we took all the plastic off and ran it to see if we could find a spark and could not find it sparking any where

Sounds like a jetting/carburetion problem. I come to this conclusion assuming a few things: This is an earlier model with the Mikuni BSR36 (CV type) carburetor, not a newer EFI model. The air filter is clean and not over oiled (predominantly a problem with cotton gauze wire mesh type filters). No leaks or cracks in the intake system. You are running fresh, clean, proper fuel. Valve tolerances are within spec. The engine starts, idles and revs up without any issue (no black smoke or pops ect..)

With those assumptions I would say your running slightly lean at WOT (common if the machine is set to stock settings, I have this same carburetor on my DR350) Simple thing to try is raising the jet needle (lowering the clip position) inside the slide. If this improves the condition but does not cure it, it is possible you need to go to the next bigger half step main jet.

Other causes/possible solutions

Dirty fuel system starving the carburetor/main jet of fuel at WOT. Clean thoroughly.

Improper valve clearances causing low vacuum at high RPMs thus closing the fuel petcock diaphragm slightly starving the carburetor for fuel. Adjust valve train. Setting the fuel petcock to "PRI" will temporarily solve this problem.

Improper float level. Check the specs and adjust if necessary.

Could be your simply hitting the rev-limiter. Replace CDI box with one with a higher redline, not recommended unless you have extensive internal modifications to utilize the extra RPMs.

Given the assumptions above I do not believe your problem is electrical/ignition related.

*cough* SUZUKI *cough*

jk, Tuck! Its a Suzuki thing...I obviously wouldn't understand!

But at least its still on the business end of the tow rope , not being pulled back to the trailer.

Ok cleaned carb and adjusted...no change. Put carb off of Tucks on and problem went away. So his dads if fixed now Tuck has a carb problem...JK

Anyway cleaned and reinstalled same problem, but with other carb it runs fine. Anymore ideas that come to mind?

Still sounds like jets. You could see through them? I know you probley did, but did u use and entire can of carb leaner with little red staw to blow out all lines? Are the needle clips set the same on both carbs. Damn carb gremlins!

Ouch, Ric...that hurts, RIGHT in the heart!!!! Plus, you KNOW you'd love to pull me back, all damsel in distress!!!

Check the CDI box. Make sure it is fully connected. If it is and if you have another Z laying around switch CDIs and see if they quad works then. That is what was wrong with mine when we installed the new CDI, it wasn't connected enough so in High RPMs it would break up and once the motor was hot(like normal running around the dunes, not over and above hot) it would die and not start back up. Then all the sudden it would work again.

Yes we cleaned and re cleaned every port and passage. No the carb needles are differant just because 1 has a jet kit, other is stock. This problem just came on all the sudden, not after any other work had been performed. I've checked hoses and diagram's for leaks and we've reset all the clearances and adjustments. It happens at the top end of every gear, but does not cut out like the CDI box does, and yes we swapped CDI boxes from other machine, did not fix it.

Its really getting old messing with it, and it's going to turn out to be some little thing that was over looked. Last night Tuck beat Mikes 400 with his sisters Arctic Cat 300, so after i talked Mike down from jumping off the bridge, we tore back into it.

Just put a Holley on it then!

Now your talking! If i could do that problem would be solved! I got a 400 in my Bronco, and this is a 400 wonder if they make an adapter plate