Johnson 50 Mid 1970s

I have just finished a farely comprehensive rebuild on this motor. On the first run out the engine would not start. On removing the cowl the motor runs fine. As soon as the cowl is fitted the motor stops. There is no obstruction in the cowl breather system. I notice that exhaust fumes are coming out of a port in the lower unit about half inch diameter immediately below the neutral switch on the left hand side viewed from the boat. I am thinking that maybe I reversed the gasket between the lower and upper unit and the gasket may have covered this port. Or is the motor missing something associated with this port, I can't be sure as I bought the engine secondhand. I guess exhaust gases should not deliberately be routed into the cowling and I presume the motor is being chocked by its own exhaust. Any help would be appreciated.

Re: Johnson 50 Mid 1970s

You probably diagnosed the problem correctly. If exhaust is getting into the housing the engine will indeed choke on its own breath. I'm not sure what port you are talking about.

Re: Johnson 50 Mid 1970s

Looking at the powerhead, I can't see any place where exhaust can come out of the block on that side. Is this leak from the engine block or below it?

Re: Johnson 50 Mid 1970s

my 1973 eletric shift had this probablem and it was the seal on the wire that goes to foot .it was missing . i put sealer around the wires and ran fine . it doesent atke much . when youputthe cover it will valcum more out of the exzous into the carbe and there is no oxygen .

Re: Johnson 50 Mid 1970s

Thanks chaps, I appreciate your interest. This afternoon I tracked down a bit of a Cornish expert on outboards. His diagnosis was that the leg that I have is from an electric shift model, the port that has been creating my problem is directly below the neutral switch on the motor and is the channel for the electrics to the original electric shift gearbox. He has known of people using the engine components that I appear to have and have overcome the problem by blocking off the port with a wooden dowel, or similar. This explanation seems to fit very well and restored my faith in my own logicical thinking. The moral of this tale must be that if you buy secondhand never take anything for granted or at face value. Thanks again I know where to come next time I have a problem, great community!