I have a Crest Pontoon. I had always wanted a bigger motor than the 75hp that came with it. I purchased a 150hp 1990 force to put on it.I had the paperwork where the powerhead had all been replaced. I had it put on the boat about 6 yrs ago; due to various life situations, I wasnt able to use it until late last summer. Took it out in the fall, ran great except the idle needed setup. Well, took it to the big lake, backed off the trailer and it started cutting out, and smoke rolling from the cowl. Finally got it back to shore, it was extremely hot to say the least. Found out later the waterpump had disentergrated due to dry rot. Well over the winter rebuilt the carbs and installed new waterpump. Got it cleaned up and ready to go a couple weeks ago. The starter would turn, then would semi lock up and start smoking. I finally got it started and water was shooting out around the head gasket. Went around and tightened the bolts, which stopped the water leakage. Well, I thought maybe the starter was bad? Replaced the starter, battery, and solenoid. I finally got it started and running on muffs...idled and ran good, sounded good when revving up the throttle. But, now wont start. It will turnover great at first, then as compression builds up it starts locking up and smoking the starter. Pulled the plugs, the bottom 4 had oil and fuel on them, the top was appeared to almost be rusty. Well, I checked compression. All the bottom 4 cylinders were exactly 130psi to the tee. The top #5 cylinder was only 110 psi. I pulled the head gasket cover off, fully expecting to see a burnt up top piston. Much to my surprise it looked brand new. Some of the top head bolts had water come out of them when i removed them. I guess my first and main question is: Would replacing the head gasket solve my problem, or do you guys think i have other issues?? I have included some pics of what it looks like. Im so so as a mechanic and really dont have the extra money to take it to a marine mechanic to pay a fortune. I have thought about going and putting my old motor back on and being done with this thing, but I have so much time and money tied up in it already..sorry for the long post..any help would be greatly appreciated!!!! Also, there are small rubberlike tubes between the cylinders. the one between the top and #4 was burnt up, the others are all stopped up with something which appears to be hard water deposits? Is this a problem and what are they? I have included pics of them as well?