1990 Merc 40hsp

[jbaranski111]

I went to buy a1990 Merc 40 Hsp and did a compression test. Motor was super clean, seller nice guy. Top Cylinder 146, next 146 next 149 I was like cool. Bottom cylinder 126. Uhhh ohhh. I did not buy it. Thoughts?

 

[racerone]

Parts are precision made these days.-----That variation speaks on internal problems !!-----They should all be within 5 PSI.

 

[jbaranski111]

Thanks. I have been reading that it could be many things. Possibly carbon on the rings or maybe from sitting for 8 months....... I don't want to take a chance but wish we could have started it then took a reading. Would a cold reading be accurate?

 

[jimmbo]

In the early 80s I saw a few Mercs that had lower compression in the lowest Cylinder. Engines ran fine and had good power. Upon disassembly couldn't find anything obviously wrong. No scored/scuffed bores or pistons, no broken rings either. I started to think it was sloppy technique when doing the Compression Testing. The lower pan of the cowl did make for a tight fit for getting the gauge fitting threaded in.

 

[Faztbullet]

Could be lower seal allowing water intrusion and cylinder has damage....20psi difference is not carboned rings

 

[jbaranski111]

Thanks all for the input. Anyone else with more input?

 

[QBhoy]

Fire some tcw3 oil down the bore and leave it for a day. Try again.

 

[jimmbo]

Place the engine so the cylinders are as close to vertical as possible, rotate the crankshaft until the exhaust port is closed then spray in copious amounts of Engine Tuner and let sit for a day or two, purge it, then put some oil in and try another compression test

 

[Faztbullet]

Oil and cleaners will not bring back 20psi...that cylinder has a problem.

 

[jimmbo]

Doesn't hurt to try. Pulling it apart would be the next step, if nothing changes

 

[QBhoy]

I only said about throwing oil down it from experience with a 96 2 stroke 90 merc I had. Same cylinder was 40 psi down compared to the rest. Hadn’t been used in 3 years. I bought it cheap. Left oil down it for 2 days and turned it over a few times by hand on the flywheel during that time....brought the compression right back up again.