1975 Merc 7.5 thunderbolt hard starting

[Kennym2022]

Still thinking you have a fuel problem. Did you run a heavy dose of fuel system cleaner (of your choice) in the fuel? 15 minutes is not very long, but probably OK, if you had fuel system cleaner in the gas.

Time to open the Toolbox.

Maybe a full rebuild of the carburetor - replace internal screen, float, etc.
Disassemble/inspect/fix fuel pump

I ran enough b-12 for like 15 gallons, in 5 gallons of gas. I talked to an old timer who told me to take the prop off and try to start full trottle in gear. Low and behold, 3 pulls fires right up, slows down, and idles.

 

[Kennym2022]

My 1969 7.5 Mercury is a '5 pull' start, but it always starts. I did at one point have to fully disassemble the carburetor (Tillotson carb IIRC) and do a re-build kit on it.

Also, assuming you have done the primer bulb thing to get fuel up to the carb. I usually have to start the engine on full choke, unless it is fully warmed up.

Make sure the fuel pump (diaphragm pump) is working. Sometimes the diaphragm gets perforated . . . easy fix.

You could run some Mercury 'PowerTune' through the engine (squirt into carb at high idle). I have also run Mercury PowerTune through an auxiliary fuel line to get it through the carburetor internals.

Welp, dont know what it was, but the motor runs nice now, still starts kinda hard, but what merc doesnt, runs nice
Thanks for all the help

 

[tpenfield]

I suspect there was something 'stuck' in the carburetor (or reed valve) and running it with the fuel system cleaner got it worked out. BTDT.