Ariens Mower Compression Too High?

turkeyvulture · Sep 7, 2016

Ariens riding mower with Briggs & Stratton 1-cylinder engine. Gradually got harder for the electric starter to pull past TDC. Replaced battery, then starter, then relay. Checked compression 210 psi. Seems too high right? My little push mower is 110 and I think cars are around that. Engine spins freely with spark plug out, BTW.

Figured combustion chamber must be full of carbon, pulled off head, no carbon, just light soot. Put back together with extra head gasket to lower compression. Still hard to pull past TDC, hard to start, still 210 psi.

WTF. How can the compression not be lower with an extra .037" thick head gasket in? Why is it that high anyway?

Advice anyone?

Dutch445 · Sep 8, 2016

some briggs motors have a decompression system in the engine, sometimes on the
cam gear, spring loaded so that at low rpm it releases compression for easy starting,
here's a video someone posted:
How the compression release mechanism works on Briggs and Stratton OHV Engines - YouTube

CaptFerd · Sep 9, 2016

Briggs uses a plastic cam shaft in a lot of their engines. The lobes wear down on the exaust resulting in high compression like you described.

ruffdog · Sep 10, 2016

Also check the OHV valve adjustment. If they are loose, the decompression mechanism won't be able to open the exhaust valve to release the compression pressure.

Mysfyt · Sep 10, 2016

I wonder if the key sheared on the flywheel and the timing is advanced?

turkeyvulture · Sep 10, 2016

That sounds like the problem. Thanks I never heard about this compression release. I'll get in there and take a look.

turkeyvulture · Sep 10, 2016

Thanks but when I added the extra head gasket I adjusted the valves back to .004".

Helitech · Sep 10, 2016

A few of the Briggs single cylinder engines suffered from bad a bad compression release mechanism on the camshaft. Can't remember the series though. 98% of the time it is loose valve adjustment. Pop the valve cover and double check.

LD1 · Sep 10, 2016

Not familiar with the Briggs, but 210psi certainly don't sound too high IMO. Speaking in terms of modern autos....110 would be a well worn out engine, and likely a hard to start engine