I have a 97 TRX400FM I put a cobra pipe on because the stock muffler fell apart. It doesn't seem to run as good and the exhaust is running really hot. I turned the mixture screw out 1 turn and seemed to help a little but doesn't run like I feel it should when it's floored. Do I need to rejet it after doing this or will the mixture screw actually do anything?

 

i would jet it
i would also put an aftermarket air filter in it to prevent it from running too rich

 

Ok so I put a main jet of 135 in it and it runs better but it's only topping out at 45MPH. Previous to the exhaust it did about 55. It gets really slow about 42-43. I was thinking about adding a better filter and opening up the box lid to see if that helps it. According to the DJ site their jet kit only has a DJ124 which is a 132.5 Keihin jet. Did I go to far with a 135? What do you guys think to problem could be?

 

it definately needs more air
i would go with the k&n air filter
if you want even more power i would get the thunderproducts air box lid valve
i have two of them on my quad
or the k&n powerlid
look on www.rockymountainatv.com under intake
you will find what you need

 

What main did it have in stock? Usually for just an exhaust, no intake or engine mods, you probably wouldn't have to go up more than 5 or 10. In other words, if your main was smaller than 125 she may be to rich. Rich engines will generally burble and sputter in the range they are rich. Unless you plan on adding the filter now, I would just jet to what you are running. Best bet- pull the plug and take a look, go from there. Oh, and jets are pretty cheap. It won't hurt to buy a few and play around some. Don't forget to include at least the next step or two up in pilots as well. Just adding a filter is just as likely to double your problems right now. Get it sorted out first, then continue from there.

 

Stock was 130 so I went up 5. Brand new plug. Ran really good when I got it two months ago but the muffler rattled like crazy and was kinda clogging up from the thing falling apart. Put the Cobra muffler with spark arrestor on as that was all I could find and it ran really hot immediately to the point were after an hour or so the paint on the pipe before it gets to the muffler is melted off. I played with the mixture screw and that helped the response a little upping the main jet helped but still doesn't seem right. Usually you shouldn't have to do anything with the pilot for just minor mods. The Dynojet kit doesn't even include a pilot jet just a slide drill and adjusting washer. Of course I've already got a bigger main than what that kit comes with.