My go-to man can't understand lawnmower problem

CelticRambler

Ride-on mower with a Briggs & Stratton OHV engine, has been acting up for a long time when trying to start (took two, three or four turns of the ignition for it to get going). The guy whose brains I usually pick for engine problems was here on holidays for a few days, and admitted defeat when he found that he could jump start the engine as long as the spark plug was removed; putting the plug back in and the engine tries, but can't turn over.

Any of you guys know what the problem might be?

For info, he checked the following:
- mower battery OK, holds 13.7V charge, drops to 9V under load
- mower battery turns the engine with no spark plug
- 110AH car battery won't turn the engine when spark plug is in place
- starter motor removed, bench tested, bendix lubicated, reinstalled, alignment checked, etc, all OK
- ring gear checked, not particularly worn.

Mower last used successfully at the beginning of August; refused to start beginning of September; has been sitting outside but under cover (its usual place) ever since.

Gooser14

Is it possible that the valves are stuck in the closed position for some reason. Check the valve movement.

CelticRambler

I don't get on well with the internal workings of engines! What valves where, and how would I find/check them?

_Brian

Yea. Maybe calves stuck or timing has slipped causing valves both closed building too much compression.

Hocus Focus

Check the connections on the battery, the one connecting the battery to the starter, the one connecting the battery to earth (the frame/body), etc., the battery while fully charged and turning the engine with the plug out might not be able to transmit enough voltage to the starter due to a bad connection somewhere, when asked to turn over the engine with full compression.
The fact that it turns over when uncompressed rules out any problem with mistimed valves. If there is a pull-start mechanism,does it start with this?

CelticRambler

_Brian wrote: »

Maybe calves stuck or timing has slipped causing valves both closed building too much compression.

Definitely no calves stuck - they're all with their mammies in the field across the way! As for valves ... I

Hocus Focus wrote: »

Check the connections on the battery, the one connecting the battery to the starter, the one connecting the battery to earth (the frame/body), etc., the battery while fully charged and turning the engine with the plug out might not be able to transmit enough voltage to the starter due to a bad connection somewhere, when asked to turn over the engine with full compression.

All done by my friend. He also swapped out the diddly little lawnmower (bought new earlier in the year, to replace the original 10-year-old on the ground that it might have been getting weary) with a fully charged one I use in my 2.8l diesel motorhome. It made no difference. To show me what he couldn't understand, he wired the MH battery directly to the starter motor (6mm² jump leads) and demonstrated how placing the spark plug barely one full turn into its housing was enough to stop the engine turning over.

Hocus Focus wrote: »

The fact that it turns over when uncompressed rules out any problem with mistimed valves. If there is a pull-start mechanism,does it start with this?

It's a key-operated ignition; I forgot to mention that it's a four-stroke engine, if that's of relevance.

Gooser14

Have a look at the following:
https://www.youtube.com/watch?v=KM1HJrGZwa4

CelticRambler

Thanks for that link, Gooser. Several things in the video and the comments section that I can follow up on in due course.

CelticRambler

Well, that was the best 2€20 I ever spent. Several weeks and one set of feeler guages later, the engine is running better than it has in a long time. Thanks again to all of ye who pointed me in the direction of valves I didn't know existed!