1980 4.5 running poorly now wont start

The back story. Engine came with a sailboat I bought this spring. To show how good the engine was, the seller had it running when I got there, without any water. I shut the motor off and thanked him. I was more interested in the boat. Since the motor was running I put in a new impeller. I also went thru the carb, which was clean. I'm using premium without ethanol. Engine ran good and moved the sailboat until about a week ago then at high power it just faded away and died. After a few minutes I of letting it sit I was able to start it again. The other day the boat was beached for the day and I noticed fuel leaking out the carb with the engine tilted up and the gas tank pressurized by the sun. Hours later it was very hard to start. The next day I put the engine in a tank and could just get it started then it died. Fuel leaking out the throat of the carb. I pulled the carb apart just to check and put in a new float, needle and seat. Float adjusted to level with the carb upside down. The cork float didn't inspire confidence. Good long blue spark from the plugs. Using the recoil started I was able to do a cold compression test. Top cylinder 58, bottom 55. I pulled the intake manifold to look at the reed valves. The open and closed as the engine is pulled thru. What is that little brass plug with a screen in the lower middle of the intake manifold?
What am I missing that it won't start?
Thanks for reading this long description.

Re: 1980 4.5 running poorly now wont start

The compression numbers are very low. Take the motor off the boat and lay it down on it's face (plugs up). Take the plugs out and spray some penetrating oil in the cylinders (automatic transmission fluid mixed with a little acetone works pretty good). Pull the starter cord a couple of times and let the motor sit for a day or so and re-check the compression.

Re: 1980 4.5 running poorly now wont start

Thanks for the tip. I soaked the engine over night. Tested it cold and got 65 on both cylinders. I was able to start it after many pulls. After 10 minutes I did another compression test and it was 65 again. It is a bear to start, takes many pulls and playing with the choke/throttle. Floods easy too. I have tested the carb float with the primer bulb and it doesn't leak. Is there a way to speed up the process of getting the carbon out if that is the cause of the low compression?

Re: 1980 4.5 running poorly now wont start

Well, you got to get it running first. I would suggest doing some diagnosis on the ignition. Get a spark tester and check for a bright blue spark on both cylinders. Sounds like you are getting plenty of fuel so check the ignition first and if that's good, then move on to the carb. Make sure you are using fresh fuel and mixed at 50:1.

Re: 1980 4.5 running poorly now wont start

I got a spark tester and both packs jump a nice blue spark 1/2 inch. When running if I do a cylinder drop test, the top cylinder has very little change when the lead is pulled. When the bottom lead is pulled the engine dies. So the top is the one with the issue. Now if it is jumping a good spark, is the spark breaking down under pressure (what little it has)? I did swap the plugs and the problem stayed with the top cylinder.