Tecumseh 10HP Compression - Snowblower Forum : Snow Blower Forums
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post #1 of 6 Old 09-17-2014, 08:41 PM Thread Starter
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Tecumseh 10HP Compression


My new ariens 24" platinum will be delivered Friday.

My old Craftsman, circa 1993, I am going to have the shop go thru it, new belts and adjust and or replace the carb. It runs OK, but have had to leave the choke on a hair, so it must run somewhat lean. Anyhow I have a new carb for it.

I did a compression check on it and got 127 PSI, I assume that is decent for 20 yr old engine with 500-600 hours on it I am guessing. I made the mistake of checking it on SUnday, but stupid me left the throttle closed and only got 70 PSI, guy at the shop thought it should be 120 or so, so I thought about it and it dawned on me I had the throttle closed.

He mentioned having the vlaves adjusted, how much time does that take, they have never been done. Only thing I have done to maintain it is yearly oil change with Amsoil, clean the carb, have replaced the carb about 5 years ago, some gease on the disc fitting and auger fittings, lube the chains and replace any broke buts like cables, and fresh plug.

I want to keep it as a backup since my neighbor moved away and all the other folks have plow guys, I need to get out on snowy days so cant wait for a plow guy.

Any how I am happy since this probably means any money I throw at it will be worth it.
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post #2 of 6 Old 09-17-2014, 09:50 PM
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Compression numbers are kind of useless since Tecumseh never published any. Briggs & Stratton used to say in the service manual "Spin the flywheel counter-clockwise by hand, if compression is adequate the flywheel should rebound sharply" I can't remember what tecumseh said in theres, but I believe it was something similar.

A valve adjustment can take a little while since the only way to do it is to grind the valves.
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post #3 of 6 Old 09-18-2014, 12:16 PM
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The other issue with compression tests is the cam shaft opens the exhaust valve slightly during slow rpms to make the engine easier to start. I think I read somewhere that the only true way to test compression is to turn the engine backwards.

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post #4 of 6 Old 09-18-2014, 03:09 PM
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127 psi is awesome compression. these are small low CR flathead engines. any more than that the rope would break starting it. that engine is like new yet for ring seal.

the best way to check ring seal is leakdown. get the piston up on TDC and put 100 psi into the plug hole with an adapter, then read a gauge that tells you how much pressure is there holding. it's like a compression gauge but has 2 gauges on it instead of only one.

but with 127 psi you are good to go, good spare machine

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post #5 of 6 Old 09-18-2014, 06:57 PM
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A valve adjustment would take between one to two hours and require new head and breather cover gaskets. Unless your compression is low or you get a popping sound out of the muffler when the engine is hot, your valves are probably ok.

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post #6 of 6 Old 09-18-2014, 11:35 PM
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In my experience if it feels like a engine when you pull the rope then it's an engine that should start. If it can't start with spark and fuel then compression may be in question.

If it does start but becomes unstable after warming up for a few minutes, eventually stalling and feeling easier to tug on the starter rope then the valve have hobbed the seats. The valves get too long to seal when heat expands them, especially the exhaust valve. When this happens compression crashes and the engine dies out. Eventually they won't even seat well enough for predictable starting if you can nurse it along long enough.

The remedy is to grind the valve stems to restore lifter clearance. You will get to clean the head and piston top at the same time. Unless the cylinder wall has something significant going on leave it alone. Put it back together preferably with fresh gaskets and you have a good shot at the block going for decades more. Remember to torque the head bolts to spec. Many carburetors have been needlessly rebuilt chasing this since it can seem like the engine is running out of fuel.

Unless the valve spring retainers give you a hard time it should fit into a 2 hour session.

Compression testing is a real wild card. Temperature and variable cylinder lubrication can provide any reading you want. The leak down test is more objective but still subject to measurement error. Rebounding when spinning the flywheel backwards is another good sign.
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