1983 KZ750 L3. Really Struggling. No start WFO

My 1983 KZ750 refuses to start. Fresh plugs, the carbs are clean ... there is fuel in the float bowls and a brand new battery in it.

After I almost killed the battery trying to start it normally with the choke on, I sprayed some starting fluid in the airbox and gave it a whirl.

It started then but ran WFO until I killed the engine.

I re-checked the idle speed screw and the throttle cable. I pulled the top of the cable off so there was no chance the cable was holding the throttle open. Felt inside the airbox and the slides are down.

Re-started it no starting fluid and it again ran WFO. I think there was still some starting fluid vapors in it. When they were gone it went back to not starting.

Pulled the plugs and squirted some fuel in each cylinder. Put them back in and tried to start it.

Again the engine went WFO for a moment then after it burned the fuel went back to not running.

Triple checked the cable and screwed the idle speed screw back so far it came off.

I really, really do not want to pull the carbs off it again. Anyone here have any thoughts?

Thanks

Air leaks. Either between the carb holders and the head or between the carbs and the holders, or both.

Could be torn diaphragms in the carbs.

Might be stuck timing advancer, though it should start anyway.

Did you bench sync the carbs and set the fuel level using the clear tube method?

Has it ever started for you? Could also be bad coil(s), bad secondary wires or caps.

Sounds like dirty carbs. Did you fully break them down, soak in carb dip or use an ultrasonic cleaner, and then rebuild each carb using new O-rings?

Here is a carb rebuild tutorial that you might want to look over to see if you missed anything... zeus.mtsac.edu/~cliff/storage/gs/Mikuni_...Rebuild_Tutorial.pdf

Thanks for your thoughts.

It hasn't run since around 2000 or 2001 but has been in dry storage.

I replaced the rubber from the carbs to the airbox.

Carbs to head rubber seem to fit okay.

I can check the diaphrams,

How does one find air leaks on either side of the carbs?

Coils are original.

If those carb holders are 20 or 30 years old, they are likely hard, cracked and leaking. There really is no way to test without the engine running, and only the really big leaks show up then.
Nessism is right about the carbs. the low speed passages are really tiny, and only an ultrasonic cleaning followed by poking through the passageways with tiny pointy things (strand from a wire brush or a tool made specifically for the purpose) and then followed by spray carb cleaner followed by compressed air, can insure that they are clear.

When you turned down the large idle knob could you tell if it was lowering the slides? I ask because some folks who have taken the carbs apart to clean them don’t set the idle screw near the center of its movement range when they reassemble the carbs. If the screw is set out too far it won’t lower the slides when it is turned. If that happens to be the case you can lower each slide individually through the adjustment screw near the top of each carb under the cap. Ed

Also true, though I believe the 1983 had cv carbs, in which case the adjustment screws are on the throttle shaft, not under the cap.

650ed wrote: When you turned down the large idle knob could you tell if it was lowering the slides? I ask because some folks who have taken the carbs apart to clean them don’t set the idle screw near the center of its movement range when they reassemble the carbs. If the screw is set out too far it won’t lower the slides when it is turned. If that happens to be the case you can lower each slide individually through the adjustment screw near the top of each carb under the cap. Ed

I peeled the airbox boots back to get a look at the slides in action.

So do I have it right, with the CV carbs on the 1983, when I twist the throttle should the slides go up?

When I twist the throttle nothing happens with the slides.

No. On cv carbs, the throttle cable opens throttle plates down stream of the slide. Vacuum opens the slides. With the engine off, there is no vacuum.

This may help

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www.manualslib.com/manual/817908/Polaris...an-400.html?page=156

Here's a video that shows CV carb operation. Ed