1974 Mercury Thunderbolt 50 HP

Alright friend of mine is having a problem that he cant figure out so he asked me to post it on here..

Its a 74 Mercury Thunderbolt 500 blue band, the carbs have been cleaned, the fuel pump is cleaned, and cranks right up in the yard on the muffs. He took it to the lake tonight and got it off the trailer and said it was a problem getting it to crank, he finally got it to crank and ran it for about 10 minuets he said and turned it off. When he went to crank up the engine he couldnt get it to crank,or turn over, at all tried it for 2 hours to crank in the water. He said that when they got back to the launch and he tried it again it still wouldnt crank. They then loaded it up on the trailer and got it out of the water and said it fired right up with no problems. So my question is. What would cause is to not start while in the water, but crank right up while on the trailer? TIA

Re: 1974 Mercury Thunderbolt 50 HP

If he couldn't get it to even turn over when he was on the water, it sounds like something electrical. Could be a loose ground or a bad ground connection in general. Have him check all his grounds and see if something is loose or in a bad location. This seems possible to me because if the ground was questionable, it wouldn't work in the water, but it would work when the boat was trailered and grounded out better.

Re: 1974 Mercury Thunderbolt 50 HP

Everything seems to tight and in good spots an whatnot, we added a idle screw to it because the one was to short for it, but after cranking it in the driveway, when it cranks it revs up on its on for a few seconds every so often. What would be causing this and how would you fix it?

Re: 1974 Mercury Thunderbolt 50 HP

Are you talking about the Idle stop screw? The one that controls where the throttle cam sits when the motor is in neutral?

As for the intermittent revving, it sounds to me like that could be a fuel pump issue...If your friend has not cleaned and rebuilt the carbs and the fuel pump, I would recommend doing so.

Re: 1974 Mercury Thunderbolt 50 HP

yes im talking about the idle stop screw, the one that was in there wasnt touching it at all in either direction, and he just got it out of the shop last week, having the fuel pump rebuilt and the carbs cleaned

Re: 1974 Mercury Thunderbolt 50 HP

Crank and start are two different things. crank is the motor turning over.
start is firing up.
What is the compression on the cylinders?

Re: 1974 Mercury Thunderbolt 50 HP

it cranks fine, starting it in water is the problem.. and its 90 on all 4 cylinders

Re: 1974 Mercury Thunderbolt 50 HP

It could be exhaust blowback putting pressure on the motor. In which case your friend would need to increase the idle speed. Also, make sure he checks the starter and solenoid for tight, clean connections.