Compression?

jezza2011 · 10 April 2015, 14:56

Ok I have a 1991 Mariner 75hp 2 stroke and I just tested the compression and it appears to be 50psi across all the cylinders.

I warmed her up and pulled all 3 plugs opened the throttle all the way and cranked her over.
is this the correct way to do this and is 50psi low?

Nos4r2 · 10 April 2015, 15:36

If it's running, I'd suggest either your gauge or method of testing isn't right. It probably wouldn't even start at 50psi.

Did you carry on cranking til the gauge stopped rising?

jezza2011 · 10 April 2015, 15:38

Well that's what I was thinking.
yep cranked until it didn't go up any more.

it starts first crank

nocando · 10 April 2015, 15:39

if it runs good then i'd assume the gauge is faulty. you would normally be in the 120 ish range if my old 75 mariner is anything to go by.

on an engine i had once i did drop a cylinder down to 70psi and i lost about 15mph, you will know when you open it up if you really have 50 psi, i doubt it though.

cheers

Festinghouse · 10 April 2015, 16:31

yeah should be 120 on all 3 cylinders. have you done anything to this engine like a rebuild, or is it as is?

jezza2011 · 10 April 2015, 17:48

No its not had a rebuild. its just very old
still runs well
I will test the gauge

Xk59D · 10 April 2015, 18:50

why did you test it?

i assume there is a reason, if it is down on power and you wanted to check then it could very well be correct.

cheers

uncle al · 10 April 2015, 18:51

The correct way to do a compression test,
1/ power head as close to operating temprature as possible,
2/ remove all the spark plugs ,and connect the H.T. leades to a spark checker to earth them safely and not damage the stator,
3/ throttle wide open,
4/ Spin over on the starter untill the gauge needle stops,
6/ make sure you have a good battery.

Nightfisher · 12 April 2015, 06:16

Do a wet test. Remove all plugs, squirt a little oil in each hole. turn over a few cranks to spread the oil around the commpression rings. Put one pug in and do the test, note the reading. Remove plug and install other plug into other cylinder, repeat. If the difference between the dry and wet test is more that 10-15 lbs Ring problems. If the wet test shows a difference in reading of more that 12 lbs Ring problems. Rebuild in order.

jezza2011 · 13 April 2015, 09:45

I am assuming the gauge is out. It starts and runs well.
Pulls well skiing and ringoing as well as hauling 6 divers and kit.
I was testing it to make sure its all ok as its very very old ( 1991)

I am fairly happy that its ok as like you guys say at 50psi I doubt it would start and run let alone pull well.

Nos4r2 · 13 April 2015, 09:53

Quote:

Originally Posted by jezza2011

I am fairly happy that its ok as like you guys say at 50psi I doubt it would start and run let alone pull well.

At 50psi, if it did start out of water on muffs , it probably wouldn't start in water. If by some miracle it did start, it'd die as soon as you put it in gear.