'99 Mercury 50HP ELHPTO: carburetor idle screw adjustment procedure

[ironring1]

Hello, all. I have a 1999 50 HP ELHPTO 2-stroke, 3-cylinder outboard (s/n 0G905364). I recently pulled the carburetors, disassembled, cleaned (I love my ultrasonic cleaner!), and reinstalled. I recorded where each of the carbs' idle adjust screws were before disassembly (all around the recommended 1 1/4 turns out from bottom). The engine fires right up, but I want to properly set the carburetor idle screws

I understand the link & sync process, engine idle adjustment, etc. What I don't understand is the process to adjust the individual carburetor's idle screws. How do tune this? I know how to adjust the carb on an engine with just one, but I've never done this for a multi-carb engine.

 

[Chris1956]

In the past Merc idle adjustments have been a compromise between idle quality and acceleration. That is when the carbs were set for best idle, the motors would not accelerate smoothly. This is because of design and the lack of accelerator pumps on the carbs. If your motor does not have the accelerator pumps, you will need to set the carbs a bit richer to achieve acceleration.

So, if you do not have acceleration pumps, set the carbs to initial adjustment. Warm up the motor in the water. Now, idle along in the water, and set the carbs to make the smoothest idle. Now try to accelerate. She may stumble and not accelerate. If she accelerates smoothly, you are done.

If not, open the top carb 1/8 turn and try to accelerate. If no go, open the next carb below 1/8 turn and try to accelerate. Repeat with all carbs, top to bottom, testing acceleration after each adjustment. If that don't get it, repeat the process, top carb to bottom.

it might take some time to dial it in. The inline engines tend to run a bit richer on the bottom cylinders due to gravity on the oil and fuel recirculating systems. They are likely to use less mixture for idle.

Once you have good acceleration, and the idle smoothness is acceptable, you have adjusted it properly.

 

[ironring1]

In the past Merc idle adjustments have been a compromise between idle quality and acceleration. That is when the carbs were set for best idle, the motors would not accelerate smoothly. This is because of design and the lack of accelerator pumps on the carbs. If your motor does not have the accelerator pumps, you will need to set the carbs a bit richer to achieve acceleration.

So, if you do not have acceleration pumps, set the carbs to initial adjustment. Warm up the motor in the water. Now, idle along in the water, and set the carbs to make the smoothest idle. Now try to accelerate. She may stumble and not accelerate. If she accelerates smoothly, you are done.

If not, open the top carb 1/8 turn and try to accelerate. If no go, open the next carb below 1/8 turn and try to accelerate. Repeat with all carbs, top to bottom, testing acceleration after each adjustment. If that don't get it, repeat the process, top carb to bottom.

it might take some time to dial it in. The inline engines tend to run a bit richer on the bottom cylinders due to gravity on the oil and fuel recirculating systems. They are likely to use less mixture for idle.

Once you have good acceleration, and the idle smoothness is acceptable, you have adjusted it properly.

Fantastic! this is exactly the kind of information that I was looking for. Thank you

 

[Mc Tool]

What Chris says is true , but the speed at which you slam the throttle open does have some bearing on things too .. Without accelerator pumps there is a flat spot off idle that can be unnoticeable when opening the throttle slowly but very obvious if you floor it . Carbs have progression ports in the throttle bore to help with the transition from idle to maybe 1/4 throttle but they dont work if you yank it wide open . Basically the flat spot will get worse the faster you open the throttle
Adjust your idle screws ( they should all be the same setting ) until you get a nice idle and then open each screw 1/8 turn ( this will give it a slight rich idle as Chris says ) and go for a drive . If it misses and farts on a large ish throttle opening from idle try giving it a bit less than 1/4 throttle untill it starts lifting its skirts and then give it a smooth push to full throttle . This modulating of the throttle is pretty much what a CV ( constant velocity ) type carb does for you automatically .

 

[ironring1]

What Chris says is true , but the speed at which you slam the throttle open does have some bearing on things too .. Without accelerator pumps there is a flat spot off idle that can be unnoticeable when opening the throttle slowly but very obvious if you floor it . Carbs have progression ports in the throttle bore to help with the transition from idle to maybe 1/4 throttle but they dont work if you yank it wide open . Basically the flat spot will get worse the faster you open the throttle
Adjust your idle screws ( they should all be the same setting ) until you get a nice idle and then open each screw 1/8 turn ( this will give it a slight rich idle as Chris says ) and go for a drive . If it misses and farts on a large ish throttle opening from idle try giving it a bit less than 1/4 throttle untill it starts lifting its skirts and then give it a smooth push to full throttle . This modulating of the throttle is pretty much what a CV ( constant velocity ) type carb does for you automatically .

Thank you for the additional insight. This motor is on a 1974 Bayliner Quartermaster 17 (17'8" V-hull) that I picked up used and have been fixing up. I finally got it in the water yesterday (ocean off Victoria, BC) for the first time. I was really worried that it would be underpowered but it jumped right up on a plane at about 25% to 30% throttle. Now I've got the confidence to really finish this project and get to some fishing!