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post #1 of 6 (permalink) Old 11-19-2018, 09:05 AM Thread Starter
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Briggs & Stratton Compression Release

Do any, or maybe all, Briggs and Stratton small engines, for snowblowers and riding lawnmowers, have compression release during cranking?

I have a single cylinder and a V Twin Briggs [each on a Murray lawn tractor] and both show around 130-135 lbs during electric start cranking. I thought this was about normal cylinder pressure but someone told me you can't check compression during cranking because of the compression release feature.

I turned down buying an older used snowblower because compression during pull cranking was only about 50 lbs but now I am second guessing my decision.
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post #2 of 6 (permalink) Old 11-19-2018, 10:18 AM
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Its a roll of the dice for sure when buying used. I bought a running old machine and actually used it once. $75. Then it wouldn't run. Pin had cut a big groove in cylinder wall. I did not test with compression. If I had and it had 50 psi wet and dry it still might have been the same.
If you can get something really cheap I think its worth the risk......my humble onion.
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post #3 of 6 (permalink) Old 11-19-2018, 11:24 AM
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If I saw that much compression in a briggs, I'd look to the sump being full of gas with the fuel and oil, standard briggs leaking carb. Check your oil I'd guess it's high and gas smelling.
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post #4 of 6 (permalink) Old 11-21-2018, 01:02 AM
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Quote:
Originally Posted by Robin View Post
Do any, or maybe all, Briggs and Stratton small engines, for snowblowers and riding lawnmowers, have compression release during cranking?

I have a single cylinder and a V Twin Briggs [each on a Murray lawn tractor] and both show around 130-135 lbs during electric start cranking. I thought this was about normal cylinder pressure but someone told me you can't check compression during cranking because of the compression release feature.

I turned down buying an older used snowblower because compression during pull cranking was only about 50 lbs but now I am second guessing my decision.

You were correct in turning down the snow blower with 50 psi, compression.
That is below the minimum starting compression of 65 psi.
You could probably get it to start and run with startin fluid and good gas but 50 psi. is too low for normal starting condition.
I normally junk anything I am trying to repair for resale that has lower than 85 psi.
The only thing that releases the compression on mower engines is the exhaust valve.
There is no pop off or pressure relief valve on any Briggs or Tecumseh I've ever seen.
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post #5 of 6 (permalink) Old 11-21-2018, 10:48 AM Thread Starter
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"If I saw that much compression in a briggs, I'd look to the sump being full of gas with the fuel and oil, standard briggs leaking carb.

Maybe my compression gauge is faulty......it's old enough, thats for sure.

Those engines continue to work just fine with no issues, one after 3 years, the other after 12 years. I was referencing that compression as properly working engines. There is no fuel in the oil of either engine and to my knowledge never has been. I even added manual fuel shut off valves to avoid the problem of fuel leaking through carburetors with the engines are shut off.

[My wife even uses the fuel shut off on 'her' machine...although that took some training. ]
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post #6 of 6 (permalink) Old 03-15-2019, 02:15 PM Thread Starter
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I had forgotten I posted the question above...

A search just now on Youtube brings up several videos which discuss that Briggs OHV engines actually DO have compression release...via the intake valve....I don't know why my two riding lawnmowers registered as high compression as they did unless the compression tester reading is off.....

A really good compression at a compression ratio of around 8.5:1 would suggest a running pressure of about 8.5 x 14.7 PSI [atmospheric pressure at sea level] or about 125 PSI. Other sources say during starting you only measure something in the 70-90 PSI range.

Who knows??
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