Mercury 2004 90hp 2 stroke compression

[Dive101588]

Hi,
I have been trying to find out what compression I would expect from a 2004 Mercury 90hp 2 stroke , what is good what is bad pls?

 

[Texasmark]

The service manual covering '94 thru '05, 3 and 4 cylinder 2 strokers in the Jet 60 thru 125 HP specifies: "Expect problems when compression drops below 120 PSIG. Variance between cylinders, 10-15% max.". Continuing on they said that cylinder to cylinder uniformity is more important than actual pressure within limits. One performing the test should have an accurate meter and use it properly. Throttle should be open to allow for maximum air to enter the cylinders, all plugs out, engine at operating temperature, and the battery capable of spinning engine at 200 rpm. Oil injected into the cylinders will increase the reading and possibly give one a false sense of security.

 

[Dive101588]

What does the SW mean at the end of ELPTOSW.
Is there a big difference between power of a Mercury ELPTOSW to a Optimax same year 2004 ?

 

[QBhoy]

I bought a boat for pennies about 4 years ago. Sold as parts or repair, with a 1998 mercury 90 2 stroke (same engine)
It was deemed as this by the owner after his mechanic told him all compression had gone.
I bought it for half the worth of the trailer and parts of the engine, so worth a gamble.
The compression on each of the 3 cylinders were from memory 75, 90 and 90. Or something similar, when I tested it cold as soon as I got it back to my house. No big deal and was as expected after the deal was made. But I threw some 2 stroke oil down each bore and turned her over by hand for that afternoon every hour or so. Then left them full of 2 stroke over night. Hadn’t even tried to start it.
Next day, I put a battery in it to see if it would run with fresh fuel. It did. Ran her for half an hour on the muffs after the smoke cloud cleared !
Did a compression test again after stopping her. 115 approx across all three. Put her in the water after fitting a new steering cable...she ran like a champ. She is still in use now with her happy owner, years later.
Id say over 100 psi would be the result you are looking for...and remember that most cheap gauges that aren’t calibrated will and can show up to 20, even 30 psi low or high. If all three are showing similar results within about 10% of each other...and read a little low...suspect the gauge more than the engine. I’d say.

 

[Dive101588]

Thanks,
2004 Mercury 2stroke 90hp 225 hrs Got 95,95,96 PSI compression , 2 mechanics have said all good with these numbers and equal across all three. Took it for a run the other day for 25 mins at 5500 rpm got 30 knots no flat spots through acceleration. Started straight away, no white smoke, light blue using oil good. So maybe good for 17 year old technology.

 

[Texasmark]

The Loopers use wedge shaped pistons that expand under the compression produced when the fuel air mixture explodes in the cylinder. If the rings aren't free to move and produce the designed sealing function, your compression will be low.

Sea Foam is available everywhere in the 16 oz can for about 7 bucks. Regular treatment is 1 oz per gallon of gas/fuel mix. You can't over do it per can instructions, in fact, the aerosol is used to check engines that don't run in determining if it is fuel or spark.

Accessing the carb venturis and shooting SF into the carb while cranking will start the engine if fuel is the culprit.....so it is combustible and contains light mineral oil so the 2 stroker cylinders do get oil if premix type of system.
What I'd do if I were you, and what I have done on occasion over the years, is to dump a whole can into 6 gallons and get the boat out and run it hard for an hour or something of the sort.

Then take the rig out again in a few days, week, whatever, and run it hard again. When you get home, pop the cowl and plugs, put the throttle at WOT and check compression again. May be pleasantly surprised!