Ok, Im doing some work on a friends 2005 TRX 450. The quad starts nicely and idles perfectly. It will even putt around perfectly. The problem is that as soon as you start get on it and rev above 4000 or so (total guess, no tach) it starts to spit and pop and will not rev any higher. It does not matter the throttle position, gear or load. It will do it even sitting still in neutral. I took the carb off and cleaned it (it really didnt look too bad), just a little junk in the needle jet. No change. I tried flicking the choke on while riding to see if it was starving for fuel, no change. The spark plug looked fine, but I did not change it. Im stumped. Any insight out there?

 

You either have a fail CDI or a failed camshaft.
To check the cam shaft, you can remove the valve cover and check valve travel.
See that both valve travel the same amount from fully closed to fully open.
Lay a straight edge across the gasket surfaces for a measuring reference.

Side note.
You didnt mention if this is an electric shift or manual shift.

 

Its manual shift.

I was starting to lean towards an electrical problem. Is there any way I can test the CDI? I dont have a book for this thing, like I said, it belongs to a friend and he had me take a look at it. The thing is beat to hell and severely neglected.

I was wondering if the timing chain could have worked itself loose enough to jump a tooth, and if it did if it would cause this issue. It just seems to run so good on the low end I didnt think that would be the problem. I had an older Suzuki and the timing chain jumped once, but it was too long ago and I dont remember the symptoms.

As far as a failed camshaft, what acutally would have broken? I can't imagine a cam lobe would break off.

Im really leaning towards an electrical problem, I just do not know where or how to start testing things.