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I have an old 28D7070123-01 11.5HP engine on an MTD riding mower. When I try starting it, the flywheel only turns part way then stops. I also hear a wheezing sound that sounds like pressure being released. I tried starting once with the sparkplug removed and the the flywheel rotated normally. I removed the cylinder head and rotated the flywheel by hand and it turns smoothly and the valves seem to work. From some internet research I think it might be the compression relief part, but I can't find one in the parts manual, and I'd really like to be sure before taking the whole engine apart. Does anyone have any advice for diagnosing it?

Do you Know your battery is good?
With the head off, rotating the engine by hand look at the exhaust valve on the compression stroke, it should bump open a little, then close.and then open when the piston comes back up on the exhaust stroke.

Scrubcadet10 said:

Do you Know your battery is good?
With the head off, rotating the engine by hand look at the exhaust valve on the compression stroke, it should bump open a little, then close.and then open when the piston comes back up on the exhaust stroke.

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I know what you meant.....

A lot of the flat head engines didn't have camshaft based compression release mechanism. they used a cam lobe that held the valve open just a little longer than normal that operates like that all the time. I would check valve clearances and see if they are too wide or not. Most of the L head engine normally close the gaps from seat and valve face wear and will float the valves causing low compression not the issue that you are having.

ILengine's right, looked at a picture of the Cam, disregard my previous post.

ILENGINE said:

I would check valve clearances and see if they are too wide or not.

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Can you recommend a link with good instructions on how to do that? Most of the stuff I find online is for overhead valves

MowSlow said:

Can you recommend a link with good instructions on how to do that? Most of the stuff I find online is for overhead valves

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If it's a Flathead, most flatheads, you take the valve cover / breather assy off, that is where the lifter meets the valve, there are no push rods. That is where the lash is measured and it is not manually adjustable like a OHV.
When I used to build Flathead racing engines for racing go-karts, I use to file the end of the valve to get the proper measurement but, I didn't care about the lash, I cared about when the valves opened and closed in relation to TDC, on a degree wheel!

not your specific engine but a good guide on what to do what you'll need

I checked the valve clearances and they're good per the manual. Anything else I should check?

can you spin the top of the valve in the seat? place your thumb in the middle of them and try to spin them back and forth.

MowSlow said:

I checked the valve clearances and they're good per the manual. Anything else I should check?

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Check your battery, I mean really check it, with an ohm meter as you engage your starter and read that #, what is that #?
If battery tests good, check flywheel key, if it sheared, it moves the Ignition timing and it can "kick back" when you try to start it, kinda like you are describing....but 1st, BATTERY!

Grab a pair of jumpers and jump the starter directly from your car.
Spins normally = starter OK
Now jump directly from the mower battery
Spins normally = mower battery good
Jump with only + lead
Spins normally = ground connection/ cables good
Move jumper to the starter side of solenoid
Spins normally = starter cable is good
Move jumper to battery side of solenoid
Spins normally = solenoid & battery cable is good

mower king said:

with an ohm meter ...If battery tests good, check flywheel key,

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An ohm meter or a voltage meter? And how do I check the flywheel key? Do I need to remove the flywheel for that?

Scrubcadet10 said:

can you spin the top of the valve in the seat? place your thumb in the middle of them and try to spin them back and forth.

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I can spin them but only when they're open (the exhaust valve is a little tougher). Is that the way it's supposed to be?

MowSlow said:

An ohm meter or a voltage meter? And how do I check the flywheel key? Do I need to remove the flywheel for that?

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I use an OHM meter set to 12DC.
To see the flywheel key completely, yes, remove the flywheel...but 1st...check that battery volts under a load of the starter....like I explained earlier!

MowSlow said:

An ohm meter or a voltage meter? And how do I check the flywheel key? Do I need to remove the flywheel for that?

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voltage meter is good enough, set it to DC volts, 20v. you may need a helper, check the voltage with everything off, key off. then have someone hold the leads to the battery posts while you try to start it, if it drops down to about 10.5 or lower volts, most likely the battery is toast.
this is similar what i use,

Scrubcadet10 said:

voltage meter is good enough, set it to DC volts, 20v. you may need a helper, check the voltage with everything off, key off. then have someone hold the leads to the battery posts while you try to start it, if it drops down to about 10.5 or lower volts, most likely the battery is toast.
this is similar what i use, View attachment 54008

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2 wires with alligator clips on the ends = 1 man testing operation.

I bought a set of those alligator clips.... Have no idea where they ended up though

Scrubcadet10 said:

if it drops down to about 10.5 or lower volts, most likely the battery is toast.

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It meaures 12.7v with the key off then drops down to ~9.8v when I try to start it. But it still spins better than it did before - probably because I have the carb. and valve cover off. I guess I need to buy a new battery

MowSlow said:

I can spin them but only when they're open (the exhaust valve is a little tougher). Is that the way it's supposed to be?

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yes

Take the head off, and remove all of the carbon built up in the combustion chamber.
The engine should crank easier after you do this.

I cleaned the innards with some carb cleaner, oiled some things, put it all back together, recharged the battery and now it runs. I have no idea what I did, but thanks for all the tips.