I ran my Onan without the muffler for awhile and was surprised to find out that it ran lousy. One night laying in bed, it dawned on me;
1. Engines run better with just a little bit of backpressure
2. Because the pipes from both cylinders were not connected it lost power because I wasn't getting the scavenging effect I would get with a crossover
3. Finally, because the muffler's outlet is offset to one side the pipes going to the muffler are different lengths

Yep..your on the right path...Take it to an exhaust shop and have them fab up a set of exhaust stacks that go vertical like a pulling tractor and then readjust the carb (cause it will affect its adjustment)..Then she should run great!!we have picked up more torque in these engines just by taking off the mufflers and putting on stacks...I think it was some 10ft that we gained...So theirs potential in these engines....You just got to find the right combination of things to release the horsepower...I know how to put a 20hp carb on you the sears tractors..What model do you have??AN ss/16?

Actionpark, all engines run better with no backpressure. Check the B&S web site: 5hp go-cart engines produce 5hp at 3600 rpm but at 6000 rpm the hp is down to 2 1/2 or so. Taking the muffler of brings the 6000 rpm hp back to a little more than 5.

Your power problems could have been due to a lean mixture.

Actually, my Onan is running way too rich. Every time I drive it i come inside smelling strongly of exhaust. The problem is that the mainjet adjustment screw is stuck and I can't tune it. Today it didn't want to run smoothly, so I took off the air cleaner to access the carb and it ran beautiful.

you guys have to realize though, I want a screaming monster, but I do landscaping with my buddies and I want it to be economical.

Sounds like you need to rebuild the carb...Its gonna cost you like $100 for the rebuild kit, but it will work GREAT!!! mine run beautifully now..Onans are real power houses..they will sit and run all day and give you great power...and some are economical..My 16hp engines are ok, but my 18hp and 19.9 are gas hogs!!

My 16hp is a PUMP SUCKER!

If it runs a lot differently without the air cleaner than with it, ther's your problem. Too much restriction in the air cleaner. You have either a dirty element or some other restriction.97XG