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Old 05-05-2020, 04:14 AM
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Default Testing compression

So... what is the procedure to test compression on a small engine that has a compression relief system for easy-pull starting? As I see it, one must be really fast to start the engine... then, remove the spark plug, and hook up the tester before the engine quits. But, then it has no ignition to keep it running.
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Old 05-05-2020, 06:57 AM
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Quote:
Originally Posted by Gadgeteer View Post
So... what is the procedure to test compression on a small engine that has a compression relief system for easy-pull starting? As I see it, one must be really fast to start the engine... then, remove the spark plug, and hook up the tester before the engine quits. But, then it has no ignition to keep it running.
Well I’m a bit confused about your description of the process. Why would you start the engine other than to warm it up? I don’t know if there is a special way to test on a relief type engine however I don’t see why it would be any different than convention.
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Old 05-05-2020, 08:10 AM
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All of the engines I've ever worked on that have decompressors; typically a centrifugal device that holds an exhaust valve slightly open, the specification for compression reflects that. A typical non decompressor engine might have a range of 135~185 psi. With a decompressor it might be 55~65 psi. A leakdown test is more accurate for checking mechanical sealing.
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Old 05-05-2020, 08:56 AM
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Quote:
one must be really fast
Now all you need is practice
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Old 05-05-2020, 09:10 AM
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Just wait until the decompressor fails, then check it.
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Old 05-05-2020, 10:31 AM
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The way I was taught, is to have the choke and throttle open, and pull the start cord to turn over the engine on compression about 6 times.
It can be done with the compression relief on, you are not trying to start it.

This is not a success story with a Honda or Chonda, because they use a valve lifter on the cam lobe that automatically reduces compression, and flies out of the way at an idle rpm. That can be defeated if you need a accurate compression number, by pulling the valve cover and backing off the valve adjustments on the rocker about a 1/2 turn.
Re-set the valve lash afterwards.
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Old 05-05-2020, 05:00 PM
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With the term ‘easy-spin’ involved I’m guessing it’s a Briggs?

Put a tester on it and pull it over 3-4 times. If you’re over 100 it should run.

Under 100, chances are a burnt exhaust valve or pooched rings.

Thirty plus years ago my dad taught me to rebuild engines on Briggs lawnmower engines. I likely did over a dozen that summer, learned how to hone cylinders, lap valves, grind/set ring and valve clearances on them, check bearing clearances and about anything else that could be done.


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Old 05-05-2020, 08:51 PM
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In the old Briggs manual I have “ to test compression spin engine in reverse direction and if it bounces back sharply the compression is good”

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Old 05-05-2020, 10:41 PM
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Quote:
Originally Posted by greywynd View Post
With the term ‘easy-spin’ involved I’m guessing it’s a Briggs?

Put a tester on it and pull it over 3-4 times. If you’re over 100 it should run.

Under 100, chances are a burnt exhaust valve or pooched rings.

Thirty plus years ago my dad taught me to rebuild engines on Briggs lawnmower engines. I likely did over a dozen that summer, learned how to hone cylinders, lap valves, grind/set ring and valve clearances on them, check bearing clearances and about anything else that could be done.


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Yup, Briggs. Tester registered only 30 psi when I pulled the rope a few times. So, I thought that was low, due to the easy-start feature that keeps an exhaust valve open a bit, until it gets to idle. Then, a centrifugal device allows the valve to fully close.

Tester, also, showed 30 psi when I tried it on my riding mower, but went higher when I checked it by hooking it to my air compressor... verifying that tester is good.

Friend is going to go ahead and order a new pump, 'cause that's what a pro told him is bad. A splitter build has been on my bucket list for years, so I told friend that if the new pump doesn't solve the problem, I'll just buy it from him, so he will have more money to replace or fix the engine.
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