Briggs and Stratton, valve lash

m610

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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
 

ILENGINE

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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.
 

m610

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

bertsmobile1

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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
 

ILENGINE

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

m610

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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
 

m610

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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
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?
 

ILENGINE

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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?
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.
 

bertsmobile1

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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
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
 

m610

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