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CRF muffler operation

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From the first day I had my CRF250M it was clear that something needed to be done to the exhaust. It doesn’t sound like anything and it’s too quiet. The easiest choice would be to order a slip-on but since I live in Thailand shipping from the US is a killer on top of the $300 to $500 purchase price.

Also I kind of liked to leave the bike looking stock and keep the plastic heat shield which looks better than a shiny aluminum or titanium can. The good thing about living in Thailand is that if you find the right people and the right shop you can get some real good work done for very little money.

So I decided to go step by step and see what happens.

There are two walls in the muffler which are connected by small pipes, there’s the exit wall with the end pipe, and then there’s the catalytic converter.




I just wanted a bit more sound and volume, no open, roaring race pipe, so I went to a shop and had the wall close to the exit wall taken out. The other wall and two small pipes stayed in.  I cut the end pipe short so that it only stuck out to the outside and then closed everything back up. The result was about a 20% increase in volume but the sound was still crappy, like a stock bike. Can’t recommend to do this. This is how it looks when you cut off the end wall:


(see attachment on the bottom)


This is how it looks after taking out the first wall with one pipe:


muff 4.jpg


Okay, so the second wall has to go, then there’s only the cat in there, see how that sounds! I had a new end pipe made from a 1,5” steel pipe and some holes drilled into the part inside the muffler. The now empty muffler body had only the cat to slow things down and a bigger end pipe. Weld it up and hope it sounds acceptable! Well it didn’t. It was too loud and sounded like a big hollow steel can which it effectively was. At least now I made some noise, I could hear the sound with my helmet on and was surprised that the bike ran like nothing had happened – no hickups or backfiring at all, just business as ususal. Amazing.


Stock end pipe, (Thai version):


muff 2.jpg


The one I had made:




Now I decided to go all the way. Cat out, core with muffler packing in! I read up on how other people did it, they went with a 1,5” core and 1.5” end pipe, and locally available fiberglass muffler packing. Several shops here in Thailand offer the complete package for 1,200 to 1,500 Baht which is roughly only about $40 to $50. But I wanted heat-resistant high-quality muffler packing which is not available here so I had to order it which takes time to ship and costs more. Then about the size of the core: the muffler body is 4” at the end wall and 6” at the widest part, quite voluminous! This ain’t no light, slim, short aluminum slip-on, this is a heavy, twin-walled steel body. A 1.5” core seemed to small, then I'd have to make up for the room around it with stuffing – 2.25” at the widest part, too much! So I decided to go with a 2” core, still enough space around it and a 1.25” end pipe, a bit smaller, keeps the volume down and provides more back pressure. (I’ve seen some 2” end pipes!)

So off to the next shop, get the core made; the FMF Racing muffler packing had arrived and we’re ready to go.


part 1 - to be continued!

muff 1.jpg

Edited by MD38E
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part II




The perforation of the core seems a bit coarse so I got some stainless steel moskito netting to wrap around it.


This time we went in from the rear, not from the side (note the catalytic converter):


(see attachment on bottom)


Only the first time we opened the can from the side which I don’t recommend:


muff 6.jpg


muff 7.jpg


Here a picture of the culprit:




Then we have to cut the core so it fits into the muffler body. Cut at about 10” to 12” and reweld the shorter end in a slight angle:




Here I already wrapped the sucker and tightened things up with wire:




to be continued



Edited by MD38E
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part III


After positioning the pipe and cutting it to the right length so it fits snug to the end wall we weld a 1” long piece of 2” steel pipe to the end wall. The 2” piece has to be a bit smaller in diameter than the core so it fits snugly inside:




Now to the fun part: stuff the muffler with packing! This packing sold by FMF Racing is designed for motorcycle exhausts and holds up longer under heat. Normal fiberglass or steel wool gets brittle, breaks down and blows out after only a few thousand miles. Since I have to weld things up I want it to last as long as possible.






I stuffed it fairly tight and used one and a half bags – the muffler body is about 18” long without the cat!

Here’s the stuffed muffler body, 4” in diameter with a 2” core, and that’s the slim part!




Now we fit the end wall back on and weld it shut.




In Thailand everybody works on the sidewalks but who cares – I paid only ten bucks for labor!

I’ve done only about 20 km with the new muffler today, it sounds great, like an aftermarket slip-on from Akrapovich, only that I paid less than $100 total for labor and material.

I can highly recommend doing it this way, wherever you are!





Edited by MD38E
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This is a good write-up for those in states where aftermarket mufflers could become an issue.  It's also saves some money if you're handy.


So are you using a fuel controller?

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Nope, the bike ran fine even with a gutted muffler, no hick-ups, now it has less flow so it should be okay.

I didn't change the intake, so no major modifications.

Next oil change I'll have a look at the plug, see if it's nice and brown.

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:D lol  yes, my mother used to watch it religiously. :D i remember, they brought it back, repeats, i got tired of it. ...Father Jack, yes, i can relate to that, i can understand, why someone would want to just stay in a chair, get pissed, and swear everyone. :D 

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