I have a 03 400ex with a brand new high compression piston and also a stage 2 cam, i have been messing around with some jetting and found that the best jet in my kit that i was able to go with was a 170, but when i get on hills or get under a big load my craps out and stutters, any ideas out there??? please help!!!

 

Probably because you have a stage 2 cam. Its not intended for lowend grunt. Other than that, your needle or needle jet and/or slide choice could be refined.

 

JustRandy correct me if I'm wrong, if you were to drop the needle clip down one notch it would richen it up and maybe take care of the problem. Another problem that I have seen and acts like what is described is not enough air flow. I have seen guys put on 38mm carbs on Blasters that had 26mm stock on them and use the stock air box. The ones I've seen don't work, they get under power or in the upper gears and fall flat on their faces.

 

Yep, dropping the needle clip down a notch would richen it up, but it could make some other part of the throttle opening too rich. Its a real can of worms! An engine is just an air pump... And with a stage 2 cam, suddenly the engine doesn't pump air so good at low rpms. So, there is less vacuum on the carb when trying to pull a hill at part throttle. The best way out of this may be to just lean everything out and get a dial-a-jet.

Once you enter the realm of stage 2, everything has to be high flow. You need a good flowing carb and air filter and a good header n muffler. Stage 2 takes HP from the bottom end and adds it to the topend. So, if you don't modify the air flow for topend ability, the whole deal is a net loss of HP.

I don't know that much about 2-strokes except they use bigger carbs and richer jets than an equal size 4-stroke. But I'd never figure a 200cc 4-stroke would need anything near a 38mm carb! Kinda senseless to have a carb bigger than the intake valve in my opinion. Maybe a 38 could be made to work on a blaster, but it would be WOT all the time and good for little else I think.

 

I agree with you on the 38, I think it is CT racing that builds a Blaster motor and they use a cylinder based off of a 250r. They run a big carb, as far as a stock ported cylinder about the best that I have seen is a 28 or 30

 

That sounds about right.... Good results come from running a 28 or 32 on a 230 quadsport. It also had a 26mm stock carb. I'm in the process right now of porting the head to match a 32mm carb with its stock 32mm valve. Something like this: