I saw this talked about a few months ago and I don't remember what the details were. 2 of my friends have 2000 400exs and they want to know about this. Something about remove the end cap drill some holes etc. Any help would be much appreciated. Thanks in advance.

 

Educate me, what does one hope to gain by drilling holes in a stock muffler?

 

If someone who knows how to do it would like pictures of the end cap (thing that you unbolt, remove from muffler), I have a couple pics that I could scan and post on my web page, just to make it easier to visualize.

 

It has to do with removing the spark arrestor screen to get to the quiet core, then remove the quiet core and replace the spark arrestor screen.
I don't see where this would gain much on a 400EX (except increase the SPL outputted by the bike), given the rest of the pipe design, whereas it may make a difference on an XR400. If you get a chance, look into how the mufflers are designed on both, and you'll know what I am talking about.

 

http://www.valleycity.net/pete/modex.jpg

YES it does work, power is increased nicely, we rejetted to a 158 main, 40 pilot, airbox lid removed

 

From your picture I can't tell what you've done. It looks like the quite core is still in there.

I can't see where modifying the cap would have any affect on HP, the restriction is before the cap in the muffler. Perhaps your HP increase came with the jetting and intake mods.

Here is a dyno run with the cap completely removed...

 

look on the outside of the core, around the outside part of the inner core

also, I seems to me the restriction is the small outlet pipe on the muffler, which this mod bypasses.

 

Here's what I did I ground off the welds on the the ring that held the sparck arrester screen on and at each one of the 6 rough braze spots I drilled a 3/8" hole and left the sparke arrester screen off. I didn't notice much of an improvement so I putt 6 more just a little more towards the front of the quad if the cap was mounted, and got more of an improvement. This was all along with the airbox proped open and a 150 main and 40 pilot. It wasn't a real big increase in sound or power but worth it. I think a 155 might be better but I havn't tried it yet. If you shine a flashlight in the muffler you can see that the hole in the plate is bigger tahn the hole in the spark arrester. So making that hole bigger would increase flow and give more power. Also I bought a new spark arrester from somebody to modify incase I didn't like the modified one.

 

If you shine a light into the muffler there is an equally as small a hole ahead of the chamber the endcap is bolted to.

If you go out and run the machine really hard, you will also notice that the front chamber gets really hot, which can be noted by the discoloration from the front of the muffler to about half way (where the rear chamber begins).

 

Again, I point to the dyno run linked above. The EX is jetted lean from the factory to keep the EPA happy, it's even documented in the owners manual. I wonder if the power you folks are seeing if from jetting alone...

Have you tried the virgin SA with the jetting changes?