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B&S 16 hp OHV won't turn over when hot

15 years ago

A friend is a very experienced small engine mechanic. He is working on a 16 hp B&S OHV engine, a model 310707, 98 year, that will crank and start just great but after it reaches operating temp, the starter will not turn it over for maybe an hour then it will crank and start just fine again. It runs good until it is intentionally stopped. It is as though the compression release doesn't work when the engine is hot.

It has a new heavy amp battery. I was there and we check the operation of the cam shaft as well as watching for the "bump" indicating the compression release was working. Valves have been set and re-checked. He has had the head off replacing the head gasket and said the cylinder wall was perfect. It has a little chafe in the cooling fins but nothing serious.

I suggested he try jumping directly to the starter and even switching starters thought I doubt that is the trouble but he has an extra new starter anyway. There was one connection that I questioned, perhaps not carrying enough current when engine is hot and cranking requirements are greater.

I did notice something that was not right. The aluminum push rod was on the exhaust valve instead of the intake as it should have been but I can't figure out how that would cause the problem. He hasn't had time to try the things I suggested yet.

Any suggestions?

Thanks,

Walt Conner

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Comments (6)

bill_kapaun
15 years ago
Well, time for the "off the wall" suggestions I guess? :-)

Push rods-
Is the compression "bump" on the intake valve?
IF SO, maybe the steel push rod doesn't expand enough with the heat, (compared to the barrel) resulting in an effective increase in valve lash?
Aluminum expands about twice as much per degree as steel.

I think that's an avenue worth pursuing. It might be only a few thou difference, but it might be enough.

IF that doesn't do it, I swap the starter, unless he's "literate" with a voltmeter.
In that case, I'd check for VOLTAGE DROP across the various connections to look for an obvious trouble spots.

I'd divide the circuit into 3 sections-
1 Battery + POST to + starter motor POST.
2. Starter motor POST to starter motor CASE.
3. Starter motor CASE to battery - POST.
Check voltages with the engine cranking.
A "perfect" reading (doesn't exist) would be 0 volts drop.
If any (one of three) circuit is excessive, one can then narrow that part of the circuit to its smaller parts.
Example circuit 1-
Battery post to cable connector, to cable, to connector, (on other end of cable) to solenoid stud, to other solenoid stud, to starter motor stud.
Typically, you'll have a lot of small voltage drops (one at each "junction") that adds up to a more substantial total drop.
Circuit#
1 tests + side
2 tests the motor
3 tests the ground side.

Also make sure starter is well grounded to the block.
Am I rambling yet? :-)
lkbum_gw
15 years ago
Sounds like the starter motor (most likely windings) are bad. The insulation on the windings will degrade over time, the di-electric value of the insulation on the wire will decrease as the insulation gets hot, causing the windings to short internally. Depending on where the short occurs, the motor may turn over, but will have no power at temperature. When the short occurs, the amperage draw is near what the motor will draw when working fine (it's not a dead short, just breaking down enough to keep the full windings from recieving the power they need).
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walt2002
Original Author
15 years ago
Well switching the push rods and trying a new starter did not help.

Walt Conner
njdpo
15 years ago
i had several experiences with "intermittent bad" starters in the last year ... im wondering if this might be your problem... it drove me crazy - the moment I replaced it I could instantly tell I solved the problem on the first startup.

good luck - Dave
tomplum
15 years ago
I would assume that cables from a known good battery went to the starter lug and then ground with the same results. I would also assume that it cranks well pulling the spark plug when it is having one of it's fits. I had a 28 that I put a new cam in 2 years back that didn't play well after any amount of running. Checked the rocker studs, adjusted the lash etc to no avail also. The cam was of a different design than the original btw. I assume a 31 CI could have the same issue.
For an easy thing to try, one could try switching the coil nad checking the flywheel key. Maybe the CD is throwing the spark at an inconvenient time hot...
ray_okla
15 years ago
Somebody may have installed oversize rings on a standard piston. Check the ring gap.

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