Merc. 1974 85hp serial Nr.3871943 looking for some input????

I need alittle input on another project motor.A buddy of mine gave me a 85hp merc. that hasnt run in 5 to 6 years or more.All the wiring harness was rotten so pulled it all off and got a new one.In the process of getting this thing cleaned up I soaked the cylinders in 2 cycle oil overnight and today I hand spun it around without spark plugs no problem.Put the compression tester on each cylinder and I am not quite sure what to believe my tester shows on each cylinder.Each cylinder shows above 160 psi.But when I put the plugs back in just to crank her over She will barley crank over(Very Hard).Good battery,heavy duty jumper cables and a good starter.If I take the spark plugs back out and crank her over she will spin great.
Any Ideas as why she wont crank over nice with the plugs in??.

I would suspect the starter is bad.---------------Lower unit binding ???------------------Timing belt on correctly????

In addition to the above - totally plugged exhaust? maybe a "critter" build a home in there while it was sitting around..

How are you Mr.galamb?How I get mtself into these bucket of bolts I dont Know but is there a simple solution to maybe cleaning up the exhaust. I have tried two different starters without success and its not the timing belt as its free spinning no distributor hooked up.As much oil as I have dumped into the cylinders I dont see much of it coming out of the bottom (no lower unit attached as of yet).Maybe more seafoam soak??

You might be able to get a look at it by pulling the lower unit. Personally, I tend to pull powerheads and get a look (but I do have a powered lift in my garage that makes moving a heavier powerhead easy). Pulling it as a complete unit usually only costs you a couple of gaskets (base and exhaust maybe) and it really lets you see what's going on...