How do you test for compression? What is leak down

[GhostFlame]

Hi I thought I'd start another post to determine weather my valves and rings are good.
Tecumseh sno king 7 hp, side valves. the engine runs fine but stalls under load. when it hits snow it bogs down then stalls.

How do you check for compression? leak down? or are they the same thing?

Here's my other thread http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=2172009#Post2172009

 

[Quest]

Easy! Get a compression meter and then when the engine is warm, stick in the meter onto the spark plug hole and then yank the engine (if yours is the manual pull type).

Squirt oil into the cylinder bore from spark plug hole and then once again, attach the compression meter (tester) and then yank it.

compare the readings to the factory manual/spec.

Q.

 

[The motor guy]

Do you ground the coil out on these small engines or does it not matter? I never found an answer to this.

 

[bullwinkle]

Originally Posted By: Quest
Easy! Get a compression meter and then when the engine is warm, stick in the meter onto the spark plug hole and then yank the engine (if yours is the manual pull type).

Squirt oil into the cylinder bore from spark plug hole and then once again, attach the compression meter (tester) and then yank it.

compare the readings to the factory manual/spec.

Q.
And block the throttle wide open!

 

[Quest]

block the throttle wide open, ground the coil, the usual.

 

[Artem]

Why the throttle? First time i hear of this on a Compression test.

 

[punisher]

On a small engine it makes no difference as the governor holds the throttle wide open during cranking.

On any other engine the throttle is always held wide open during a compression test. Closed throttle compression tests show artificially low readings as the cylinder cannot pull in the full volume of air on the intake stroke.

 

[MrBeachcomber]

To answer your other question, a leak down test is a static test wherein the combustion chamber is charged to a specific PSI while both theintake and exhaust valves are closed and the piston is at TDC. Then the pressure is monitored over a set time frame to determine how much pressure loss there is due to leakage past the rings. (Some leakage is normal, e.g. typically 5%, but the exact amount determines the overall "health" of that particular cylinder.)

Since the compression check is dynamic and the leak down test is static, they are not the same. Hope this helps!

 

[dorkiedoode]

my favorite educational video from eric the car guy :]

http://www.youtube.com/watch?v=WgrfT0LFMhc

 

[boraticus]

Compression tests are normally performed with a cold engine. Pull the plug, ensure throttle is wide open, install compression tester and keep cranking the engine until the gauge needle no longer moves. That's your reading.

One thing to acknowledge is that most snow throwers have an automatic compression release to assist starting. If the decompression actuator is not disengaged, it will be difficult to get a proper compression reading.

I did a compression test on my snow thrower with a 305cc B&S Kool Bore engine using the electric start and got a good reading. So, I'm wondering if the electric start can spin the engine fast enough to disengage the decompression mechanism.

Another point to consider is testing your compression gauge before taking a reading. I just had a experience where I tested two engines that showed lower than factory spec readings and was about to embark on some serious work. Due to both engines reading the same low figure, and following a recommendation from 660mag, I tested the gauge using a set air pressure on my garage air compressor. I found the gauge to be faulty. It consistently reads 20 psi less that it should.

 

[660mag]

To add to the post about leakdown, you can also pinpoint where you are loosing air. If your engine fails a leakdown leave it hooked up, and listen to the carb, muffler, and oil dipstick tube. If you here hissing coming from any of these it tells the story i.e. intake, exhaust valve, or rings. If the sound is from none of these spray the head gasket w/ soapy water to test for a leaking head gasket.

 

[1 FMF]

on small engines there is an exhaust valve start relief thing, forget the exact wording they market it as. But when you pull the starter cord the engine is rotating slowly enough that the cam bumps the exhaust valve open on the compression stroke. this is to relieve pressure so you can pull the starting rope. but it will result in a lower compression reading than what you should expect. I think most small engines with the start relief only get 70-90 psi cylinder pressure on the gauge when pulling the start rope.

it's most likely bogging and stalling because of the carb needing a cleaning and adjustment.
I just gave away my old 8hp tecumseh powered 2-stage snowblower,
those side-valve engines just don't put out the rated power even if the carb is functioning, and you may just be overloading the engine. Mine was old and ran good, but it would overload way too easy. Also look at the throttle governer spring location, set it so it gives the most throttle opening, you may not actually be getting full throttle. when it bogs down the slower engine speed is seen by the governer spring which opens the throttle more to bring the speed back up

 

[doitmyself]

Many decades ago, in a small engine class, I "thought" I remembered them telling me that measuring compression in small engines is difficult for some reason, or required a different type of tester.

Is this not true?

Edit: maybe its what 1fmf is talking about....the cam lobe is shaped to release compression at "pulling" rpms??

 

[660mag]

Yes that's what he's talking about. But compression testing tests the flow through the combustion chamber and only somewhat gives the story. It tells you if the valve timing is good, and if the carb or exhaust are blocked. But a leakdown tells you more info as far as wear on the cylinder.