86 Black Max 150 Compression Test

[techhome]

Hi, New to screamandfly - looking for feedback on my comp test.

86 carb'd Black Max 150 XL. This is on my fishing boat, not a go fast machine.

Engine tested warm, fuel disconnected until motor stalled, all spark plugs removed

Compression readings on my cheapo tester: 100, 90, 100, 98, 95, 95

Retesting yielded identical readings

Motor starts easily and runs fine, idle is a bit rough and lopey, which I think may be normal for this engine?

Should I be concerned with these results?

Doug
Annapolis, MD

[gregpro50]

If they are accurate the numbers are a bit low. It should be up around 125 or so but it could also be the gauge you are using which is pretty common for the cheapie ones to read low. Rule of thumb is that you want the results to be within 10% of each other which it seems you are on the border line.

[techhome]

Much Thanks, I knew the variance was not good news, but good to know I'm not over 10%

I have some follow up questions
I have two other Black Max 150s sitting in my garage. I bought em cheap and was told that both were running motors. Both have had the oil injection removed.

#1 Was used on a crabbing boat w/ lots of shifting- reverse was blown, but motor supposedly runs good
#2 supposedly starts, but dos not idle, was told lower unit on #2 is good. but it needs a re-seal at minimum

Guy I bought them from had planned on swapping the lower from #2 to #1 but never got it done.

I don't have an engine stand nor extra controls to try and start these motors. What I'd like to find out is if either of these 2 motors has a powerhead that is in better shape than the Black Max 150 on my boat
Q1 Is it possible to do a cold compression test on these used motors without a set of controls?
Q2 If so how useful is such a test?
Q3 Is there a better way to evaluate these powerheads, perhaps pull the heads and have a look?

Thanks,

Doug
Annapolis

[gregpro50]

You can spin the motors by touching the starter leads with some jumper cables and a car battery.I think it would be useful to get a general idea of the internal condition.If you get a reading that doesn't make sense it it real easy on these motors to pull the heads and have a look.If the motors have been sitting you'll also want to go through the carbs and fuel system too.

[techhome]

Good deal, thanks. I will dry test and see if I have a powerhead that gives more consistent readings.

Doug