Dlock

It sounds like you have an air leak somewhere. I have been told by Traxxas and other that if your carb has the two o-ring set up that whenever you remove your carb you need to replace the lower o-ring. I just rebuilt my engine after a good cleaning and I used some RTV to seal up everything. RTV is a silicon based gasket maker. It comes out in liquid form and basically makes a rubber/silicon gasket when it dries. I put this around the backplate of my engine, along with around the carb. Just take a little of the RTV smear it where you want the seal and put it back together. It needs to dry for about 24 hrs but after that you should be good to go.

So here is what I would do. Go get some RTV from a local auto parts store, while you are out get yourself a new o-ring for the carb. Take the easy start off and then the backplate. Put a little RTV around the backplate o-ring and seal that up. Try not to get too much on there so the RTV does not get in the way of the internal parts. Also thread lock the screw for the backplate. Then remove your carb, replace the lower o-ring. Once again put a little RTV around the carb and slide it back in. Be careful when you do this so you don't tear the new o-ring you just put in. Also remove the cooling head and make sure the shim is seated correctly. When you tighten it down go in a star pattern, tighten them slowly. Get a little tighter each time. The cooling head is also another place you can have a leak.

I hope this helps. I went through pretty much the same thing you did a while ago. It turned out to be my carb o-ring was bad. It basically was shaved in half. Good luck, and come back if you need any more help.....