Compression Test for small engines

Discussion in 'Mechanic and Repair' started by fixer67, May 18, 2004.

  1. fixer67

    fixer67 LawnSite Silver Member
    Posts: 2,098

    I keep reading post after post about doing compression test on small engines. The thing is the type of compression test you do on your car does not give you the data you are looking for when it comes to a small engine. In fact to my knowledge no small gasoline engine manufacturer even gives compression data for there engines. The test that is used instead of a compression test for small engines is called a leak-down test. A tester is screwed into the spark plug hole with the position at TDC with both valve closed and 100 pounds of compressed air is pumped into the cylinder and a percentage is given as to the leakage you have from valve and ring blowby. If you are doing auto type compression test on an engine with a ACR like the Kohler K-341 you will get a very low reading on compression because the ACR does not drop out until the engine is running. And with a leak-down tester you can feel the air blowing by the rings and out the oil fill hole or out the carb or muffler so you know where your problem is. I did compression test on small engines for years before I found out just how useless there are.
     
  2. Mikes Machines

    Mikes Machines LawnSite Member
    Posts: 112

    I agree completely, a compression test on an engine with a decompressor is useless because it will be way low. The only good test is a leakdown. This tells us what shape the engine is in. I almost always do a leakdown on a no start that comes throught the shop. This way you are not chasing your tail with an engine that almost starts but refuses to run
     
  3. ducky1

    ducky1 LawnSite Senior Member
    Posts: 252

    I agree to a certen extent. But a compresson test is handy on twin cly to see if both sides are working and how each compare with each other.
     
  4. sdwally

    sdwally LawnSite Senior Member
    Posts: 385

    We use both depending on engine. Neither test is complete with out the other.
     
  5. xcopterdoc

    xcopterdoc LawnSite Senior Member
    Posts: 752

    A compression test is only good if you know what the specs are. As fixer67 said, most small engine manufactures dont even give compression specs. The leak down test is more accurate anyway. It will pinpoint exactly where the problem is. With a comp check you would have to do a dry and a wet test IF you had specs. Way too much time.
     
  6. mower_medic

    mower_medic LawnSite Member
    Posts: 77

    There may not be any specs for small engines available; but I have found through experience,that a small 4 stroke (2-8HP) with a compression reading of as low as 45psi will run but will not have any power. Above about 75psi for a small 4 stroke is decent enough to run good and have plenty of power.90-110 is excellent. On the other hand, a compression test is the first thing I do on any 2 stroke that comes into my shop before I do anything else. Anything below about 90 psi and it isn't going to run very well and will be very hard to start. I also recommend anyone contemplating buying a used piece of equipment do a wet/dry compression test as part of your inspection and demo prior to buying the item. It can go a long way in determining the amount of life left in the engine.Just my 2 cents.
     
  7. repair_guy

    repair_guy LawnSite Member
    Posts: 4

    A leak down test on these newer engines that has got a comperesson relase is all well and good.But don't forget the older engines that are still out there that wasn't made with a compression relase.On these you will still to have a compression guage to see if cylinder/piston/rings/valves are ok,weak or what.
     

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