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Hi. I just inherited a Snapper Zero-turn mower (model 7800795) that has a 21 hp Briggs and Stratten motor (model 33M777-2117). It ran great for the previous owner until he did a little work on it, got disgusted, and bought a new electric motor to replace it.
"My New Snapper" has a new battery and starter and cranks, but gets stuck. By stuck I mean the engine turns for a bit then it stops. I know the started has power because the battery cable starts to smoke. My guess is the valve adjustment is out of whack. If I remove the spark plug the engine cranks just fine.
How is the valve adjustment done? (I can't find a service manual online.) I'm sure there are timing marks on the engine but I have no idea where they are, and since the motor is on the mower, I probably won't have access to them anyway.
I'm tempted to try to set the valves by slowly turning the engine by hand and adjusting the valves so they never have more than 0.04 mm lash, or whatever the value is supposed to be.
Suggestions?
Thanks in advance.
Mike

The proper way to adjust the valves is to remove the spark plug and turn the flywheel to 1/4 inch past TDC on compression stroke which should allow both valves to be closed. And then set the clearances to .003-.005 for both valves on your engine. The dirty way to adjust them is to turn the engine until one valve is being held open and adjust the other valve and thn turn another revolution to reverse the open valve and then adjust the now closed valve. Then while turning the engine over by hand observe the valves to make sure they are opening and closing properly and also looking for the little valve bump just before TDC compression on the intake valve which is the bottom valve. No valve bump off if the valves are properly adjusted would mean the compression release part of the camshaft is laying in the oil sump. You should be able to handle the valve adjustment the camshaft replacement may require professional help.

Thanks. How do I find TDC? Is there a mark on the starter/ring gear?

Screwdriver down the plug hole
When it stops moving out you are at TDC.
For B & S engines you keep going till it moves 1/4 inch back in

m610 said:

Thanks. How do I find TDC? Is there a mark on the starter/ring gear?

Click to expand...

Remove the spark plug and either shine a light into the hole while watching the piston move when the flywheel is turned or insert a screwdriver into the sparl plug hole and when the flywheel is turned the screwdriver will move out of the hole and when it starts going back into the hole that is TDC

I work on Opels a lot and that includes building engines. To find TDC we put a degree wheel on the crank and a dial gauge on the piston, then turn the crank while watching the dial gauge. As the piston gets near the top the dial gauge slows to such a degree that you can't be sure when TDC was reached. So what we do is note the peak value on the gauge and keep turning the crank until the piston drops by 0.050". We note the position of the crank on the degree wheel, then turn it in the other direction until once again we are at 0.050" below the top value on the dial gauge. We then note that position, then we turn the crank until it is midway between the two 0.050" marks on the degree wheel. That puts us at precisely TDC.

The method recommended here could easily put me 5 degrees or more off true TDC. Is that good enough?

Mike

bertsmobile1 said:

Screwdriver down the plug hole
When it stops moving out you are at TDC.
For B & S engines you keep going till it moves 1/4 inch back in

Click to expand...

Just to confirm what you are saying, the 1/4-inch that ILENGINE was referring to is a 1/4-inch movement of the piston, not some mark on the flywheel or another part of the engine. Correct?

m610 said:

Just to confirm what you are saying, the 1/4-inch that ILENGINE was referring to is a 1/4-inch movement of the piston, not some mark on the flywheel or another part of the engine. Correct?

Click to expand...

Correct. You want the piston started down on what would normally be the power stroke about 1/4 inch. The reasoning being it is so the compression release has time to close the intake valve so you can adjust both valves at the same time. Placement isn't really that critical just makes sure the cam lobes are not trying to open or holding the valve open when trying to make adjustments.

m610 said:

I work on Opels a lot and that includes building engines. To find TDC we put a degree wheel on the crank and a dial gauge on the piston, then turn the crank while watching the dial gauge. As the piston gets near the top the dial gauge slows to such a degree that you can't be sure when TDC was reached. So what we do is note the peak value on the gauge and keep turning the crank until the piston drops by 0.050". We note the position of the crank on the degree wheel, then turn it in the other direction until once again we are at 0.050" below the top value on the dial gauge. We then note that position, then we turn the crank until it is midway between the two 0.050" marks on the degree wheel. That puts us at precisely TDC.

The method recommended here could easily put me 5 degrees or more off true TDC. Is that good enough?

Mike

Click to expand...

When I was in training to buy the mower repair run, the previous owners regular comment was "It is a bloody lawn mower not the frigging space shuttle.
There are better methods that are far more accurate .
I use piston stops on my old motorcycles and timing discs.
However mowers have FIXED timing and the only reason for finding TDC is to set the valve lash so +/-10 deg will make zero difference
On top of that very few home owners would have dial gauges and of those who do only 1/2 would know how to use them properly
Remember the KISS principle , it is only a lawnmower

Thanks, all. As soon as it stops raining I'll give it a try.

Oh it is so much easier to adjust each valve independently in my opinion instead have to hunt up the extra tools. I just turn until one is fully open and adjust the closed valve rocker.

My understanding is that the B&S has an extra little centrifugal widget that opens a valve at low rpm to decompress the engine to make starting easier. Apparently it only opens the valve a tiny bit, because if the valve gaps are slightly too big it doesn't open it at all and it becomes hard for the starter to turn it over. It's worth spending time adjusting them to get them spot on. Took me a few attempts because the gap changed when I tightened the locking.

All Single cylindre inteks, he outer nut is the adjuster and the inner grub screw is the locking device
There are a lot of You Tube videos showing it the other way round because the poster is a brain dead moron with a massive ego
Twins can go either way
If the adjuster is over the pushrod then it is grub screw to adjust & nut to lock
If the adjuster is on the mounting stud then the nut is the adjuster & the grub screw is the lock .

The valves are adjusted. They were actually really close to where they should have been.

With the spark plug out, I cranked and the motor spun fine. After I put the spark plug back in the motor would barely turn. The mower has a new battery and starter. I made sure the battery had a full charge by leaving it on the charger for a few hours, then tried again. No change. The motor barely turned. It was too dark to continue so I'll look a little deeper tomorrow.

Jump directly from the battery to the starter + terminal and to a good ground near the starter ( I use the oil drain plug )
If it spins normally then investigate battery leads terminals

With the valve cover off turn the engine over by hand. Check the intake valve (I think) for a slight movement just before TDC. If no movement then it looks like broken decompressor. Think it's needs a new camshaft to fix that.

If you need to use the machine sometimes it will start if you manually rotate the engine backwards against compression then try starting it. This gives the starter a bit of a "run up" to clear the next compression stroke.

Thanks for the suggestions. I cleaned every electrical contact, including the grounds, then shorted the starter cable straight to the battery, and the engine cranked great, for the few seconds I gave the starter power. I thought the problem had been solved. When I tried to crank it again the starter would engage but the engine would not turn. I put a wrench on the crank and turned it myself, and every other rotation was much more difficult than the others. I turned the crank past that point again and fired the starter, and the engine turned twice and stopped at that difficult spot again. Too much compression once the rings got a little wet with oil and/or fuel? (If I ever get this thing cranking again I'll measure compression.) Not enough starter? The battery is rated for 330 CCA, which is considerably more than what I have on my Craftsman riding mower/tractor.

I tried the run-up idea, turning the crank until just past the difficult spot, and that didn't help.

That really sounds like the decompressor isn't working. I'd have the valve covers off again, recheck valve clearances and look for the little extra lift when turned over by hand.

The intake valve does not do the tap dance, so yes, it looks like I'll need another cam. Per the video, it looks like a very doable procedure. I'll let you all know how it goes.

Thanks for the tips, and the video is fantastic.

Mike

Another good vid..