My Lawnmower Forum banner

B&S 3.5hp wont start after warmed up

5641 Views 6 Replies 7 Participants Last post by  barterjoe
I have a pair of 3.5hp B&S engines on push mowers. Both exhibit the same symptoms.

Engine starts and runs fine when cold.

Engine runs smooth at low and high throttle. (let it warm up for 10 mins)

Shut down

Wont start again, any throttle position.

Once the engine cools down, start no problem

I have cleaaned the carbs, tanks, air filter, etc.

I did notice that the choke flap always closes, after the engines run, no matter how warm (or hot) they are.

Any suggestions?

1 - 7 of 7 Posts
If these are older engines replace the points and condenser (one part Briggs number 294628). What you are stating is common symptom of a bad condenser. If the motors don't have points never mind. Roger
Hi Owen2008x1
Those older 3.5HP Briggs have a carburetor diaphragm that controls the choke. It is supposed to work like an auto choke. It sounds to me like the choke is not opening up correctly If it is that style that would be the first thing I would check
Yes, the automatic chokes were an idea that really didn't work as well as planned, I don't think. There might be a way to adjust them but don't have the info at hand. Check B&S website for your engine model and see if there is something available.

I used to have a mower with this setup. I found a screw driver holding the choke open solved the problem, tho it was a compromise.
If this engine is a 1981 or newer it will have electronic ignition. The symptoms for a bad ignition coil are that when it gets hot it shuts off then after cooling it will return to normal opperation. Eventially it will quit rurnning regardless of temp. The choke that is controled by the diaphram in between the carb and the tank is supposed to remain closed when the engine is not running. It uses the vacume from the running engine to pull the choke butterfly open. You can check it by removing the air filter and pulling the starter rope. If you see it open and close several times then it is OK. If it dose not open and close then the diaphram is most likely torn/damaged or the linkage to the diaphram is possibly unhooked or damaged.
Test for spark after it stalls, if no spark is present replace the points/condensor ($5) if your engine does not have points try removing the kill wire from the coil, if that doesn't fix it you have a bad coil, try getting a used one, they're rather expensive.

More likely however you have a Briggs engine with the "Choke-o-Matic" carburetor, this is a carb with a vacuum controlled choke, on this carb the choke is always on when you start the engine regardless of whether you need it or not. Try sticking a screwdriver or something in the carb to hold the choke open, if it starts it's too rich and you should lean it out a little bit.
The auto choke if equipped is most likely the problem. The diaphragm has to be "preloaded" so that as the choke works properly.

Here is an excerpt from the 1919-1981 service manual.

The following link is for a model like yours.

you can get a hard copy of the manual from Arco Lawn Equipment | Home or download it here:

I hope this was helpful
See less See more
1 - 7 of 7 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.