Most people will tell you the only way to quiet a generator is to not use it.
This awesome method, however, could be a game changer!
I bought a 10HP lawn mower muffler and welded it between the factory “spark arrester” and the muffler mounting flang.
Then I stuck the thing back on the generator and braced it back to the motor like it was.
The muffler has to be mounted back to the MOTOR, not the frame, so that the muffler moves as the motor does.
I know the lawn mower muffler was designed to be stand alone and let pressure out the other end, and it's probably under more pressure feeding exhaust into the Spark Arrester, But several professional mechanics have said the muffler will hold up and that adding a little more back pressure won't adversly effect the generator (Other than delimiting the power a smidgen). If there's a difference in power, I can't tell it.. it just runs MUCH quieter.
UPDATE, 16 January 2007:
I went to Advanced Auto and bought a generic “Thrush” muffler for $19.95.. it's on the Isle with their muffler pipes and fittings. Then I bought a 2.5 inch adapter, and the Flex hose to fit between the adapter and the muffler… also buy a few 2-1/2 inch clamps…
Weld the Adapter pipe OVER the exhaust outlet hole on the Spark-Arrestor, and then put the muffler back on the Genset… this allows you to use the factory set-up on the Genset, OR add the Car Muffler if you need the generator quieter… the adapter doesn't even stick out if you have a wheel set on your generator.
Now Go Ahead and Put the Flex hose on the muffler and clamp it down.. you can store this set-up with/near the generator.
As I said, you can use the generator with the normal factory Spark-Arrestor, OR you can clamp on the pre-assembled flex hose with the Muffler attached for MUCH quieter operation.. with this, I can put the Generator in the Shop (with a box fan blowing on it to cool it) and pipe the muffler outside.. you LITERALLY CAN NOT hear the generator running from the front of the house (with the Genset in the shop at the back of the house) when you do this.. the building blocks the motor noise, and the big Car Muffler quietens the exhaust to almost nothing.
If you do this in conjunction with the inline-10HP muffler, you can WAY outdo the exhaust noise from a Honda generator, about ALL you can hear standing beside the generator is the Clanking of the Generator engine, and the “SHHHHHHHHH” noise of the generator head (A noise I never knew it had until I added the bigger car muffler).. WITHOUT the inline Muffler, it's still quieter than a Honda Genset.
People have been asking how quite this REALLY makes the Generator, since the Coleman is among the loudest portable genset out there (among the popular brands), so I made the video Below… You can't even hear it from the sides or front of the house.. NO WAY my neighbors would hear it now, even late at night (When this video was shot).. as the video shows, just turning your back on it makes it almost non-hearable.
While admittedly, these instructions are kind of spotty, it is not hard to get the concept of what you can do.
You can also consult a professional welder to get their advice if welding is not in your personal tool kit or hire them to do the basic welding work.
To learn more about this amazing way to fully quiet a generator, please visit Alpha Rubicon.