77 KZ 750 B

bgulsvig wrote:

Voltage is 13 at the battery, 13 at the fuse, 13 at the connector in the head light, and 11.5 at the ignition switch and kill switch. None of the wires or connectors are corroded. They are like brand new?.....

If not already done, would study the wiring diagram, especially the connections inside the ignition switch and kill (on-off) switch.

To access on-site wiring diagram, click here > kzrider.com/index.php?option=com_docman&...d&gid=282&Itemid=108

Voltage enters ignition switch and exits to kill switch where it exits to ignition coils

With 13 vdc going into ignition switch, is supposed to be 13 vdc exiting and going into the kill switch, with the same 13 vdc exiting the kill switch and going to the coils.

Perhaps losing voltage in ignition switch and/or kill switch.

Such voltage loss might result from corrosion inside the ignition switch (or maybe the kill switch).

Good Fortune!

Well I did the "coil mod" and now have improved voltage at the coil, but that did not help with the starting problem. Then I repaced the coil, plug wires and plug caps I got from Z1. Still would not start. Here's the wierd part. I pulled the right spark plug and grounded it to head to check for spark and cranked it over. It started and continued to run with one plug in and one plug dangling out of the hole. Is that even normal or possible? I put the plug back in and it started and idled fine. I thought the new coil had fixed it. I took it for a ride and it ran better but would still cut out and "buck" violently when I tried to accelerate. Came back and pulled the left plug. Black and sooty as always. I let it idle and tried adjusting the carbs. Once again I was able to tighten the adjustment screws all the way in and it continued to run without any effect. That can't be normal. What do those screws adjust anyway?.....

If those screws are having no effect either the circuit for them is plugged, or you have ripped diaphragms. Have you even checked the diaphragms or cleaned the carbs?? I'm to lazy to reread entire thread.. heh.

Air mixture screw for the idle circuit is "what it does" and if turning them in or out is having no effect and you have soiled black plugs from excessive fuel, it is definitely a problem you'll need to correct.

Best of Luck.

I have removed and cleaned and inspected the carbs numberous times and even put a kit in both carbs.
The last time installing them a found a small slit on the seam of one of the rubber air ducts running to the air box. Could that small slit make it run rough at speed but idle fine?....

Well the slit could definitely contribute a lot of air if it expands when the vacuum increases. Yes it could be a major factor in your headaches as far as stumbling, sputtering throttle near the midrange. It wouldn't explain the excessive fuel you seem to be getting to make your plugs sooty black.

I'm still a bit baffled by the air screws having no effect, if you cleaned carbs and inspected diaphragms. Even with open carbs (some effect), or pods on (more effect), or a leaky box (better effect)... the air screws should still have an effect, in my experience.

You could, for a test at least, set the air screws to stock staring point and tape up that boot good with some duct tape or electrical etc... should be able to get a good enough seal to determine if that is causing all the ruckus.. if so start looking for replacements. If one is shot most likely the other is near it's end as well. (Filter is in and cap on air box of course.)

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More questions for ya, sorry heh:

Do you have the needles on the 4th notch down from the top???

With the carbs all buttoned up before you put them back on the bike, when you lift the slider up to the top with your finger and plug off the vent with your other thumb, how long does it take your sliders to fall to seated?? It should be about 10 seconds, and they should be fairly even.

If you didn't do that test, you could undo the 2 bolts holding the air box to the frame and should be able to push it back enough to get velocity stacks off carbs and spin them out of the way to do the above test. Sometimes a visual inspection of diaphragms won't expose pinholes.

When you did your kit in the carb, you didn't by chance forget the part # 37 in diagram did you? tinyurl.com/242849c

I repaired the small slit in the boot but it made no difference. I did notice that I am using NGK B8ES spark plugs when my service manual says to run B6ES plugs. Could that be the cause of the black plugs and rough running?...

If you haven't made any mods to the stock bike then the jetting and everything else should be close.

You still haven't talked about how you fixed the low voltage at the coils.

If the voltage is lower then 12.65 vdc at the coils then it is to low. Alot of people are against the WG coil mod here and for goood reason. But it is a cheap and easy diagnostic tool.

If you follow their advice and still get more then 12 volts at the coil then you'll be fine but according to the last time you posted you only have 10 volts. That isn't enough. You need 12.65 VDC at the coils to be within factory specs. 12.80 would be best. I still think it is an electrical problem since you are reporting sooty plugs and low numbers at the coils. This bike will start under stupid bad battery conditions and run all day. I know. My bike drove 300 miles with a bad battery last year.


But if your switches are corroded and shot then you won't be able to produce enough voltage to clean the carbon off your spark plugs. They'll look sooty and act like the carbs are rich.

You need 12.65 volts at your coils. For older bikes like ours it may not be as important but in the shop I work I'll tell you right now when a bike comes in with an electrical problem we imediatly check battery voltage and if it isn't up to snuff we won't even test anything else.

BTW 12.65 is normal Battery voltage.


I will admit that the BS 38 carbs are a whole nother animal to diagnose. But they follow normal carb tuning rules. Except jets can leak through the float bowl gasket and that is why the are a pain.

Here is a list of things I did over the summer:
- In-line fuel filters
- Coil/relay mod
- New coil, wires, caps, plugs
- Carb kit in both carbs

Idles fine but still cuts out or stalls when I try to ride it. I pull the plugs and they are black. Should I be using B8ES or B6ES plugs.

Hello everyone...I am reviving this thread as the problem hasn't been solved and I'm back at trying to get it running. Any new ideas occur to you over the winter?.......

bgulsvig wrote: I was tinkering with it last night and tried to adjust the carbs. I let it idle for a full ten minutes. During that time I tried different settings of the pilot screws. At one point I had them both screwed all the way in and seated. There was no change in idle speed. Is this normal?

It's not normal. If the pilot circuits are functioning at all, it's not even POSSIBLE. Your carbs are really screwed up if they do that. You need to fix the fuel system before screwing with the coils. The basic fuel system cleanup:

1) Clean fuel tank, remove rust

2) Clean petcock/screen assembly

3) Install inline filters

4) Set fuel levels in bowls with clear tube

5) sync carbs

6) Check intake manifolds for leaks

7) Check diaphragms CAREFULLY for tears or slits

Clean carbs, clear jets and all passages

IMHO, 10 - 11V at the coil terminal is plenty to run the coil. The system is designed to tolerate some voltage drops. Obviously no voltage loss is ideal, but if the engine starts OK, you are getting enough voltage to run. The battery drops an additional 3-4V when cranking the starter, reducing coil voltage to minimum. If it starts, it should run fairly well.

I have done all of the items except 4 and 5. What is involved in syncing the carbs? Also what does the pilot circuit do anyway?

bgulsvig wrote: I have done all of the items except 4 and 5. What is involved in syncing the carbs? Also what does the pilot circuit do anyway?

The pilot circuit feeds the engine at idle and small throttle openings maybe up to 1/4 throttle.

Syncing means making sure the carb throttle butterfly plates are open the same physical distance at idle so they will be balanced. You loosen the clamp between the throttle shafts to adjust this.