'85 Merc 50HP 2-Stroke - Rich/Lean Adjustment

[joewg3]

On the adjustment screw - does turning it to the right make it rich or lean?

I've been trying to tune my carbs for a little while. My plugs still seem a bit wet and I'm getting some stuttering. Overall, I'm running a million times better than before, but I want to get rid of the stutter/misses and get the air/fuel mix right. All of my plugs look like this. Im guessing that i have to be pretty close.

 

[Chris1956]

Most Mercs use a gas screw, vs an air screw. That means that tightening the screw leans out the mixture.

Usually you start at 1-1/2 turns open and adjust from there under load in water, at idle, with a warm engine. Normally you need to richen them up from there a bit to allow acceleration. If the idle mixture is too lean, she will stumble, die and hesitate.

I would check your fuel pump diaphragm as that plug is pretty wet. When were the carbs last cleaned and rebuilt?

Are those the correct spark plugs? What brand and model?

 

[joewg3]

I bought and downloaded what should be a Service Manual. Looks like Clockwise is Leaner. Any confirmations would be appreciated! Thanks!

 

[joewg3]

Most Mercs use a gas screw, vs an air screw. That means that tightening the screw leans out the mixture.

Usually you start at 1-1/2 turns open and adjust from there under load in water, at idle, with a warm engine. Normally you need to richen them up from there a bit to allow acceleration. If the idle mixture is too lean, she will stumble, die and hesitate.

I would check your fuel pump diaphragm as that plug is pretty wet. When were the carbs last cleaned and rebuilt?

Are those the correct spark plugs? What brand and model?

Carbs were both cleaned last month. I changed the fuel pump, etc, and gaskets on the top carb, doing the bottom this week, but all of the plugs look the same. The plugs are NGK BUHW. Book says it should be BUHW-2, but I've also tried the replacement Champion L78V. All are flat like that, which thew me for a trip!

 

[BWR1953]

Here's the carb adjust procedure, straight from the manual.

 

[joewg3]

Here is what's going on and what I'm trying to fix this time -

'85 Mercury 50HP 2-stroke- WOT stutter

 

[joewg3]

Here's the carb adjust procedure, straight from the manual.

View attachment 399961


View attachment 399962

Thanks!!! Really appreciate that. I've been doing the adjustments hooked to the muffs, not in gear. Can I remove the prop and put it in a tub so I'm getting back-pressure?

 

[Chris1956]

No, the engine must be in gear, with a prop installed to set the carbs. On the water is the best for those of us without a professional test tank.

The design of those plugs look a bit different than what I expected, and I have worked with a lot of BUHW spark plugs. I don't remember the intermediate ring on the electrode of the plug. It was always a center electrode and a ring. The BUHW-2 had a slightly fatter ring, and a smaller "gap".

If I remember correctly BUHW-2 matches a Champ L-78V. Champs were the original recommended plugs as were a Delco brand which came with the motor. NGK is a late change to the recommendation.

Anyway, you set the carb float height correctly? You should always replace the inlet needle and seat and gaskets when cleaning the carbs. Replace the floats, if they look bad. Check the fuel pump diagrams and gaskets for leakage.

 

[joewg3]

No, the engine must be in gear, with a prop installed to set the carbs. On the water is the best for those of us without a professional test tank.

The design of those plugs look a bit different than what I expected, and I have worked with a lot of BUHW spark plugs. I don't remember the intermediate ring on the electrode of the plug. It was always a center electrode and a ring. The BUHW-2 had a slightly fatter ring, and a smaller "gap".

If I remember correctly BUHW-2 matches a Champ L-78V. Champs were the original recommended plugs as were a Delco brand which came with the motor. NGK is a late change to the recommendation.

Anyway, you set the carb float height correctly? You should always replace the inlet needle and seat and gaskets when cleaning the carbs. Replace the floats, if they look bad. Check the fuel pump diagrams and gaskets for leakage.

Thanks! Really appreciate it.

 

[airshot]

No, the engine must be in gear, with a prop installed to set the carbs. On the water is the best for those of us without a professional test tank.

The design of those plugs look a bit different than what I expected, and I have worked with a lot of BUHW spark plugs. I don't remember the intermediate ring on the electrode of the plug. It was always a center electrode and a ring. The BUHW-2 had a slightly fatter ring, and a smaller "gap".

If I remember correctly BUHW-2 matches a Champ L-78V. Champs were the original recommended plugs as were a Delco brand which came with the motor. NGK is a late change to the recommendation.

Anyway, you set the carb float height correctly? You should always replace the inlet needle and seat and gaskets when cleaning the carbs. Replace the floats, if they look bad. Check the fuel pump diagrams and gaskets for leakage.

Interesting info on the spark plugs. My 1992 Merc calls for NGK plugs in the factory manual, also my Dad had a mid eighties Merc that called for NGK plugs. We tried Champion but it never ran right with them. When I had OMC engines, they always seem to run better with the Champion plugs over the NGK......no idea why !

 

[joewg3]

Thanks All - I changed out both fuel pumps, recleaned the carbs, and properly adjusted the fuel/air ratio running it on the river, in gear (son steering), per the service manual.