Briggs & Stratton mower engine races and drops like clockwork

[JM_newsgroups]

Hello,

I have a base-level 3.5 HP Briggs & Stratton engine that's about 6 years old. It's a throttle-free design with a primer bulb. Lately, after I get it started, it goes in to a rhythm of racing and dropping off about every second. This started rather suddenly. The plug is OK. I started to take it apart, because I didn't even know if mowers had "points". I assume it's some kind of induction ignition system based on the setup of the cutoff switch and the magnetic flywheel. The carb looks like nothing more than a plastic throttle body, and I don't see a float/bowl of any kind. I generally don't repair mowers, obviously. Does anybody recognize that surging/dropping behavior? Thank you!

Thanks,

JM

[SHOPDOG]

SOunds like the diaphragm, which is between the plastic carb body and tank

[George]

The governor would be a candidate. I had that with a broken governor spring.

G

[Bob F]

Or, just a mixture adjustment.

[pheeh.zero]

Try taking off the top rewind (pull-starter). You may have the classic "critter nest" under it preventing the air vane from moving. (If I remember that motor correctly)

pz

[ransley]

It has a govenor controlled by air I believe from the flywheel it could be dirty.

[Red]

A very dirty air filter can cause it also.

[JM_newsgroups]

Thanks, gentlemen. I had pulled the cover off and cleaned it. The air filter, governor, and the air-controlled assembly looks OK. I reassembled and had the same problem. I'll give Shopdog's suggestion a try, because I had seen that mentioned for another B&S motor. The condenser was kind of funny to reinstall, because there is enough play to have it stick right to the flywheel.

[Stormin Mormon]

Condensor? Older Briggs motors, the condensor was under the flywheel in the aluminum covered center. Newer Briggs don't have a condensor.

[Stormin Mormon]

When a small engine races and slows, sometimes the problem is that the governor spring has gotten stiff. They are supposed to be springy.