1976 20 HP Mercury Thunderbolt

Hey All,
My dad gave me this motor which I'm planning to sell to offset the cost of a used boat.
The motor did not run for about 10 years, nor was it fogged or the lower gear case oil changed. I pulled out the carb and gave it a really good cleaning and replaced the gasket from carb to block only. I also changed the plugs which are surface gapped (I think that was the term...no prong, just the exposed electrode).
Anyway, it started right after that with little effort. I kept spraying throttle body cleaner and carb cleaner in the motor to really clean it out. I ran it from slow RPM's to high for about 5 min's unit it started backfiring with puffs of smoke as it wanted to stall, but it wouldn't. Well it finally stalled and I'm having a hard time getting it to start up again. I even tried spraying 'quick start' directly into the carb, but nothing. What do you thing the problem is?
I decided to check the compression and found the top cylinder was 120 and 60 psi on the bottom. So what do I have to do to get the psi up on the lower? Do I have to send it in for a rebuild? Is it worth it? Or can it run as is and what is the risk.

I'm going to take a video and post the link in the next while so you can see it in motion.

If you have any comments that could help me out in the mean time, I thank you in advance.

Be Well All,
BB

If it is only 60 on the bottom it is probably toasted and the "quick start" may have been the final straw which ultimately finished off the cylinder (that stuff should never get close to a 2 stroke motor).

Is it worth fixing to then try and sell - absolutely not if it requires ever a partial rebuild.

A mid-70's 20 horse motor might sell for 500-700 bucks, on a good day, if it was running perfectly.

A brand new 20 horse can be picked up for about $2500.

It could cost you $1000 or more to have this rebuilt at a shop and otherwise put in working order.

So you can do the math.

Your best bet would either 1) sell it as is, you might get 200 or 300 for it or 2) part it all out which may or may not bring you a little more than selling it as a complete motor "that needs work"...

You can't "clean" a carb by spraying magic juice at it and changing a few gaskets--it has to come COMPLETELY apart and be cleaned out--all the little emulsion tubes and passages. You've good some crud in there that has to be removed.

he lower cylinder had bad compression, but it might simply be stuck rings. Mercury sells a cleaner that you run through the engine to free them up.

Jeff