starter won't spin motor properly on 17hp briggs

seekertom

this mower was running and in constant use until the mower deck rusted out. bt new mower, decided to use old one as yard mule. worked great for a year or so. then it began having trouble cranking.

for a while i just assumed it was a weak starter motor, so hooked up a socket to a drill and helped it spin that way. again, ran great for long time. then got the no-spark problem. while testing, noticed motor would spin full speed with the plug out, but not with the plug in. even hooking jumpers directly to the motor (ruling out losses in wires, solenoid etc) and same problem. even tried a new motor, no luck either.

motor spins freely by hand with no plug. put finger in the hole and it comes out dry and free of oil. thought maybe water or oil locking it up, but again, ran flawlessly after it started.

i'm stuck with this one. any idears?
thanks, st

sdsester

How many years or miles does this thing have on it? Have you ever been through the whole engine?

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seekertom

mower is probably 5 years old, has never been molested. last time it was used regularly as a yard mule pulling a small trailer of yard debris to the road and it ran fine, no extra noises, no loss of power, normal smoothness in its running.

only problem was that it began to refuse to start normally. turn the key, it would spin half a turn, then quit spinning. only way to start it after that was to give the starter motor a helping hand with a cordless drill socketed to the nut on top of the engine. no big deal, thinking starter motor was weak, and it always started right up in one or two spins. now that i've had a chance to replace the starter motor, i see maybe it's not the starter. even with battery jumpered directly to the starter motor terminals have same problem.

only way to spin motor freely with the key is to remove the spark plug.
thanks for your thoughts.
st

sdsester

Weird. Is there a safety override or something looking for a key to allow the starter to do its thing?

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DexterII

Although not conclusive, what is the voltage of your battery? At five years or older, replacing that would be high on my list. The fact that it turns easier with no spark plug is to be expected, so that by itself is not too revealing, and tests conducted with jumper cables leave a lot to chance, in my opinion. Sdester's suggestion on interlocks would be good to follow up on first though; there is probably one for the seat, one for the transmission or clutch, and one for the mower engagement. At the same time, I would inspect the back of the ignition switch for physical damage, and, if you have an ohmmeter, test the terminals for any indication of internal damage; there should be a wiring diagram in the owner's manual, or you could go to the manufacturer's site and pull one up. Again though, would have the battery near the top of my list of culprits.

seekertom

any safety device i can imagine would be there to electrically prevent the spark. in fact, the clutch, seat etc switches, ground the coil out to prevent running (for example, when you stand up off the seat, the engine loses power. it shuts off, not slams to a halt. these interlocks would not act to allow only partial spinning. the other interlocks in that line actually disconnect 12v from the starter completely so it won't spin at all when key is turned.

mechanically, there is a blade brake to halt the blades, and i've seen cases where the belts do not disengage and the result is the blade brake prevents the motor from tuning over at all. not this case at all.

this problem is none of that. remember, when i give the starter a slight helping hand, the engine runs flawlessly.

as for the battery, it's new, but just because the old one died of old age anyhow. had nothing to do with the problem.

it's almost as if there was an internal valve that normally releases the compression until the engine starts, then seals it off, but it's broken now.

this is definitely a weird one for me!
st

Fairview

Being 5 years old and 17 hp I'll assume this is a single cylinder Briggs with over head valves ( OHV ).

A typical maintenance problem often over looked is valve adjustment and I just cringe each time I read when a new starter, switch, solenoid etc. have been replaced to no avail.

The starter motor was designed for X number of amps. and that's why jumping from a mower powerful battery doesn't help in this instance. Normal cranking amps. for this engine is around 100 and if you were to place an amp. meter on the starter cable we would be seeing in the neighborhood of 300 when the starter motor locks.

The designed compression relief doesn't function when the valve adjustment is too wide so a box wrench, torques wrench and a feeler gauge to adjust the valves may solve the no cranking problem.

There are numerous videos available in case you are new to this. If I lived next door or 7 doors down I'd do it for free

Pictured is what you'll see when the valve cover is removed.

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seekertom

finally, a direction that makes sense. thanks, fairview!

in my blundering way, i had imagined something that normally lets compression off, then allows it to build up, wasn't working now. you may have nailed it explicitly with valve mis-adjustment. i can see that with wear, the valves would tent to stay closed too long, and probably need tightening down to let them open sooner and thus to effect a fix.

it may be a while before i can get to it, but it'll be on my mind until then. thanks.
st

DrHicks

I wouldn't be surprised if you have a bent push-rod, or something such as that, that is preventing your exhaust valve(s) from opening. The fix to that problem would be pretty simple, but the cause is the issue.

It could also be a loose rocker arm nut that's causing you problems.

seekertom

well, as for bent valve, rods etc, i don't think that applies since the motor ran fine once i gave the starter a nudge. i'm going with the valve adjustment idea for now. will report on progress when i can

thanks for the input anyhow!
st