not sure if this is the right forum, but.... my yz250f was kind of tinny sounding and i had to lean the idle air mix out more than usual. so, i repacked the exhaust, but i ran out of packing so there is some space in the oval can top and bottom, it is not packed tight. so i rode today, and i had to lean the idle air screw out more again. my question is, how much, and what exactly will a bike do if the exhaust needs packed, or is packed loosly like mine.....it seemed to run fine once i got things adjusted, but i was on a tight single track so other then second gear it was not opened up. it sounds fine, i am hoping i don't have to repack again???

if its not packed tight it wont last very long

i am wondering if it effects performance or just sound. I feel it only effects the sound of the bike, and a few bike mechanics in this area also feel the same, but on the internet it is said that if the exhaust is not packed right, or fresh, the 4 stroke will not perform as well. i am trying to find out the reasoning behind this, as most motocross bikes have a straight through pipe  inside the can anyway.  I don't see the logic in how bad packing can effect performance....

maybe distributes heat better?

crzyatv said:

i think that is for sure, i know if there is not enough packing the pipe can get too hot and actually come apart. but would distributing heat make the bike run better?

spinner38 said:

It is very important for a 4 stroke muffler to be packed "tightly"(more so than a 2 stroke) And it needs to be a high quality packing like Silent Sport Premium. Cheap packing can burn up and overheat the muffler can.

In a 2 stroke or 4 stroke muffler when the packing burns up it creates voids and there is a flow loss. This is due to the exhaust gasses entering the can through the baffle holes, this creates turbulence and lowers flow through the muffler.

A properly packed muffler will improve flow through the system and increase power.

455 said:

ok, that makes sense to me......

it will effect the power a little because mine got wet and it got loose. i noticed a big loss of power

so for performance, you would want to cut out the baffled section inside the can and weld in a solid pipe straight through, best performance, but very loud correct? pack the muffler tightly, too tight, best performance but too loud since the exhaust gas cannot get into the tight packing? pack it jsut right, noise supression and exceptable performance. pack it too loose, noisy will not last long and as the packing breaks up performance will drop.....how does this sound, about right?

dont mean to thread jack but would a pipe with no packing cause engine damage i.e piston hitting valve

04rmz250 said:

Nope.

I saw a guy riding a KTM with the endcap and all the guts ripped out of his muffler.  Idiot.  It sounded terrible, but the bike seems to run okay.

04rmz250 said:

Here is some food for thought. Why do exhaust manufacturers give you jetting specifications when you buy an exhaust system? Why are there different stages? Stage 1 - 2- 3?  It all has to do with flow of a motor. Air in and Air out. Have you known anyone that didn't rejet their bike after putting an exhaust system on? Or remap their EFI? Look at the header pipe and you will see that it is a lot more colorful. This is due to heat. Which is a bad thing.

That is why as others have already stated that it is best to pack your silencer tight. It will cause problems if you don't. You may even notice that it may pop and crackle more when you chop the throttle. This is because your sucking in air through the exhaust (which happens anyway) and without the packing it gets a little more.

Just food for thought...

"carry on"