Compression Question

[Tonka Boater]

I brought my boat in this week for some spring maintenance - new starter, trailer maintenance, etc. When I purchased the boat last spring I trusted my gut and didn't have them do a compression test...it looked, sounded and ran great. I started to see a little bit of black smoke from the exhaust toward the end of last season....nothing excessive or worrisome but enough that it caught my eye so I had my shop do a compression test while they had it for the other items this week. The shop I use is local but they're the best on our lake -- better than the chain dealer in my opinion.

The tests came back pretty good for an 18 year old engine -- here are the numbers:

#1=152, #2=150, #3=155, #4=150, #5=151, #6=147, #7=150, #8=152

My research shows that this vintage 502 MAG should be in the 150-160 range so everything looks good overall and my shop said it's running very strong. #6 is a little low but not way out of line compared to the others.

Questions: I'm not an engine expert, what are your impressions of the above numbers? Is #6 at 147 anything to be concerned about? As for the smoke, my plugs weren't fowled but they were in need of a change so I had them do a complete tuneup (plugs, wires, cap, rotor, fuel filter, etc.) and they said they didn't see any smoke when they ran it.

 

[Lazy Daze]

Perfect. I don't see a single thing wrong with ANY of those numbers. It would be rare, even for a new engine, to have all cylinders at the same numbers. You are WELL within spec. Have fun, go boatin'!

 

[Tonka Boater]

Perfect. I don't see a single thing wrong with ANY of those numbers. It would be rare, even for a new engine, to have all cylinders at the same numbers. You are WELL within spec. Have fun, go boatin'!

Awesome! Thanks Dennis!

 

[Lazy Daze]

You're welcome!

 

[fwebster]

If the engine happened to stop last fall with the exhaust valve on #6 open, some slight surface rust on the valve seat or valve face, could explain the slight variance in compression. If it were mine, I'd run it regularly about 1/2 a season….or all season, then recheck #6, and I bet the variance goes away.

 

[RollerCoastr]

Nice looking numbers. I wouldn't attribute black smoke to low compression anyway. Black smoke is typically a rich-combustion condition. Blue smoke (burning oil) is what might send you looking for things like low compression.

 

[Tonka Boater]

If the engine happened to stop last fall with the exhaust valve on #6 open, some slight surface rust on the valve seat or valve face, could explain the slight variance in compression. If it were mine, I'd run it regularly about 1/2 a season….or all season, then recheck #6, and I bet the variance goes away.

Thanks Frank...great advice.

As a side note, I had them replace my hour meter and read the engine hours. When I bought the boat it had 364 hours on it....when I returned from my first trip with the boat it still had 364 hours on it. Aargh...I never would have thought to check the hour meter to make sure it was working. Anyway, the engine actually has 660.1 hours it. Still not bad for an 18 year old boat. I manually tracked 40 hours on the engine last season....we had over 120 hours of boating but only 40 with the engine running. That's about in line with the history of the boat -- 36 hours per season on average. The shop checked the risers/manifolds and everything looked great. It's a freshwater boat that has been well maintained and they didn't see any cause for preventative replacement... they think I still have a few more seasons.

 

[Bob & Sue]

Is your engine carbed? I have a service bulletin #97-8 1-599 on the electric choke being set to rich. Mine was I leaned it by 2 clicks smoke went away but had to pump it several times when cold, I set it 1 click richer starts fine with just a bit of black smoke till choke opens completely. Let me know if you want the service bulletin.

Setting the Electric Choke
When servicing an engine, the choke should be reset to specifications as listed below. Add these choke specifications to all your older service manuals. The old setting were too rich for most engines. Production carburetors on new engines are set at these settings. If needed, the choke can be set slightly richer or leaner to customize it for the operating conditions that the engine will be run in.

V6, V8: 2 Index Marks Clockwise.

a - Timing Mark in Cover is Shown In Line With Center Index Mark.
b - Clockwise = Leaner.
c - Counterclockwise = Richer

 

[Tonka Boater]

Nice looking numbers. I wouldn't attribute black smoke to low compression anyway. Black smoke is typically a rich-combustion condition. Blue smoke (burning oil) is what might send you looking for things like low compression.

Is your engine carbed? I have a service bulletin #97-8 1-599 on the electric choke being set to rich.

Mine is fuel injected but you're both correct.... my shop said the smoke sounded fuel-related which is another reason they suggested the tuneup.

 

[Bob & Sue]

Mine is fuel injected but you're both correct.... my shop said the smoke sounded fuel-related which is another reason they suggested the tuneup.

Might be a sensor failing causing a rich condition.

 

[Tonka Boater]

Might be a sensor failing causing a rich condition.

Good point. There's no smoke after the tuneup and it's running great but I'll keep this in mind if it comes back.