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Using a Compression Leak Tester

W

wsmm

Member
Do I need to figure out someway to lock the crankshaft on an older 11HP Briggs and Straton engine in a lawn tractor when using a compression leak tester. Working on a no start problem with gas being spit out of the carburetor when trying to start. Will need to put the head back on as I took it off to insure I did not have a sticking valve Both intake and exhaust seem to open and close fine when turning the engine over by hand.
Thanks for any help and or suggestions, idea.
Bill
 
Yes, you will have to lock the crank at top dead center (TDC) on the power stroke with both valves closed. There is another TDC between the exhaust and intake strokes, but that will give you false readings because of cam overlap holding the valve open. In other words, the exhaust valve is not quite fully closed and the intake valve is beginning to open.

Actually, you may need to be part way down the cylinder on the power stroke to be sure the intake valve is not on the compression release, depending on which engine it is.

Before you put the head back on, roll the engine over to TDC on the power stroke and see if you can turn either valve with your fingers. if you can spin it, its not closing fully.
 
I have found that a lot of problems with gas spitting Briggs engines to be carb related. Mainly the float.

Remove the carb and then the bowl.

Turn the carb upside down and look at the float. It should be level.

If not. Remove the float and the needle valve. Carefully heat the small tab that contacts the needle and bend a small amount. Then cool it with a few drops of water.

Reinstall everything as it should be and check the float again. If needed repeat the heating process and bending until the float sits level.

Remove and check that the Hi and Lo speed adjusting screws are not bent or damaged.

If OK. Reinstall them and turn in until they stop and then back them out 1 1/4 turn and try starting the engine. If it runs adjust the speed screws until the engine is running correctly.

=======

One last thing. Have you checked the flywheel key to make sure that it had not sheared and threw the engine out of time?
==========

I have always rotated the piston to near BDC on the Power stroke for pressure testing. If the valves are seating good. The pressure most times will force the piston down to BDC as pressure builds/
 
On the woodruff key locking the flywheel to the crankshaft, If it's even partially sheared that can be a problem also. Never thought that the few degrees of error in timing caused by such would cause hard starting and a problem in eventually actually running, but it happened to me. New key and away she went.

Just takes a second, pop the flywheel nut and have a quick look.
 
(quoted from post at 16:11:49 10/23/18) Do I need to figure out someway to lock the crankshaft on an older 11HP Briggs and Straton engine in a lawn tractor when using a compression leak tester. Working on a no start problem with gas being spit out of the carburetor when trying to start. Will need to put the head back on as I took it off to insure I did not have a sticking valve Both intake and exhaust seem to open and close fine when turning the engine over by hand.
Thanks for any help and or suggestions, idea.
Bill
Click to expand...

Additional Info. Engine is a Briggs and Stratton model 25707 type 0622 02. Electric start only, don't believe it has a compression release. Ran a leak down test on it, did not need to lock the crank. Was able to determine it was leaking at the intake valve. Both intake and exhaust had a lot of carbon on each valve. Cleaned up both valves. Odd intake valve had keepers on valve, exhaust valve had a notched retainer. Intake valve clearance was good, no clearance on exhaust valve. Clearanced and checked clearance on exhaust valve, now in specs. Will lap both valves in the next few days and then run another leak down test. If still bad will probably just switch the engine from my other lawn tractor with a bad transmission to it.
 
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