Compression.......

[mricemachine]

I have seen in many post here about compression. It seems that the number is not important but that all of them are close to each other. My question is this:<br />Example: 1993 175 evinrude all 6 at 110<br />different motor 1993 175 evinrude all at 85<br />Now this might be a very bad example but why would the compression be so low? However still good based on the "all the same" idea. <br /><br />Just curious

 

[Barnacle_Bill]

Re: Compression.......

There are many things that come into play here and I'm sure others can add more. But what immediately pops into my head are questions like this.<br />Was the same gauge used on both motors?<br />Did the starter turn at the exact same speed on both? Was there the samed anount of residual oil in each cylinder of both motors? Was the gauge tightened down the same in each cylinder? Was the motor allowed to turn over the same number of turns each time? Were both batteries in good condition and fully charged? I'm sure there are other things that apply but its too early in the morning for my mind to work so hard. LOL

 

[jfm1]

Re: Compression.......

also carbon build up

 

[itstippy]

Re: Compression.......

Taking compression readings is a very good way for a novice to evaluate an outboard he's interested in buying or fixing up. Despite little familiarity with the gauge or the outboard he can tell if there are "for sure" internal problems. Getting low (-80) readings on all cylinders or a significantly lower reading on any one cylinder means something's up. A simple tuneup will not make that motor run right. <br />As familiarity with the gauge and the outboard increases, so too does the precision in interpreting results. I always use the same venerable Snap-on gauge I got from Grandpa. I do a compression check on "Old Blue" every year when I put him away in the fall. He always runs 115/120. If he suddemly dropped to 85/85 I would freak and find out why.<br />Really experienced outboard wrenches (not me but there are some on this board) have used the same gauge and same technique on hundreds of outboards. The compression test is a very useful tool in their bag of tricks. When they evaluate an outboard they are looking at all kinds of stuff we overlook (gasket conditions, paint blisters, oil smudges, ...). These things IN ADDITION TO the compression test give them a window into the guts of the outboard. But they can't teach you to do that over the internet. I wish they could!

 

[CFronzek]

Re: Compression.......

The procedure is the same for 2-stroke engines. No spark, carb open, warmed up if possible, same numder of cranks for each cylinder. <br />But, piston and ring sealing, as indicated by the pressure readings, are much more important on a 2-stroke. That is because both the bottom and the top of the piston help pump the mixture thru' the engine. When the piston starts to lose seal (cylinder wear, bad/broken rings, carbon buildup) the burned gases can get into the crankcase where the incoming charge is and lean the motor out. On engines that don't have a reed valve the mixture can actually be blown back out of the carburator. As cylinder pressures drop the motor will lose power and run hotter to the point where there's not enough pressure to fire the engine.<br />When you do something about it is up to you.<br />Charlie

 

[umblecumbuz]

Re: Compression.......

Charlie,<br /><br />That's a real concise and comprehensive assessment of two-stroke internal health.<br /><br />Thanks.