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Hartman mod (drilling/gutting the stock muffler) (Read 1150 times)
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Hartman mod (drilling/gutting the stock muffler)
03/09/10 at 09:37:27

The Hartman mod is to gut the stock muffler and open the hind end a bit.  It's a good mod if you don't have access to anything else.

You will need to rejet so have that in mind before you do it.

After fires are common regardless of the muffler, I suspect that it could get worse which I would be concerned with as the reply to your after fire maybe a lead burp from an ak47.  When I 1st tuned my latest bike the afterfire resembled the report of a high powered rifle.  Which set me to giggling, shortly to roflmao when the neighborhood came running out of their front doors.  Nearly pulled the bike on top of myself.

A little higher idle and a little richer idle mixture should get it down to a phfft.
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« Last Edit: 07/14/12 at 10:54:54 by Oldfeller--FSO »  
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Re: Hartman mod
Reply #1 - 03/12/10 at 08:58:22
I did a Hartman muffler mod a few years ago.  I didn't take pictures but if you go to

you can see details.  There are a couple of different stock muffler baffles.  My bike is a 2003 and the muffler was as illustrated at the bottom of the page in the link above.  I used a circle cutter to cut out the baffle unit.   Instead of separating the two baffle parts and then replacing the pipe with the holes in it, I just discarded the entire unit.  I then fabricated a 1/8" thick aluminum disk with the center whole the same size as the original exhaust hole and fastened it with 4 large aluminum rivets.  I made the fit as perfect as possible to prevent edge leakage.  I didn't use any fiberglass packing.  

You can see the finished mod in this picture:

I turned the mixture screw out a turn or so but no other changes.  It runs well and sounds great.

I experimented with a couple different hole sizes but settled on the original size for back pressure and it just sounded the best.  Nice fat sound with no whistles etc.

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Re: Hartman mod (drilling/gutting the stock muffle
Reply #2 - 09/28/21 at 09:10:39
I have done very little besides raising the needle in the carb:   Stock air cleaner and two 3/8" additional holes in the back plate of the muffler.  Hardly any louder, but, the other day I came around a corner and was surprised by a horse and rider.  I let off the throttle quickly and the back fire was loud and sharp!  Thankfully the horse remained cool and didn't buck.  I think the idle jet is ~2 turns out.
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