1958 Evinrude Lark 35hp hard to turn over

Hi everyone,
Over the winter I rebuilt a 1958 Evinrude Lark 35. I bought the manuals and followed them through the rebuild procedure. I have everything back together now and am just about ready to hook up gas and test the motor in a tank. The motor is still on a stand for the time being because I wanted to test the starter before running gas through it. So this is what I have done, I took the spark plugs out and hooked a battery via jumper cables to the terminal on the starter and grounded to the bottom of the starter with the other. With the spark plugs out the motor turns over flawlessly, but when I put the plugs back in the motor can barely turn over. It is as if the compression of the motor is too much for the starter motor to overcome. I have read many other posts and watched started repair videos on the subject and since then have pulled the starter motor apart and cleaned it, got rid of all the dust inside, inspected the brushes and scuffed up the commutator with emery cloth. I put it back together and the outcome was the same. My next thought was the battery so I pulled the battery from my truck and hooked the started motor up as before with the jumper cables and got the same outcome. I even tried it with the battery charger attached in the quick start setting. I'm now at a loss for what to do. What could the possible problems be? Is there some step that I have missed before trying to start the motor? Will it not turn over at the correct speed until I have it wire into the console of the boat? Please Help!
Thank you in advance.

Starter motor / cables are defective !

I would rather not go out and buy a new starter at $142 to later find out that isn't the problem. Also, the cables are relatively new, in great condition and large gauge so if anyone else has any advice that is helpful please chime in.

Starter is defective.----Did you do any testing on the armature?----------Maybe it needed more than just a cleaning.

I agree with racer the starter is getting weak, when you cleaned the commutator did you clean out the grooves good with a small screwdriver and clean it good with electrical cleaner? Those old starters are getting hard to find new do you have a starter rebuild shop close?

Those starters were just adequate when new.

I did clean the grooves in the commutator with a small screw driver. I haven't looked into a rebuild shop near by. I didn't test the armature and haven't even looked into how to test that. I will look into both those and write back.

That is why they put the compression release on the pull starter. I am guessing you replaced the impeller also it will loosen up some once you get it running good. They will be able to test the armature on a growler I suppose you could make one yourself?
https://www.youtube.com/watch?v=yXQ0heuV6VE

Look for thrown solder from the armature communtator. They did that from overheating due to lugging it when trying crank it with a low battery or bad connections. Or from excessive cranking without allowing time to cool.