running with the fuel cap open tomorrow and see if anything changes. If there's no change, I'll pull the tank off and investigate the petcock.

 

Gotta be fuel starved, and more reason to rebuild the carbs, buying thre kits here:
Z1 Enterprises, Inc. - Specializing in Vintage Japanese Motorcycle Parts
K&N Pod Filters, with an aftermarket exhaust or stock exhaust buy a Stage 3 Jet Kit:
Dynojet Jet Kits - Offering Motorcycle Jet Kits & ATV Jet Kits

You have fuek coming out the drains, but where is the level inside? Float heights set? Carb bowl orifices cleaned for breathing? Vents clear? Needle and seats sticking, or not working(these come in the rebuild kits)? What jetting is in there now? Do you have a good manual?

 

sounds like #2 is the only one that is running close to properly, Kawasaki has these bikes running lean to meet smog regulations. If someone put a pipe on that bike and K&N filter pods i bet they went over board on the main jet size. I myself would start with a carb clean & inspection, float hight, put (*stock*) or 1 size bigger main jet, raise the ( needles .040 -.080 from stock position ) and then most likely you should have your plugs white/tan color. The #2 vacuum port is used to open the fuel tap, if the diaphram on the fuel tap is no good it will not deliver fuel in the run position and that cyclinder might have a small vacuum leak / running way to lean condition.

 

You can test the diaphragm on the fuel tap by taking the tank off and putting a pipe on the vaccum connection and with the tap set to ON not prime suck gently and if fuel comes out of the fuel tap then the vaccum part of it is working fine.........

 

Just went out and fired the bike up (with the fuel tank cap open this time.) Did the same thing it did yesterday: ran awesome for about 10-15 minutes (smooth idle, crisp throttle response etc.) Then it started getting unwilling to rev until it would only run smoothly at idle, then even at idle it would start to falter, until it finally stalled. My main jet size is 165 (thats the number on the jet, where you unscrew it); the Haynes manual says the stock size is 110. The floats are moving freely and set at the correct height, and the throttle valves are moving freely and not sticking anywhere. Going to try testing the fuel tap diaphragm later today. The Dynojet Stage 3 provides 185 main jets for running aftermarket exhausts and individual filters. Should I bite the bullet and order the jet kit and rebuilt kit, or should I only get one or the other?