TOPIC: 1978 70 HP Throttle Issue

Alright you Gearheads, I could use some help. I recently picked up a cherry 3 cylinder 1978 70 hp. Great compression 125 across the board. It was recently serviced and idles like an absolute top.

I installed it on my boat and changed out the throttle/shifter cables and took her out for the maiden voyage this past weekend. She fires right up and idles great but once I advance the throttle she'll get to about 5-10 mph but dies if I advance further. It will start right back up. I did notice that the primer bulb never really got hard or stayed hard, and my female fuel adapter was leaking fuel constantly at the ball seal when removed at the engine. I think it may be sucking air? Would this cause it to die under load. I made sure both the barrel locations were in the same positions when installing the new cables.

I also think I need to change my prop to a larger diameter. It's a 1961 17' Renken Overnighter (convertable). I'm running a 73138A40 14 pitch. Would that have an effect?

Thanks in advance!!

Yeah, if the fuel fitting is leaking, the engine can't pump fuel properly. This leads to fuel starvation, lean-running which can cause high temps and burn up the pistons/rings. Something you could try is have someone back aft to squeeze the fuel bulb when you're taking off (providing the fitting isn't leaking so badly that fuel sprays all over the place). See if the engine responds to that. It's also a possibility you could have a weak fuel pump, or debris stuck in the check valve(s).

I don't think you can get any parts to recondition the female fuel fitting, best to replace it with a Mercury part, or a good trusted aftermarket such as Sierra or Aqua Power. You'll find a lot of cheap non-branded parts on Amazon and ebay and likely they will be made of Chinesium.

The 700 Triple is a small-cubic-inch engine (49.8 c.i.) and it will not tolerate being lugged or overpropped. Your prop selection should be based on WOT (Wide Open Throttle) rpm with a light load (i.e. only the driver). Prop it to the high end of the mfr's recommended rpm range (recommended: 5000-5500 rpm).

I suspect you'll likely find that you need to go to a prop with smaller pitch on your beautiful 17' classic.

Last thought, pull the clamshell cowling and advance the throttle to full. Make sure the arm that advances the ignition timing is moving along with the throttle, if it's stuck at a retarded timing setting the engine isn't gonna run well up-throttle.

You'll find plenty of parts info here, including comprehensive parts diagrams:

www.marineengine.com/parts/mercury-outboard/

HTH................ed