vintage 1970 50 hp thunderbolt ign

Hi guys I'm new to this forum so please bear with me...My Merc 50 runs strong for 40 seconds on the stand then shuts down I had it to the local merc guru to ck it out and 150 bucks later he said it was running fine in his tank and for me it hasn't run one day on my boat or on the stand for 45 seconds since i got it back from him...so I guess I need some serious help from anyone willing to impart some knowledge upon me...could be fuel ....could be electrical....so thanx in advance for any help I will be getting from you guys...Al deep in the adirondacks

Whose fuel source/tank did he use? You may need to replace the fuel hose assy. on the tank.

thanx for the link it will help a lot..i'm electrically adept i do house and some auto wiring so hopefully no problem...if you mean the kill switch that you hook to your vest...it doesn't have one...too old i guess...a winter project to sell in the spring thanx again al