Outdoor Power Equipment (Lawn Mowers, Snow Blowers, Chain Saws and more) Discussions

I borrowed a  pressure tester from the local auto parts store and tried testing the compression on this Yamaha 8hp snowblower engine.
I opened the choke and set the throttle to full open.  But I'm confused.
When I tested by pulling the recoil starter just through one cylinder of compression I got one reading ( 47 psi  ):

Then when I pulled the recoil starter cord fully (like you would to start the engine) I got another reading (72 psi  ):

Then when I plugged in the electric strarter and really spun the engine over I got a third reading  ( 100 psi  ):

I  repeated each of these tests several times and always got the same results (pretty much). Could I be doing something wrong?  So which one am I supposed to be looking at?

The owner's manual listed a recommended pressure for the cylinder on this engine of  49 to 78 psi at sea level (my Yamaha YS-828TM snowblower sits 129 feet above sea level) I also did the "pour oil into the cylinder" test.  The compression barely went up at all with the oil added to the cylinder.  I'm at a loss as to how to make sense of these numbers. And who's ever heard of too much compression? :

Read the last statement in the post... Read the Compression above max statement...Large carbon deposits will increase compression in the motor..

Friiy

friiy wrote:

Increase the compression?  That is a strange concept for me to understand.  Like "an increase in the amount of sleep you get at night" or "an increase in % of healthy foods you consume" or " an increase in the cleanliness of the air you breath?" 

What could be wrong with a little more compression?  This is a combustion engine.  Compression is a good thing.     I did the test because I thought the cylinder did not have enough compression.  (lack of power)

Too much compression?  This engine is 20 years old. It should be on its last leg. Hanging onto a thread. Leaking compression at every bolt, gasket, and o-ring.  I hope I don't sound indignant.  The previous owner sold me the snowblower because it was (in his words) "lacking in power" and this is true. It starts and runs fine but does not blow snow with  the gusto that it should.  This symptom pointed me to a lack of compression.  That would make so much sense.

Too much compression,   may cause a ping,  knock or rough running.. The carbon in the combustion area is taking up space causing the air/fuel charge to compress to a smaller area.  Higher compression means faster burning of the fuel /air charge..

Friiy

The engine does vibrate more than it should at idle.  I could take the cylinder head off and look in there to see if there is a large deposit of carbon. But I'm not sure that a buildup of carbon would relate to the "lack of power issue."  Would these two be seperate?  "carbon buildup" and  "lack of power"  ? 

I think that I may have hit a dead end here. I picked up some books over the weekend from the library and I've started reading them in hopes of learing something that might help me.

The if your engine has that much cabon it has other problems  (oil burning/ smoking)...  I would not tear off the head yet... I would check the vavle clearance first...  If the engine has preignition (knock and pinging) it is fighting itself buy buring fuel before top dead center..

Friiy

Thank you.  Is this task "checking  valve clearance" reasonably straight forward?  Any rare or unussual or expensive tools required?  My manual provided me with this guidance:

You'll need a feeler gauge.  A good set is less than ten bucks. 

You say the engine vibrates a lot while idling.  I'd check the motor mounts.  If your compression is as high as you say it's reading and you don't have a problem with pre-ignition (pinging - particularly under load) I wouldn't be too worried about it.  Did you do the compression test with a cold engine?   The correct method, is cold engine and crank until the gauge no longer reads further increases.   

borat wrote:
No, I did not do the test with a cold engine. The manual (see above) specifically said to warm the engine up first.   So I did that. I ran it for roughly five minutes. The spark plug was hot when I pulled it out to put the gauge in.  I really truly thought the compression test was going to give me a reading in the low 30's.   I'm still in shock/denial.  

Well, that's a first for me. 

Everything I've read about compression testing has always been with a cold engine.