'80 KZ750: Wont start after it's warm

Long-time lurker but first post. Hi KZRider! Been riding for 2 years now on this bike, my first, & I've gleaned a lot of knowledge already but Ive got an odd problem with my 750H1:

I bought my '80 KZ750 two years ago from a retired mechanic. For the most part, the bike runs great. Even as winter is rapidly approaching here in Chicago & some mornings its in the high 40s, it starts up with relative ease & runs like a champ once its warmed up. This summer I had a shop check it out after it wasn't running well. They put in new plug wires & tuned it up a bit. I got it back & it ran great. However, if I go for a ride & want to stop somewhere, the bike wont start back up. I'll have to wait 45 mins or so for it to cool down before it'll come around. Again, only does this when the bike has been running for a bit. Because of this problem, I think I might have also damaged something in my starter cause it makes a horrible scraping sound that it never did before. But not being able to stop somewhere quick & hop back on to continue riding is a total drag.

Ive cleaned the carbs out 3 times this summer, put in new pilot jets, new plug wires as mentioned, and just put in a new battery this weekend. Any help is much appreciated, I'd love to fix this myself since this late in the season if it goes to the shop I'm likely done riding till spring.

I'd start with the coils. Some people have said that the original coils were marginal when they were new, so a 30 year old set of coils could easily be getting to the point that they no longer produce spark.

The theory is that coil primary resistance goes up as the coil warms. Since E = I * R, since the voltage stays constant, as the resistance goes up, the current goes down, Since P = I^2 * R, the power (P) the coil can produce will be dropping as the resistance increases.

The test is pretty simple - pull one of the lead wires from you coil (you can leave one of them) and measure the resistance across the primary. You should get something like 3.3 ohms. The resistance should be low - 5 ohms is still serviceable. If the primary resistance is 5 ohms or below, you are good cold. If it's above that, your spark is starting to get marginal.

Once you have the baseline, heat up the coil (hair drier or heat gun) and test the resistance. If the coil is good, the resistance will climb a little. If the coil is bad, it will climb a lot. If the resistance gets above about 10 ohms, replace the coils.

If the coils check out, start looking at the other ignition components. I don't remember when the 750 went to electronic ignition, but if you have points, the condenser could be going. If you have electronic ignition, there are other parts that need to be checked.

To summarize - I think your coils are shot and need to be replaced with a good (Dyna, for example) coil with the appropriate primary resistance. If it isn't your coils, a lot depends on what you've got for the rest of the ignition system.

+1 on checking the coils. And what is the number on your plugs? If the coils are bad Z1 has replacements
at a reasonable price. If the resistance on the coil primary and secondary sides check out it could be to tight
on valve lash.

.

I'd clean all of the electrical connections for a more reliable bike, especially if you've never done this:

Cleaning Motorcycle Electrics

Get some of the De-Oxit DN5 electrical contact cleaner and figure on spending a good day going from the front of the bike to the back. It’s a plastic safe cleaner/preservative. www.deoxit.com is their website, and can be purchased at most electronic supply stores.

On the older Kawasaki's, a majority of electrical connectors are inside the headlight housing requiring removal of the headlight, then the fun begins.

Do one set of electrical connectors at a time to avoid mixing up what connects to where. Usually disconnecting, spraying with De-Oxit DN5 and reconnecting is about all you'll need.

However, when encountering the green crud of corrosion, a brass wire brush may be needed on the pins you can reach. Some 400-600 grit wet and dry sandpaper strips rolled into a tube should reach the male and female pins in the more difficult to clean connectors.

Smoker’s pipe cleaners, cotton swabs and wooden toothpicks work as cleaning aids.

Really small electrical connectors may require the use of a welders tip cleaning tool assortment.

Most pins in the connectors are coated with a thin plating of tin, and others may be nothing more than copper or brass.

If moisture is added, the resulting corrosion lowers the voltage/current being carried causing dim lights, slow engine cranking, slow turn signal response and lower input voltage to the ignition coils resulting in weak spark.

The left and right handlebar switch pods will need attention too as they have circuit functions like turn, horn, run/stop, and start. If you discover the soldered connections at the switches are crumbling, I’ve got a repair procedure for this, just ask.

Usually a spritz or two with actuation of the switch is about all needed for these switches unless corrosion is detected and then careful disassembly is required.

The ignition switch may or may be not sealed to allow spraying the internal contacts. I urge caution if attempting to open this up as springs, and ball bearings may fly out never to be seen again!

If your bike has the older style glass tubed fuses, I suggest replacing them as vibration can cause internal failure. AGX is the type used, and most auto parts stores can get them for you.

Clean the fuse holder clips, looking for signs of overheating(discolored insulation, signs of melting). I use metal polish on a cotton swab, followed by spraying another clean swab with the De-Oxit DN5 and then rubbing the inside of the fuse clip.

All battery cables must be clean and tight for maximum current transfer. Check the cables going from the Negative(-) battery terminal/post to the engine mounting bolt

Also the one going from the Positive(+) terminal to the starter solenoid and from there to the starter motor.

If any battery cable feels ”Crunchy” when flexed, replace it as possible corrosion is inside the insulation.

Each "Bullet Connector" will have to be sprayed to ensure good connectivity, especially the ones going to the energizing coil of the starter solenoid.

The alternator output “Bullet Connectors” are usually behind the engine sprocket cover and will need inspecting and cleaning too.

The turn signal light sockets will benefit from a spritz from the contact cleaner along with the tail light/brake light socket.

Some brake light switches can be sprayed on the actuating rod, with the spray running down inside to the electrical contacts, others may be sealed requiring replacement if the switch is intermittent in operation.

Some people put the Di-Electric Grease on cleaned terminations/connectors, I don’t, as I’ve read/heard it can cause problems when it gets hot, actually insulating the connections, so the choice is yours to use or not.

I think I've covered about all of the electrical systems on the bike.........


“I spent a weekend going through every electrical connection and switch on the bike with a little scotchbrite pad and DeOxit - what a difference! Everything was brighter, gauge backlights, indicator lights, turn signals, I was getting a nicer spark, it fired up quicker, etc. Well worth my time. WELL worth it! “
From a forum member at www.kzrider.com


Re: Cleaning the many and various Motorcycle electrics.
I used the De-oxit D-5 spray on all the electrical connections of my 78 Kz1000 that had sat for twenty years .

When I parked the bike in 92 I was starting to have troubles with some of the electrical system, but after pulling everything apart and giving it a spray , reconnecting the connectors and working them back and forth a few times, then gave them another shot of spray to wash them off before the final reconnection.

Everything still works perfectly and has for over 7000 miles this year.

I am extremely happy with the results , a lot of the connectors were green when I started and after the cleaning they looked new ( shiny gold )

I found the Deoxit D-series, D-5 spray in a 4 oz can at a big electronics store called Frys in Illinois, don't get the small cans at Radio Shack , they are different and meant for computer type stuff.

store.caig.com/s.nl/sc.2/category.188/.f

Trust MFolks , the guy's a true rocket scientist and really knows what he's talking about.
JD

Check coils first, and if they are old ones, just replace. There is no way to accurately bench test them.

If your bike has the electronic ignition it might be the ignitor module or the sensors.

A too tight valve clearance may allow a cold engine to start and run, but then cause compression loss when the engine is warm.

Good Fortune!

Thanks for all the advice, people on this forum are so helpful. I'll probably end up just replacing the coils. Not sure how long the ones on there have been but they both have sizable cracks in the outer housing & besides, why not throw brand news ones on anyway when in doubt. I think I've read elsewhere on the forum that there's a small amount of drilling into your frame if you get Dyna's though? Or am I making that up?

I do have electronic ignition.

I believe I have 8s for plugs, which if I remember right is the standard/recommended plugs for my bike.

Checking the valve clearances is the next item on my maintenance schedule, hopefully this weekend. Cleaning out all the electrical connections is a great idea, have to get a space heater for the garage & make that a weekend project in the near future.

Dynas have screw in terminals instead of the bayonets that are on your current coils. When I replaced mine, I removed the old ends and replaced them with soldered on eyes. A little heat shrink tubing on the exposed part of the solder connection (not the eye, just the arm) and you should be able to just bolt them in place where your old coils were. Some people find that adding some additional (or a slightly longer) spacer makes for a good fit. I used an extra nut on mine, which worked very well.

Follow up for someone else who might have the same problem: Patton nailed it, it was the valve clearance. After they were put into spec I'm happy to say the bike starts right up no matter how warm it is.