Logo

75 H.P. Mercury Mariner

clamb

New member
Just joined this forum today. Searching for info.

I have an older boat with a 75 hp 4 cylinder Mercury Mariner engine.

I purchased the boat from my Father, & he purchased it new in mid to late 80's. It hasn't seen much use in 30 years, & was stored inside. Possibly never used at all some years, & maybe 4 or 5 tanks of gas on other years.

The engine seems to run great on the muffs in the driveway. It fires right up, instantly once warmed up, but it stalls at idle, & won't troll at low speeds, when out on the lake.

I have bypassed the oil injection, & been mixing 2 cycle oil at 50:1

Seems hard to find any info on these larger horsepower Mercury Mariners.

Carbs were removed, & found that the air fuel mixture screws were not set equal.
top carb was screwed out 1 full turn, bottom carb was screwed out to almost 1.75 turns.

Any idea where the screws should be set.
Heard that they behave differently when adjusting with muffs than when actually on the lake.

I cant find a model number, on my motor but there is a Mercury SN starting with Letter A, & 6 digits.

Spark plug currently in it is an Autolite 2872.
I am not sure if it uses a resistor or or inductive resistor.

I believe that the correct plug should be a NGK BUZHW-2, & that they have been discontinued.

Not sure how important the Inductive resistor is, & if it could be causing problems, or more likely my air fuel mixture screw being out of adjustment.

I plan on taking a screwdriver & trying to adjust the air fuel mixture on the lake.

Canada Day Stat holiday tomorrow, so hoping someone has some info for me.

Thanks
Calvin
 
Would a plug make that much difference? How do I determine if I have the ELO or ELPTO?

1 recommends the BUZHW-2, & the other says BUHW-2

I also seen another person recommends drilling out the holes in the butterfly valve. Not sure if this was something that needed to be done with this engine.

This motor has always been a bit of a dog and stumbled at low idle trolling speed.

There should be an idle adjustment to allow me to open the butterfly valve up slightly, rather than drilling out holes.
 
Don't start drilling anything !!-----Set the idle mixture screws with boat in the water.----Set at 1-1/2 turns out.----Then you must fine tune.
 
thanks. I will try it out on the lake.

was just out on tuesday, and wouldn't idle.

When I got home and put the muffs on it ran perfect.

I guess just need to spend some time and fine tune it on the water.

I have adjusted the idle mixture screws, because they were not in sync, so going to start adjusting it from baseline setting starting at 1.5 turns out from seated.

I want to try to get it to run as lean as possible, by turning in the screws until it runs best, or do I turn them out? To me it seemed like it was flooding. Boat would run good at anything above 1/4 throttle to wide open
 
Last edited:
They only sound " perfect " on a hose to the untrained ear.------The motor does no work when on a hose !!-----When an outboard like that is at it's leanest it will idle real smooth.-----But then it will NOT accelerate.-----You have to open slow speed mixture screws 1//8th turn at a time till stalling / bogging goes away.
 
Back
Top