compression test, reeds or rings?

[arnieb]

Hello group,<br /><br />I have a 1975 Evinrude 70hp. Just did a compression test. Top cyl 80 lbs, middle 90 lbs, bottom 90 lbs. Seems alittle low? I then put some oil in the top cyl and tryed again. It stayed at 80 lbs. So what does this mean? Reeds, or are these piston ported motors? Any ideas would help.<br /><br />Best Regards<br />Arnie

 

[Walker]

Re: compression test, reeds or rings?

Well, it ain't the reeds.<br />Do a de-carb treatment on the motor and test the compression again

 

[Silvertip]

Re: compression test, reeds or rings?

Just a technical point. Reeds have nothing to do with porting and all two strokes are piston ported. Neither of these has anything to do with compression. Piston, cylinder wall and ring condition determines compression. If the engine runs smooth and seems to perform well, you have nothing to worry about. Although the one cylinder is on the borderline of being within 10% of the others, it's a sign some attention is needed. As Walker points out, decarbing (as in decarboning) using SeaFoam or similar product may bring that cylinder in line with the others.

 

[JB]

Re: compression test, reeds or rings?

All outboards use reeds for crankcase intake valving, Arnieb. <br /><br />Engines that use the skirt of the piston to time crankcase intake (piston ported) make low torque at low rpm and are almost always used with multispeed transmissions, like motorcycles and snowmobiles. Rotary valve intakes are too heavy and bulky for outboards.<br /><br />Absolute compression readings vary too much from guage to guage and technique to technique to set criteria. Comparison among the cylinders is valid, and 10-15% is the usual criterion, meaning that your engine meets the standard at 11% variation.<br /><br />I agree with the above advice. You can probably improve the comparison by decarbing the engine, but you are not in compression trouble.

 

[arnieb]

Re: compression test, reeds or rings?

Thanks for the advice. I will be looking for some<br />de-carb. I just bought this boat and motor. Have to try it out on the water SOON. Lots to do before then. I have a service manual coming for the motor and hope to change the water pump adjust carbs, change plugs and bottom end oil. Its been 20 years since my last boat and can't wait to get it in the water! <br />Had a dream last night that I put the boat in the water for the first time and left the drain plug out, lol. Hope this dream helps me remember the plug when the real time comes.<br />I'm new to this forum and have looked back at most of the posts. I have learned alot from everyone. Sure glad I found a place to ask questions. Now if I could find out how to hook up this old Airguide tach?<br /><br />Again thanks for taking the time.<br />Best Regards<br />Arnie