73 (ish) merc 7.5 wont run past idle

[Skidooosl]

I have a 1973 ( give or take a year ) 7.5 Merc I purchased used, when I put it on the boat it will start but the only way it will run ( and it will run for a LONG time and sound great ) is with the throttle turned all the way down to slow, the second I give it any gas it will bog out and stall, sometimes backfire... If I turn it back to slow and give it a pull it will fire right back up and run great till u try to accelerate, even while in neutral. Any ideas? I already pulled the carb apart and it was spotless, also replaced plugs.

 

[Skidooosl]

Re: 73 (ish) merc 7.5 wont run past idle

anyone? i need some help!

 

[Barnacle_Bill]

Re: 73 (ish) merc 7.5 wont run past idle

Try pumping the bulb while giving it gas. If that lets it accelerate any at all, rebuild the fuel pump.

 

[Skidooosl]

Re: 73 (ish) merc 7.5 wont run past idle

tried it.... no luck

 

[70seven]

Re: 73 (ish) merc 7.5 wont run past idle

I have the same motor only mine would not run at lower throttle , problem was not fuel but ignition. faulty breaker points will cause this problem, You obviously have steady spark at low or closed throttle , try checking for spark with throttle in the higher range , if spark is not apparent or irregular one of your set of points are probably faulty or dirty.

 

[Skidooosl]

Re: 73 (ish) merc 7.5 wont run past idle

so as you can see from the previous dates I like to take my time when I work on my older motors..... This was just a spare motor in my outboard collection and now that I live on a lake full time instead of just the weekends I have some extra time to tinker with it and try to get it running again.

I believe the issues with this outboard is related to ignition..... I assume the points are under the flywheel? I will hopefully pull it this weekend and give a look. Are the points hard to change?


How big of a gap should the spark have the ability to jump?

 

[7920hpMercury]

Re: 73 (ish) merc 7.5 wont run past idle

Assuming your carb is clean and your fuel pump diaphram is good, with no pinholes, I would:
-pull the flywheel with a puller, auto steering wheel puller can be used.
-pull off points cover and set the point gap to .020, the points can look really pitted and still work ok with Thunderbolt ignition
-flywheel nut torque to 35ft-lb

If this does not fix it, there is also a low and high speed stator coil that need to check out:
-undo wires and check the high speed coil, yellow and blue, 185-205 ohms
- low speed coil, red and yellow, 3950-4300

Not sure of max spark plug gap, should have L78V perma gap plugs, also last forever, with a nice spark.

Also, might pull the big ignition coils, continuity on the + and - terminals, and 500 to 600 ohms on the plug wire and ground wire. Be careful with the tiny ground wire on the back side of the ignition coil. Clean the aluminum surface that the ground wire contacts to and seal with dielectric grease.

Hope this helps, good luck! great little motors!!

 

[Skidooosl]

Re: 73 (ish) merc 7.5 wont run past idle

Thanks for the tips.... I am going to see if I can get a carb rebuild kit for this outboard and go from there..... last time I just cleaned it and put it back together. If that fails I will remove the flywheel and try everything else

What year did these motors stop using points?

 

[7920hpMercury]

Re: 73 (ish) merc 7.5 wont run past idle

1973 for the 9.8 and 7.5 was the last year for points and the aluminum cover. 1974 used a fiberglass cover and CDI ignition.

1972 for the 20 was the last year with points.