Logo
FULL SITE MENU

Compression #s for 70 hp

A fresh rebuild would be around 140 psi. A used one 20 something yrs old I like to see around 120 psi depending on condition. Minimum to run would be around 100+ anything under that is iffy and the cylinders need to be within 10% of each other no matter what.
 
Normal is over 100+, 130 psi, something on that order, and fairly even on all cylinders.

It is best that you tell us what psi reading you are obtaining on all of the individual cylinders... not just a note that the compression is great or some such statement. Let us know what you find.
 
Last edited:
Couple more questions. I saw very close to 100lbs on all 3. This thing had not been cranked in about a year and wondered if because of dried out oil in cylinder or something if the number might go up after it runs a bit. Also because I did'nt think to manually open carbs while spinning it over (neutral safety switch) if that might matter. I justified that in my head by thinking that the 2 stroke was getting enough air thru the crankcase and open plug holes. Value the opinion of folks that know more about it than me.
 
The throttle does not need to be open for a compression test. The motor is not pulling air in through the spark plug holes Once you plug the spark plug hole it acts like normal and pulls air through the carbs. If the cylinders were all even I would suspect the compression tester. I would spray a 50-1 mix into the cylinders and let it sit over night. Then I would do another compression test with a different tester.
 
flyingscott said:
The throttle does not need to be open for a compression test. The motor is not pulling air in through the spark plug holes Once you plug the spark plug hole it acts like normal and pulls air through the carbs. If the cylinders were all even I would suspect the compression tester. I would spray a 50-1 mix into the cylinders and let it sit over night. Then I would do another compression test with a different tester.
Click to expand...
Duh.. guess there are pistons blocking the holes..... but..... would,nt it get some air from the other 2 with the plugs out thru cylinder port and the crankcase? I'm not very knowledgeable about 2 strokes...
 
ss396fish said:
Duh.. guess there are pistons blocking the holes..... but..... would,nt it get some air from the other 2 with the plugs out thru cylinder port and the crankcase? I'm not very knowledgeable about 2 strokes...
Click to expand...

Don't overthink it... it'll drive you nuts.

No need for open butterflies.... the air for compression is there. Where does it come from... who cares? :)
 
Yea I'm bad for that... I did run the boat and even on the old gas it ran fine, at least till we got ready to load up when it started a spell of dying when I put it in gear and just getting into the throttle a little. Maybe just fresh gas? Or could a little varnish have something fouled? Carbs were supposed to be cleaned after it was run the last time. 18 gal tank was left brim full but probobly not treated. Ran great for about 30 min. and idled fine but started its light throttle problems. When it did decide to go it pulled hard and ran good. Will try compression test again.
 
Istalled a new impeller ?---Updated the cooling system ?------There are known issues with the center cylinder running hot !
 
Does the tell tale hose come off the side of the BLOCK ?-----Or was it moved to the top of the block per a 1994 service bulletin ?-----Latest factory water pump kit has all the parts needed to modify the cooling system / thermostat.
 
flyingscott said:
A fresh rebuild would be around 140 psi. A used one 20 something yrs old I like to see around 120 psi depending on condition. Minimum to run would be around 100+ anything under that is iffy and the cylinders need to be within 10% of each other no matter what.
Click to expand...
I just bought a compression tester kit how do I run the compression test on my 70hp Evinrude I think a cylinder has low compression
 
Remove spark plugs.----Install compression tester in #1 and crank till gauge stops going up.-----Repeat with #2 and #3 cylinder.----Note---There is n need to open throttle plates for a compression test.----Post the actual values here.----Perhaps start a new thread.
 
Back
Top