Briggs/Stratton 3.5 Hot Start Problem

[wbrian63]

I have a McLane lawn edger with a Briggs & Stratton 3.5hp engine. I'm not sure of the year model - bought it used. New-ish, I'd say - has a paper filter element instead of the old oil-foam filter.

When I got it, wouldn't run at all. 1 carburetor kit later and it fires up and runs 95% - there's a miss here and there at full throttle, and it backfires if you cut the throttle too fast.

However, once you've used it for a few minutes, if you stop the motor and try to restart it a few minutes later - no dice.

My typical experience with B&S (and other small engines) is that once hot, you don't need any choke to get them to restart.

I've tried all combinations of choke and throttle position - nothing. Not even a pop or an attempt to start.

I'm wondering if I've got a vapor lock problem. I'd expect to smell gas if I was flooding it with too much choke.

Suggestions welcome...

 

[Davefr]

You need some more diagnosis:
1. When it fails to restart pull the plug. Is it wet?
2. Now, ground the plug body and see if you get a nice blue spark across the tip when you pull the cord.

You can also put a teaspoon of gas in the cylinder and see if that makes it pop.

You need compression, spark at the correct timing and fuel. Find out which one you're not getting and go from there.

 

[M6erfan]

Airfilter clean?
Correct spark plug in good shape with proper gap?

If in doubt on the above, I'd get new replacements and start over from there. Need to have a proper baseline to work from

 

[laser3kw]

you may have a valve lash problem. The valve seats will sink over time and get to a point of zero lash. You should be able to check it cold. It should have at least .005" up to .009". If it doesn't, do a good valve job, cut the seats and lap, and straighten the head and block out before you do the valve job.
I had another similar incident.
The shop that had it before me put a new carb on it - didn't help, so I got it.
I dissected it and found it plugged with grass under the shroud. I would get hot enough that the exhaust valve seat came loose. When it cooled down, several pulls on the recoil would knock it back enough to where it would run. It was un-savable - it was run long enough to where I couldn't peen it back into place. The other option was a custom size valve seat and the cost that goes with it.

 

[redmondjp]

Not quite as likely, but it could be heat soak causing the ignition module to fail - as stated above, check for spark when it is hot.

 

[MTW]

I'd say you have a intake or exhaust valve that is not seating when hot. Too little valve lash clearance, once the valve heats up and expands, creating less lash between the valve stem and the tappet, the valve starts leaking combustion pressure.

Symptoms- Starts fine cold, looses power when warm, backfires when hot (leaking) , wont restart when hot, very little compression when hot (spins over too easily). Once it cools to room temperature the cycle repeats.

The fix is to remove the head and valve tappet cover and check the valve to tappet clearances. Compare to the factory specs for your motor. Remove the valve that is too tight and grind the appropriate amount of material from the valve stem end to obtain the proper clearance. Do a small amount at a time and remeasure to avoid going too far.

Clean the carbon buildup off of the valve and seat with a soft wire wheel. Reassemble with a new head gasket and valve cover gasket.

I don't bother re-cutting the valve seat, its worn already and that's why there is not enough valve lash. Cutting the seat removes even more material. I just clean it if it's not pitted from blowby, and give it a light lapping with some compound. I would suspect that yours is in decent shape because it seals properly when its cold. Continued operation in this condition will cause the seat and valve face to become pitted from the leaking combustion gasses.

One B&S mower that I have is on it's 3rd valve adjustment in 20 years of service.

MTW Ω

 

[theoldwizard1]

Vapor lock. My trusty old Toro with an early B&S Quantum will do it from time to time. Just wait it out.

 

[laser3kw]

One B&S mower that I have is on it's 3rd valve adjustment in 20 years of service.

My "daily driver" push mower I bought brand new 18 years ago, Ranch King with Briggs motor.
I just did it's first valve job! Ran good before, just a little sluggish. Pulled it apart, the head gasket was blow from the deck sagging and head warping. The valve lash was almost gone (from the deck sag).
Cut the deck, cut the head, cut the seats, install new valves and springs, set the lash. Starts on first or second pull.