trouble shooting Compression test

[ShawZ]

I have a 1976 Merc 650, ser# 7074122, 3cyclinder

I bought a compression gauge today, and did a test on compression. my boat have been lagging in power since I bought it last year. Top cycl. was at 130ish for compression, middle was 140 and the bottom was at 115. I was told the magic number is 125.

but was wondering if the lower number could be another issue other then a bad cyclinder?

I have a small hole on the left side of motor just above the throttle linkage the spits out a steady stream of water, just wondering if that could cause lower compression?

just wondering if a doing a tune up on the motor is all it needs, or I have bigger problems and should start looking for another motor, any help is greatly appreciated thanks.

 

[racerone]

The small hole is the vent for the poppit diaphragm.----That has nothing to do with compression.------Means the diaphragm is torn or leaking.---That should be fixed !-------Remove cover on other side of the motor to inspect pistons / rings.--Compression numbers are not encouraging.

 

[Chris1956]

Your compression is fine. Look at your spark plugs, after running it. If one plug is cleaner, your motor may be getting water into a cylinder causing it to misfire.

Check spark plug wires for arcing to ground. Make sure motor has spark on all cylinders.

 

[Faztbullet]

That compression is not fine!!!

 

[ShawZ]

How do I check plugs for spark and wires for arch?

and what cover do I remove to check pistons and rings?

so is compression fine or not? getting some different answers on that?

 

[racerone]

Some will argue.-----Your compression is very suspect.-----------On the starboard side is a large cover.---------Remove it to look at pistons and rings.------------Or to replace the diaphragm you have to remove exhaust cover.-----that also allows a look at pistons / rings.

 

[flyingscott]

As a rule your compression #s need to be within 10% of each other. That means all the cylinders need to be within 13 psi of each other. That bottom cylinder is suspect, I would not replace anything on that motor before I looked at the pistons and rings. Do what racer said in post #2 and #6

 

[ShawZ]

I will have look tomorrow hopefully, what I'm I looking for exactly?

 

[flyingscott]

You will know when you see it. The piston will probably be scored.

 

[Chris1956]

Your spark plugs look pretty good. I would expect the bottom plug to be darker. You may be getting some water into it. But first start the motor on a flusher and observe the spark plug wires. Listen for a snapping sound which indicates arc to ground. Put a timing light on each one and see that she sparks consistently. You may only need to replace a spark plug wire or other simple ign component.

Your compression is a bit low in the lower cylinder, however, considering the motor is 40+ years old, I am not sure it makes $$ sense to repair. I would try to make sure she runs up to her potential, and run it. JMO....

 

[racerone]

The poppit valve diaphragm must be looked at / replaced !------Motor may overheat the way it is now !

 

[ShawZ]

I will open it up and have a look


I'll take pictures at well.

 

[ajgraz]

I think your plugs look fine. If you were getting water intrusion...and it would be on the bottom cylinder on that motor...the bottom plug would look either steam-cleaned OR (once bad enough) wet, foamy and nasty. Question though: fresh water or salt?

I also think your compression numbers are OK for a motor that vintage. Salt water killed my 1978 3cyl 70 (basically same motor) after ~8 years of my ownership, ate right through the exhaust baffle plate (under the portside coolant cover plate), started as a trickle then became a flood into the bottom cylinder, knocked compression on that cylinder down to 30psi and ate the lower crankshaft bearing. You'd know if this was happening, you'd have REALLY crappy power and some serious "clanky" noises. Not to mention the steam-cleaned plug.

(Wound up replacing that powerhead in 2012 with a 1988 NOS powerhead, still running strong)

When's the last time you cleaned (and I mean REALLY cleaned!) and/or rebuilt the carbs? That was always my bugaboo with that motor, until I put a water-separating filter on it. Went from cleaning carbs every 3 to 6 months to I haven't had to clean them in 5 years.

 

[ShawZ]

I bought boat combe last year off a guy. Told me it was launch ready. I don't know much about boats and motors so I'm learning on the fly.

The motor lacks power big time. The 7hp kick motor pushes the boat faster.

I am guess I have a carb issue as well. But one thing at a time

 

[Chris1956]

If the kicker out performs your 650, you have an issue. Your motor is likely running on 2 cylinders, and running poorly on those. As was suggested, check for spark and arcing on the plug wires.