i would like to say that i have looked around these forums here and they are great. i have an issue with a 98 300ex that i just got from a friend. it runs fine when its started and ran cold, but after a few minutes when it warms up it starts spuddering and acting up. i replaced spark plug, changed gas and cleaned air filter. it will idle fine even through the rough running but its acting up. i noticed that someone has tried to weld a place in the pipe close to where the muffler meets the pipe and its i can feel exhaust through the hole when its running. if anyone could point me in the right direction i would really appreciate it..thanks alot

 

What color is the plug after it has warmed up and starts running bad. I would warm it up, run it to the point that it starts acting up and then hit the kill switch while you're still moving. Pull the plug and see what color it is. If it's white it is running to lean, nice tan color is good, black is to rich. One of my atv's was doing the same thing to me. I had been running it with the air box lid off and it was getting to much air...when taken up to speed it would loose power and start running like crap. I simply had to put the air box lid back on to restrict the flow which fixed the problem but the only way to tell is by doing the above.

 

My money is on the imulsion tube.... basicly its the long brass tube that the main jet is screwed into..

The tiny holes on the side of it are probably clogged up.
Just go ahead and do a full carb clean. Polish up Every brass peice in the carb with 0000 steel wool and liquid dishwashing soap and water. carefully roll up a tiny amount of wool and slowly spin it inside the jets and imulsion tube. toclean out the insides.
DONOT I REPEAT DONOT USE the wires to clean out welding tips inside your carb... Alot of ppl say do it. but DONOT! they are way to abrasive and will ruin a jet faster then you can imagine.
Just be shure to Blow out the carb very well, with a air compressor to get all of the steel wool out of the jets and carb.
If the imulsion tube's side holes are clogged and you cannot seem to get them clean.. use a brass wire from say.. Speaker wire.. to gently poke out the corrosion or whatever is in the holes...

BTW. the absolute best carb cleaner on the market is Berrymans B-12 carb spray... I will not use anything else!
Or if the corrosion is severe enough . soak the entire thing in lemon juice for a night of two...


One other thing that might be happening is a high rpm missfire..
unplug the spark plug boot. and unscrew it from the coil wire... you will see where the corse threaded screw has stretched the insulation on the wire... Trim just that 1/4" section off the end.
Then with something pointy (dont poke your eye out :-P) carefully spread the copper strands out and evenly space them, so that when you thread the boot back on it will screw into the center of the strands. Giving a nice uniform connection between the coil wire and sparkplug boot...
While your at it. goahead and look inside the boot and clean any corrosion out with the steel wool you used in the carb...
beshure to use some form of hardening silicone inside the boot where the wire goes in. Just enough so that it seals the connection from moisture...
This will help ALOT with throttle response and longevity of the connection..

anyways. long post..

GL on that issue