B&S 18.5 HP 31P777

[slingblade4419]

So, i have a B&S OHV 18.5 HP model 31P777 that is stopping on the compression stroke when trying to start. I researched a good bit and ended up on this forum. I have adjusted the valves several time, drained and replaced the oil, and even tinkered with the cam a bit. I have several questions because i'm an automotive mechanic, and the decompression valve is throwing me off a little. Questions are:
A-Is the decompression valve the counterweighted pin on the intake lobe of the cam?
B-Should it be opening immediately after the intake stroke? (when it does this, it is before compression has built up enough)
C-Valve adjustment specs are between .003-.005 for intake and .005-.007 for exhaust correct?

Any help with this would be appreciated

 

[pugaltitude]

So, i have a B&S OHV 18.5 HP model 31P777 that is stopping on the compression stroke when trying to start. I researched a good bit and ended up on this forum. I have adjusted the valves several time, drained and replaced the oil, and even tinkered with the cam a bit. I have several questions because i'm an automotive mechanic, and the decompression valve is throwing me off a little. Questions are:
A-Is the decompression valve the counterweighted pin on the intake lobe of the cam?
B-Should it be opening immediately after the intake stroke? (when it does this, it is before compression has built up enough)
C-Valve adjustment specs are between .003-.005 for intake and .005-.007 for exhaust correct?

Any help with this would be appreciated

A- yes that sounds right. The spring might be broken which is stopping decomp working.
B- basically it opens on compression stroke. When checking clearances, you have to be 1/4" past tdc on power stroke to compensate decomp as you could get wrong clearances. Intake valve opens then compression valve shuts then opens slightly at top. (decomp working). If not opening slightly here decomp not working.
C- yes that should be them.

More info here and a spec sheet
Servicing Engine Valves | Briggs & Stratton

 

[slingblade4419]

Just went and played with it a bit more. I pulled the spark plug and used my thumb as a stopper. It's decompressing fine, but it's pulling a vaccuum on the power stroke since it is letting the pressure off on the compression stroke. The engine is on my work bench so I cant start it correctly at the moment. It needs a new valve body gasket, which i will hopefully be able to get tomorrow so I will update once I have that on and back in the mower.

 

[Rivets]

I am confused, it may be that I am an old fart, with old timers disease. If I do have any memory left, I always thought that the compression release opens the exhaust valve and not the intake. Attached is a website where you can download a manual. I have given you 2, because I am uncertain as to which one fits your engine. Hope I am not confusing things for you.

http://dl.dropbox.com/u/12502267/B&S Service Manuals/10_276781SingleCylinderOHV.pdf

http://dl.dropbox.com/u/12502267/B&S Service Manuals/04_272147VanguardSingleCylinderOHVAirCooled.pdf

 

[pugaltitude]

I am confused, it may be that I am an old fart, with old timers disease. If I do have any memory left, I always thought that the compression release opens the exhaust valve and not the intake. Attached is a website where you can download a manual. I have given you 2, because I am uncertain as to which one fits your engine. Hope I am not confusing things for you.

http://dl.dropbox.com/u/12502267/B&S Service Manuals/10_276781SingleCylinderOHV.pdf

http://dl.dropbox.com/u/12502267/B&S Service Manuals/04_272147VanguardSingleCylinderOHVAirCooled.pdf

It can either be the intake or exhaust depending on model Compression System | Frequently Asked Questions | Customer Support | Briggs & Stratton

On the larger OHV single cylinder its the bottom valve that should slightly open which is intake.
There is 2 ways to check the decomp.
1 is to strip engine down and visually check camshaft or
2 is by checking decomp working on valves with rocker cover off.

 

[Rivets]

Learned something new today, now I can go back to bed. In all the years I have been working on small engine, I don't ever remember working on one with a compression release on the intake. Thanks for teaching me something.

 

[chance123]

Learned something new today, now I can go back to bed. In all the years I have been working on small engine, I don't ever remember working on one with a compression release on the intake. Thanks for teaching me something.

This makes 2 of us