Compression Release Mechanism


Old 12-30-06, 06:57 PM
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Compression Release Mechanism

Do all Briggs and Stratton engines have Compression Release Mechanism's?
How do they work?
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Old 12-30-06, 08:06 PM
hopkinsr2's Avatar
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Hi, Most of the Briggs I have seen have a "Bump" on the back of the exhaust cam lobe. This cracks the exhust valve open to relieve compression & makes starting easier.
Old 12-31-06, 12:10 AM
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Some do, some don't. Depends on which you have.
Old 12-31-06, 10:17 AM
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Ok. Can I tell by the model #?
I'm taking a compression reading (appx. 61) but then read the true way to test is 'spinning the flywheel in reverse and feeling for spongieness'...

Cheese- you may recall this is the engine with the Intake Valve issue.
I have continued to lap the valve and have gained additional vacuum (by feel)
at the intake port.
However, the tappet still doesn't drop out of the way till most of the compression stroke is completed, thereby sealing the valve.
Thought this 'compression release' may have something to do with the Intake Valve issue but I guess not.....?????
Old 12-31-06, 10:45 AM
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It is virtually impossible for you to get anymore compression (therefore will never run)until you resolve the intake valve issue. I wish I were there to help you.
Old 12-31-06, 10:52 AM
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Yes - realize this. Just didn't want to 'chase my tail' taking compression readings when in fact it may not be the accurate test method on a Briggs.
Old 12-31-06, 11:20 AM
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There may be another way to take these readings that I am not familiar with, but I gaurantee you if you take them in the conventional way,your reading will be accurate enough to make a decision on.
Look at these very primtive steps of compression for a three inch stroke piston. We start with the intake valve closed. Remember, three inches is full stoke and we start at the bottom with say atmospheric press of 15#. Move the piston up half way or 1 1/2" compression gage reads 30#. Move up half of the remaining distance or3/4" now reading is 60#. Move up half of that distance or 3/8" gage reads 60#. move up half that distance or 3/16" snd gage reads 120#gsh. In four compressions we now have attained 120# theorectically. Thats with the valve closed at the beginning of the stroke. There is something way out with the valve timing on your engine. I hope this helps although for anyone out there standing by to critizse, I admit this is a crude explanation. LOL Tom
Old 01-02-07, 11:10 AM
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61 is borderline bad and it sounds as though you're doing it properly. You're doing a compression test which Briggs doesn't publish guidelines for good/bad readings for the reason of the MCR on the camshaft. They do have a leak-down test and tester which is much more accurate and will indicate where you have trouble...rings, exhaust or intake. If it were in my shop and all I had was a compression reading and it was that low I'd remove the cylinder head and have a look at the combustion chamber.

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