Logo

Re: 1984 Mercury Outboard 75 HP 4 cyl Bizarre problem HELP

Re: 1984 Mercury Outboard 75 HP 4 cyl Bizarre problem HELP

I myself would not drill the throttle plate on your carb chugz as this is a different set up, the triples have on carb per cylinder and yours has one carb feeding 2 cylinders. The holes air metering holes.....
 
Re: 1984 Mercury Outboard 75 HP 4 cyl Bizarre problem HELP

I think / know the Mercury triples ( older ones ) have but 2 carburetors, ------------Facts must me presented correctly otherwise people are misled and end up chasing wrong information on their motors!!!
 
Re: 1984 Mercury Outboard 75 HP 4 cyl Bizarre problem HELP

Facts must be presented correctly otherwise people are misled and end up chasing wrong information on their motors!!!
Exactly as I was stating carb for the new 3 cylider loopers, the old crossflow triples that had 2 carbs and center cylinder scavenged off the upper and lower carb.
 
Re: 1984 Mercury Outboard 75 HP 4 cyl Bizarre problem HELP

The carbs pictured are either the same as your big four carbs OR your motor uses the older design Tillotsons. Either way, this modification WORKS. (Many of these larger Tillotsons already have the holes in the butterfly; they are TOO small, however.) Merc recognized this in the late '80s and made one of the 3 holes larger, but the other 2 holes need to be made larger as well.

This modification is easy to make, causes no harm whatsoever (the holes "aren't there" far as the carb knows above idle), and WORKS as I mentioned before. Furthermore, this trick is not confined merely to triples! Several people have done this on other motors, with other carbs--as have I--and the results have been very satisfying.

Jeff
 
Last edited:
Re: 1984 Mercury Outboard 75 HP 4 cyl Bizarre problem HELP

That's correct. Start w/the top carb and slowly turn the idle mixture screw CW until the engine starts to stumble and then turn the screw CCW until the idle smooths out; continue to turn the screw CCW until the engine starts to stumble again. Now turn the screw CW to the midway point between the two stumble points. Repeat on each carb until the idle is smooth. The RPMs may increase a little but that's OK. Idle speed is set in forward gear in the water.
Hi I wanted to ask are you setting these screws in idle or forward gear? I am having same problem with with take off I have to push choke in to keep going then it will get take off. I also have to use throttle only button everytime to start engine. I wanted to try this first before putting carburetor kit on.
 
Re: 1984 Mercury Outboard 75 HP 4 cyl Bizarre problem HELP

On acceleration this " stumble " is often caused by a too lean condition.------Try opening the low speed mixture needles an 1/8 turn at a time to see if the problem goes away.
 
Re: 1984 Mercury Outboard 75 HP 4 cyl Bizarre problem HELP

Idle speed is set in forward gear in the water.
Mixture can be set on a garden hose. Turn the screw CCW 1/4 to 1/2 turn after is is idling smooth to richen the mixture a little.
 
Re: 1984 Mercury Outboard 75 HP 4 cyl Bizarre problem HELP

..."On acceleration this " stumble " is often caused by a too lean condition.------"

Acceleration stumble is ALWAYS caused by a too lean condition.

Jeff
 
Re: 1984 Mercury Outboard 75 HP 4 cyl Bizarre problem HELP

Acceleration stumble is ALWAYS caused by a too lean condition.
Not always.... for example retarded timing from sticky/slow trigger advance or binding harness will cause this also.
 
Re: 1984 Mercury Outboard 75 HP 4 cyl Bizarre problem HELP

True, but rarely--lke findinga 5 leaf clover rartely!

Jeff
 
Re: 1984 Mercury Outboard 75 HP 4 cyl Bizarre problem HELP

Mixture can be set on a garden hose. Turn the screw CCW 1/4 to 1/2 turn after is is idling smooth to richen the mixture a little.
Thank you guys for the advice got to work on engine over weekend at lake. Adjusted both and the engine is running great. From idle to full throttle engine jumps and Seems to be starting better too. I only went about 1/8 to 1/4 more like you say. Will keep watch I dont't think it too rich. I will watch smoke and see if it does more. Thanks for the help will keep in touch.
 
Re: 1984 Mercury Outboard 75 HP 4 cyl Bizarre problem HELP

Great!

Your problem was caused by ethanol in the gas since more fuel is required ( since it has less uuumph! than gas alone). I've had to open the idle mixture screws a bit on all of my carbbed motors.

Jeff
 
Back
Top