Problem with 25hp 730 motor. Slow cranking no start... WITH VIDEO

timmat1

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  • / Problem with 25hp 730 motor. Slow cranking no start... WITH VIDEO
Background.
I've owned it about 3 months. Used it about 6 times with no issues. Then 7th time it wouldn't start. It has a broken throttle that leave it on full throttle.

I've replaced starter, both coils, and spark plugs. Compression test shows about 50psi in on and 60 in the other and both have about 50% leak down.

I'm assuming head gasket but would love some advice. Thanks

Video (not sure why it sounds like I'm pinching my nose... I'm blaming my phone) :)

https://youtu.be/hvxJMdvaXeQ
 

Boobala

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  • / Problem with 25hp 730 motor. Slow cranking no start... WITH VIDEO
GEEEZ man, that thing sounds like a "thrashing-machine" when starting, sounds like the starter is grinding, but then again, it sounds like the pistons are dragging in the cylinders, did it run out of oil ?? or overheat ?? you say you replaced the starter,.. is it the correct model, (pinion-gear ??) I would pull the plugs and with a strong flashlight, look in the cylinders for scored walls, does it rotate EASILY by hand with the plugs removed ?? In my opinion you have a SERIOUS issue with that engine, and why the huge start cart, will it attempt to start with a fully charged mower battery ?? IF you need that many Amps to roll it over you DO have a problem !!
 

bertsmobile1

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  • / Problem with 25hp 730 motor. Slow cranking no start... WITH VIDEO
no way to diagnose that over the web.
It sounds like you have the wrong starter and the gears are not meshing properly.
A lumpy turn over like that is generally too much valve lash or even a dropped con rod, but in your case it is hard to tell.
So off with the rocker covers to check the valve operation look to see if the engine sticks when one of the valves is being pushed down ( bent valve stem ).
Can you turn the engine over easily by hand once you remove the spark plugs?
Did you run it without oil ? could be a partial seizure leaving a build up on the crank making it hard to turn, but the difficulty seems to be on every second stroke which points towards a cam / valve problem.
 

timmat1

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  • / Problem with 25hp 730 motor. Slow cranking no start... WITH VIDEO
GEEEZ man, that thing sounds like a "thrashing-machine" when starting, sounds like the starter is grinding, but then again, it sounds like the pistons are dragging in the cylinders, did it run out of oil ?? or overheat ?? you say you replaced the starter,.. is it the correct model, (pinion-gear ??) I would pull the plugs and with a strong flashlight, look in the cylinders for scored walls, does it rotate EASILY by hand with the plugs removed ?? In my opinion you have a SERIOUS issue with that engine, and why the huge start cart, will it attempt to start with a fully charged mower battery ?? IF you need that many Amps to roll it over you DO have a problem !!

Without the spark plugs in, it turns easy. I replaced the old one with the same model. (maybe previous owner had wrong one on it) I just have a bad battery. It started about 5-6 times I used it this year.

I do not believe it ever ran out of oil (always has been in the checkered area since Ive owned it) or overheated (how can you tell)

I'm pulling the heads tomorrow. I'll take pics and that might be able to help.
 

timmat1

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  • / Problem with 25hp 730 motor. Slow cranking no start... WITH VIDEO
no way to diagnose that over the web.
It sounds like you have the wrong starter and the gears are not meshing properly.
A lumpy turn over like that is generally too much valve lash or even a dropped con rod, but in your case it is hard to tell.
So off with the rocker covers to check the valve operation look to see if the engine sticks when one of the valves is being pushed down ( bent valve stem ).
Can you turn the engine over easily by hand once you remove the spark plugs?
Did you run it without oil ? could be a partial seizure leaving a build up on the crank making it hard to turn, but the difficulty seems to be on every second stroke which points towards a cam / valve problem.

I will double check for the correct starter. I just took the part number off the one already on it.
I'll be pulling the heads to inspect for issues tomorrow
The engine turns over easy with no plugs in
Always had oil since I got it, but not sure about the previous owner.

Any chance head gaskets could do this? Lots of air heard when doing blowdown test
 

bertsmobile1

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  • / Problem with 25hp 730 motor. Slow cranking no start... WITH VIDEO
Blown head gaskets make the engine easier to turn over because the starter is not fighting engine compression.

Your engine is doing 1.75 revolutions, hesitating then overcoming whatever is preventing it turning over then doing another 1.75 turns.
That behaviour is typical of excessive compression in one cylinder.

The other thing that does that is sticking valves , bent cam, bent valve, valve seat dropped out holding valve open which is hitting a piston.

However those things happen plug in or plug out so we are back to excessive cylinder pressure
So start by removing the rocker cover and looking at the valves & rockers.
I will almost bet one side is decompressing and the other is not
 

zman111666

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  • / Problem with 25hp 730 motor. Slow cranking no start... WITH VIDEO
That sound is weird to me. Is it coming from the starter? It almost sounds like the relay is vibrating. Might want to try jumping straight across the relay, and see if it sounds any different. Of course, it could be that it's just the way it sounds on my speakers, or the camera is picking up noise from the battery charger.
Also, I never had an engine crank very well on a charger, even on a 50 amp jump setting, if the battery wasn't decent.
 
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