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Compression how much?


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  #1  
Old 09-24-2001, 12:33:25 PM
larry
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Default Compression how much?

How much compression should I have on a marine 2 cycle engine? Right now 12lbs is all I can get.
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  #2  
Old 09-24-2001, 04:30:04 PM
Russ Hughes
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Default Re: Compression how much?

That sounds kind of low, being less than two to one. What kind of an engine are you refering to?
  #3  
Old 09-24-2001, 06:01:26 PM
SAM
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Default Re: Compression how much?

YOUR AVERAGE CHAIN SAW SHOULD HAVE MINIMUM 100 LBS.
  #4  
Old 09-25-2001, 09:45:33 AM
larry
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Default Re: Compression how much?

Its a truscott special engine(sparkplug not igniter) 3 1/2 hp.It has a 4inch piston and 14 1/2inch flywheel.Everything looks fine no rust ,no scraches ,but can't get much out of it.There is no head and no removeable jug(cylinder) so measureing clearances is hard.
  #5  
Old 09-25-2001, 06:00:28 PM
larry
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Default Re: Compression how much?

I tried adding oil on the piston,but got no increase.Looks like I have more work to do on this one.Thanks for the replies.
  #6  
Old 09-26-2001, 03:39:32 PM
Russ Hughes
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Default Re: Compression how much?

With the piston at top dead center, using some sort of a measuring cup, fill the cylinder with some light lubricating oil. Determine how much oil it took to fill the cylinder. Next, with the piston positioned as low as it can go without opening either the exhaust, or the intake ports on the side of the cylinder, repeat the process again. Note the amount of oil it took to fill the cylinder once again. This should provide you with a theroretical compression ratio by comparing the two volumes of oil used to fill the cylinder. The actual effective compression ratio will be less than indicated because of leakage losses in a running engine.

You might also let the oil stand in each case to see if it drains out and how fast it drains. This can tell you how good the seal is past the piston. You would expect to see some oil loss over time, but it should be a slow loss.

At least this will provide you with some indication of the potential compression ratio this engine should have. If someone in the past replaced the piston with another piston from some other engine and the distance from the top of the new piston to the wrist pin is far less than what the original piston was, then the compression ratio will now be far less than designed.

I do not know anything about your engine, but throughout the years, anything may have been done to it. Not being able to access the cylinder from the head end makes it difficult to assess the situation. At least By measuring the two volumes of oil used to fill the cylinder, you can get a rough idea of the approximate compression ratio.

Most of the older two cycle outboard engines were also constructed the same way without a seperate cylinder head. Be sure to drain out the excess from the crankcase and cylinder before attempting to run the engine again.
  #7  
Old 09-26-2001, 07:37:12 PM
larry
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Default Re: Compression how much?

Keeping oil in the cylinder would be a problem.Today I dissasembled it after more testing and it looked ok.When i ran my finger through it I could feel something.Useing a hone for a while made a deep scrach appear.The piston fits good with no apparant wear so I guess I will try to hone it out.Thanks for the help.
  #8  
Old 09-27-2001, 11:20:25 PM
Russ Hughes
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Default Re: Compression how much?

You have to keep the oil in the cylinder only long enough to make the measurements.

Adding a little extra oil to the cylinder helps make a better ring seal, but it doesn't last long with the engine running. An out of round, warped or otherwise distorted cylinder, or piston can cause a loss of compression. If the gouge in the cylinder wall is large enough, you can lose a lot of compression through it. Wish you good luck in solving your problem.
 

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