I have a lawn tractor with a 12.5hp briggs I/C motor. The problem is that the motor is loping. By that I mean that the motor sounds like it is cutting out and refiring every second or two. The rpm's are up and down at a continuous steady rate. I have a new plug, air filter, fuel filter, fresh gas, muffler and I cleaned the carb. I would think that it would be an ignition problem but when I engage the blades and put load on the motor, the problem goes away and the motor runs nice and smooth. If anyone has any ideas, I would greatly appreciate hearing them.

Isolate the engines ignition from the mowers safety switches and see if it clears up. There may be an issue with one of the safety interlocks causing the ignition to cut out.

I'll give it a shot and let you know what happens. Thanks.

I isolated the safety features from the ignition and I still have the same issue.:(

I isolated the safety features from the ignition and I still have the same issue.:(

Have you tried giving it some choke? If it improves,I would guess that the pilot system in the carb is restricted.When it's running without a load,the throttle plate is actually almost closed,and the engine's running on the pilot system.Under a load,the main jet takes over.Most times if the low speed is restricted,it will not accelerate good,though.
The pilot passages are the tiny,hard to clean out ones.

Take care,
Steve

I did give it a little choke and it did smooth out quite a bit. So what do you think I should try? Thanks, Mike

I did give it a little choke and it did smooth out quite a bit. So what do you think I should try? Thanks, Mike

If it smooths out and runs better with some choke applied, then it's time to have a look inside the carburetor. May need cleaning and or a kit installed.