Honda GX620 Exhaust Pops

[HHR]

Have a Pug UTV with a 20hp v-twin Honda GX620 engine. The exhaust makes a loud pop/bang after you run wide open throttle and let off the gas. I've checked and adjusted the valve clearance and carburetor is clean. Turning the low speed fuel screw in or out doesn't change the symptom. Getting good spark. Runs and idles well. Flywheel key is not sheared. Compression is 148 on cylinder #1 and 135 on cylinder #2.
Could an exhaust valve be sticking or could there be carbon holding a valve open?

 

[cdaigle430]

Try a little sea foam. Had a smiliar issue with my Briggs and Stratton 24hp engine and seafoam in the tank made it go away.

 

[HHR]

We are using up all the gas that is left in the tank and will try fresh gas and Sea Foam next.

 

[SpudHauler]

My Kohler 27 hp does the same when you shut if off.

Excess fuel dumped into hot exhaust exploding.

Some companies dump extra air into the mix to lean it out so this doesn't happen.

Not allot you can do really.

Adjust the idle mixture screw to make it rich would help some. Letting it run at an idle for a minute or so before shutting down helps our situation.

Try moving to the idle position from wide open slower might help. It's all about fuel being dumped into a very hot exhaust system.

My 2 cents.

 

[HHR]

The carburetor has a solenoid on the bottom that cuts off fuel flow when you turn off the motor. The solenoid works properly and the only time I get a backfire is when going wide open down the road and lifting off the throttle. As soon as the engine goes to idle, you hear one large "pop"
Screwing the mixture screw in or out makes no difference in the pop condition.