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

So I picked up a 1998 Craftsman single stage with the Tecumseh HSK850 engine.  I've got the thing all apart already to do some inspection/carb cleaning/adjustments. 

I did a compression test but it appears this engine might have some form of compression release.  Using the electric start, I had it registering around 95-100 psi.  Does that sound right?  Pulled the muffler off and the cylinder looks very good.  No indication of blow-by on the exhaust side and the rings look great.  No carbon build up and they're nice and free.  

Last question, does anyone have a link to a manual for this engine/snow blower.  Craftsman 5/21 approx. 14 years old.  Going back to work.  Got to clean the carb.

Compression tests are unreliable for many reasons.   They are ok for a quick go nogo but for valid feedback the way to go is a leakdown test or run air into the piston through the plug with the piston at TDC.  Brace the belts or flywheel as the air will force the piston off TDC.  Listen at the carb, muffler, head and oil fill hole for leaks.  With listening it's an indication and rough.  With a leakdown you get a % of leak away from 100%.

For example you pump in 15psi and one gage is set to 100%.  That's the input side. 

After that gage is a check valve.  Then another gage is on the output side of that valve.  The side that goes into the head.

While 100% (15psi) is is being sourced the other side in the head will leak some even in a good engine so may read 90 to 95% which says it's loosing 5% of what was pumped in.  10-30 is ok but closer to 30 is a concern.  Past 30 is time to decide on what to do about your motor.  If all the air is comming out of the muffler you need to check that valve.  Out the oil fill hole the rings or walls worn.  They are 30 bucks from Harbor Freight and a great value.   

I think I got the ring thing.  You looked through the muffler.  You don't know they are ok you just know they are there.

trouts2 wrote:
Thanks for all the help.

The above instructions sound applicable to a four stroke engine.  No oil fill holes or valves on this engine. 

Regarding the rings.  I've worked on numerous two cycle engines and by peering through the ports, a lot can be learned about the rings and cylinder if you've got the experience to know what to look for.  If there's little or no blow-by staining on the piston and the rings are clean and free of coke accumulation in the grooves , the rings are still functioning well.  Generally speaking the engine looks pretty healthy. 

I replaced the welch plug and put a couple dabs of JB weld on it. 

I'm going to take it out tomorrow and put a tach on it to see if I can bump the rpms up a bit for a little more power.   Snow is on it's way so I'll be able to give it a work out when I'm done.

One thing that needs to be added to the discussion....   

When you check a Engine by doing a "leak down test",  you must use some sort of refrence data and calibration for the test...

 The source of air for the leak down test must be calibrated for Flow at the given PSI.

Meaning that :

IF I can hold in the engine  80 psi,  pumping in 100psi at  3 cubic feet per minute airflow.  

OR

 I also can hold  in the engine 100 psi, pumping in 100psi   at 6 cubic feet per minute......

OR

MY Briggs 2 hp, worn-out  (small piston) engine will hold 100 inside at 100psi pumped in at  3 cubic feet per minute.

BUT

My  Briggs new  8hp  (large bore piston) engine will only hold 75 psi inside  at 100 psi  the same cubic feet per minute.

Meaning that all the data you can collect is worthless unless you have a baseline of data for what you are working with...  That's why Most good Leak down testers have a built on regulator (not adjustable) and Calibrated feed inlet that only allows  (X) cubic feet per minute of flow..   Plus diffrent bore size engines have diffrent flow rates to test by....

SO unless you have GOOD data to test and compare on the engine,   The standard compression gauge is the unit I would use.  The Compression gauge will tell you what compression the motor will make in typical attempt to start...  If it will not make compression,  it will not start or will be hard to start. (isn't that what most people are looking for?)...  Your calibrated arm pulls the same every time (until tired)...

Good Luck, My 2 cents..

Friiy

Borat, did the machine reference above match your machine?

>>>No oil fill holes or valves on this engine.

   It’s got a port cover, rings and head gasket. 

 

Friiy,

>>>That's why Most good Leak down testers have a built on regulator (not adjustable) and Calibrated feed inlet that only allows  (X) cubic feet per minute of flow..

 

   Right, so I don’t know why you mention the CFI unless you have some weird set of gages. ??  Why are you dumping in 100psi on a small engine?  It will only make the test setup harder and maybe hurt yourself with piston and flywheel turning under pressure that great.

   The small engine leak down gages are low pressure sourced from a regulated tank (required and standard on most compressors).  The low psi (15) on these gages are all that’s needed and work fine across small engines without knowing the CFM.  You don’t need to know for these gages. It’s taken care of.  

 

 

Friiy:

>>>The Compression gauge will tell you what compression the motor will make in typical attempt to start...  If it will not make compression,  it will not start or will be hard to start. (isn't that what most people are looking for?)...  Your calibrated arm pulls the same every time (until tired)...

 

   This isn’t a plane or car engine.  At least that I ever heard of or read no one ever uses a compression gage related to starting pressure values on a small engine 5-13hp.  They are used to determine the general health of the components related to compression on the power stroke.  Compression gage can’t do that especially in 4 strokes where they can only give you very limited info at best.

   Some L-head engines and it’s very rare will allow a full reading on the compression stroke so for them a valid reading.  I test lots of L-head 4 strokes but not 2 stroke where the info would be less with a leak down than a 4 stroke but still more useful than a compression test i.e. port covers, reed valves, head, rings and other types of intake exhaust mechanisms.

   Wicked low compression test values are one of the few places a compression test is useful.