I have an 02 400ex with full curtis sparks exhaust high compression piston bored to a 426 with stage 2 hot cams. I have the keihin flat slide carb off of a crf 450. I just put a 190 main and a 55 slow jet in it with the needle 3rd notch down from the top fuel screw 3 turns out. From an idle it bogs down after about a 1/4 throttleIt. It will do fine if you ease into it. And on deceleration it will not throttle back down all the way. It is like it sticks at 1/4 throttle for just a few seconds. Any help would be appreciated I want to fix it so I can ride this weekend.

What's the model of carb your using?
What happens when you drop the needle on clip?
What happens if you run the ATV without the air filter on?
Sounds to me you have too much air flowing to your motor,, it's bogging down your bike when accelerating, and when you let off the throttle it's still pulling gas because so much air is flowing through the carb.
You could try and make the inlet of the carb a bit smaller with a insert ring of some sort.
Timing advance might be an issue too, you'd have to know that your carb is tuned right first.

Ok, bigger problems now. I went out to a local track Sunday and while riding my quad made a horible noise and died. I took it back to a freinds house tore it down and the exhaust valves broke off and fell into the cylindar tearing up my piston and head. Now what would cause that and how do I fix it so it does not happen again. Do you think this had something to do with the carb not being tuned in perfect. I mean it was close or so I thought so.

Hmm, was the motor ever rebuilt?,, Broken valves are a sign of not having them replaced. Valves get weak as times goes on, even if they were re-ground, or if your hard on your engine you'll snap valves, you have a very bullet prof engine, but it's not uncommon if you snap a valve on a well used thumper.

I'd also like to point out your engine is nothing like a 450r,, the only thing they have in common are they have a piston and use valves, their designed totally different,
I don't think the carb was the problem, unless it was real lean and heated up the exhaust valve, Post a pic and I'll tell you.

Anyways it looks like you need to start rebuilding,, good luck..

What are the best parts for the price to rebuild with. I want to go back with a high compression piston like a 12.1 or so do I need to get stronger valves and springs to keep this from happening again or are oem fine. I will get a pick up tomorrow. Thanks for the help.

I would upgrade if I were you, 12.1, 426cc, I hope your crank isn't warn too or you might be doing this for nothing, what do you think?
At the very least I'd have the crank bearing replaced,,

Is the crank bearing hard to replace? I have never been that far into a motor so I know nothing about it. How can I check the crank to make sure it is not bent.

QUOTE FROM ANOTHER SOURCE:
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Many different carbs out there that will fit different engines. Just have to make sure that venturi size is the same.
Venturi is the narrowest part in throat of carb body.
If venturi is too small, power will be restricted. Generally, engine manufactures make them smaller than what engine can actually handle.
If venturi is too large, engine will run lean.
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( I read this about hopping up gocart motors, and holds true to what I mentioned above about having the right sized inlet of the carb a bit smaller with a insert ring. If you were running it lean it could have heated the valve and made it easier to fail, you should look at this when your engine is fixed.)