I belive it is from both. It lokks to me like the spark arrester hole is smaller tahn the muffler hole. If you just take off the whole end cap and don't rejet It seems to me you lose power so that dyno is pretty close.

 

I don't think the combo of no airbox lid and jetted lean from the factory would make up the difference going from a 148 main to a 158. but, I could be wrong. It worked for us, and we'll continue to use it.

 

that hole is actually a tube that is about 8" long and it is 1" diameter. Why does the front get hot, but the back does not? You mention that you know the difference between the xr and the ex pipes what are they? Why doesn't the pipe sound tinney? I am curious because I tried to remake the pipe out of aluminum, but I didn't cut open the factory one to see how it was exactly made. I got it very close with feelers and a flashlight.

Liking the weight and looks of aluminum without the loud sound.

 

Hi, where do I start,ok first I rejetted to 150 main an 40 pilot -stock being 148 main pilot 38,an added a k&n with outer wears ,ok while it was winter here in the desert I ran with out the s/a ,but we all know you can't do that here or any where there is a chance of a fire , so we got to looking at the guts of the spark erester then we desided that honda an a few other companys use a screen type ,ok we desided that we had to keep the screen in there,but the whole guts of the thing had to go.After rejetting she was to rich with the s/a in ,so we ground the screen clamp welds off cut the screen "save it you need it to put back on"then take the band offnow you need to cut the baffle off 1 inch back ,you need to leave 1 inch there to screw the screen back on what you are doing is opening the flow up from 1 in. to 2in.this mod does not make it to load but will give you more powera bit loader then stock" I am going to test it to see for sure what decibles it is makeing I'll get back to you on that ,but it is about half as load as it not being there at all.
After the mod I checked the plug burning great,no problems.As for power I know it helped there too can climb better up the hills, an more grunt all the way up trough the trottle
The way I see it more air-more fuel-easier flow=more power---->just my 2 cents you don't have to belive it or even try it ,but look the thing over ,an you will see what I am saying

 

I installed a Dynojet kit, K&N air filter with Outerwears and removed the air box lid. The dynojet kit says to use the #170 jet for a bike with these mods and a good aftermarket exhaust system . The only thing is I don't have an aftermarket exhaust. I have the stock exhaust,I did take off my spark arrester, remove the quiet core insert, then re-attached the spark arrester.
This thing runs like a different quad ! So I guess the muffler mods worked pretty well in my case.

P.S plug readings look good also.


2000 400 ex

 

I was thinking about doing this to my pipe. I already have the K&n filter but I need to know what jetting I need to use here in Tulsa Ok thanks for your response.

 

just take the cap off the end of the muffler

 

just take the cap off the end of the muffler

 

"be careful in this part of the country you start a fire, you pay the bill to get it put out" you need the screen in there.

 

When you put any after market muffler on a 400ex you only gain an increase of about 8% in hp. Buy drilling the holes near the end of the tube next to the crome end piece you gain and increase of 6-8% in hp depending on the size and number of holes. The end cap must be on to creat some back pressure. The stock head pipe does not need to be changedas it is very free flowing up to any 450cc's.
Put a 40 pilot and a 152 main in the carb. I have done this to my quad and I can run even with any 400 that has any muffler on it. We have even done it to one with a 440 kit in itand you should see the looks on the people faces after he beats them in a drag race thinking that the quad is stock. You might say that the crome muffler looks better, but I like the look on the faces after they get beat better.