Dry carbon fouled plugs 120 hp mercruiser

[matti5475]

Need a little help on which direction to go! I have an 1983 open bow sportcraft with 120hp 2.5 mercruiser with 2 barrel Rochester carburetor. It's running rich and will foul the plugs after being on the water for a few hours, then hesitates on acceleration. I believe the hesitation is due to misfire on plugs.I have installed a new fuel pump, rebuilt the carb, new autolite plugs 145, electronic ignition conversion kit with 40volt coil and 12 volt power. Serial #6324320. Any help would be great. Still have to get compression test kit and proper hook ups to check fuel pump pressure. Also set mixture screw 1 1/2 turn back.

 

[Texasmark]

Well, how does it run with your mods?

 

[matti5475]

With muffs, rich and black smoke. Which I believe is fuel.

 

[matti5475]

After market fuel pump, so I have see what pressure it's pumping and see what that is supposed to be at to rule the pump out.

 

[Texasmark]

Fuel pressure isn't very much, like 3 psi at idle and 6 at WOT on some engines. If the pressure is too high, the float is unable to keep the needle valve that lets fuel into the carb bowl seated and you flood and run rich.

 

[matti5475]

Fuel pressure isn't very much, like 3 psi at idle and 6 at WOT on some engines. If the pressure is too high, the float is unable to keep the needle valve that lets fuel into the carb bowl seated and you flood and run rich.

I'll check it out tomorrow and post what I come up with. Worked on compression test today, but after further research, I didn't do it the right way.

 

[matti5475]

I think I have it figured out. When I converted over to electronic ignition, I didn't change coil over from the accel super stock coil I had on there. Although it's a 12volt, 3 ohm, 40,000volt, it's not internally resisted. I ordered a flame thrower that is, so hopefully this will correct the problem.

 

[TyeeMan]

If you rebuilt the carb, are you sure you float settings are correct? If the float setting isn't correct the needle will not seat and shut off the fuel flow to the bowl, extra fuel could be dumping down the carb.

 

[matti5475]

I'm pretty sure I set the float right. I learned how to do it watch Mike's carburetor on YouTube. I changed the coil over to a flame thrower, seems to run and sound better. I still have a good amount of smoke, but I feel it's all the carbon burning off. I also checked all cylinders for compression, all around 140 on 4 cranks each cylinder. I think I'm going to run some Seafoam and take it and run it full throttle to burn off that carbon.

 

[TyeeMan]

If you have black soot on the transom of the boat at the water line, then you are running way rich. You might need to tune your idle jet(s).

 

[matti5475]

If you have black soot on the transom of the boat at the water line, then you are running way rich. You might need to tune your idle jet(s).

I definitely have that going on. Is the needle on the bottom of the carb? 1 1/2 back?

 

[jhande]

Black smoke, black soot, dry black fouled plugs are definitely a sign of running rich.

What would cause running rich:
Carburetor issues - float/needle/seat, jetting issues, choke to name the most common.
If the ignition was to be the cause it would be cylinder specific not all the cylinders. It's obviously creating spark or else you wouldn't be able to run the motor smooth for a few hours.
The adjusting screws with the springs in the throttle plate are idle mixing screws. Basically just effecting how it idles. To proper adjust the settings you should use a vacuum gauge and tachometer.

If it was me I'd look at the carb.
When rebuilt was it thoroughly cleaned? All air and fuel passages?
Was the needle & seat in good condition and clean?
Was the float in good shape (floats as it should) and properly adjusted?
Usually though a float/needle & seat issue would result in seeing a fuel leak from the carb over filling.
If all was fine and cleaned good I would focus on the power valve and the fuel discharge ports under the venturi.

Just my 2 cents.

 

[matti5475]

Well I'm the one looking for help, so I'll take your advice and tinker with it in the morning. Thanks for your help on this.

 

[Grub54891]

Look down the carb when the engine is running/idling. The stream should be a fine mist, any droplets in the flow indicates to much fuel. Usually from a sunk float in my experiance.

 

[TyeeMan]

1 1/2 turns out is your starting point. Put the boat in the water and get the engine up to operating temp. Then adjust your idle jet richer then leaner, identify the spot in between where the engine runs the best and your done. A vacuum gage is not needed. This from one of the pros on this forum from another thread, paraphrasing, even at Merc school they teach you to use your ear and a tach to adjust the idle jet(s), no vacuum gage.

 

[matti5475]

Ok, I will do that when I go out today. I had a few different things going on that I have straightened out. One being the wrong coil, I switched over to electronic ignition and thought the accel super stock coil would work. After further inspection I realized it has no internal resistor, which I believe was causing misfire after a few hours on the water. I also realized I missed the arm on the butterfly choke valve with electronic choke coil. Now I'm going to change oil and see how it's runs on muffs.

 

[jhande]

1 1/2 turns out is your starting point. Put the boat in the water and get the engine up to operating temp. Then adjust your idle jet richer then leaner, identify the spot in between where the engine runs the best and your done. A vacuum gage is not needed. This from one of the pros on this forum from another thread, paraphrasing, even at Merc school they teach you to use your ear and a tach to adjust the idle jet(s), no vacuum gage.

Very true that you can adjust by ear.
But when you use a vacuum gauge you can fine tune it as close to 20-22. Getting the highest reading of vacuum will be the sweet spot. I've worked at car dealers and a shop building and tuning old school muscle cars & motors and that's how we always finished adjusting the carbs.
But that's alright anyone can adjust by ear and call it good enough.

 

[nola mike]

Depending on your electronic conversion you shouldn't need a resisted coil. Also, have you checked to make sure your choke is opening fully?

 

[matti5475]

Depending on your electronic conversion you shouldn't need a resisted coil. Also, have you checked to make sure your choke is opening fully?

Per Tronix and it calls for resisted in the directions. The choke butterfly arm was not centered in the electronic choke coil.

 

[matti5475]

Very true that you can adjust by ear.
But when you use a vacuum gauge you can fine tune it as close to 20-22. Getting the highest reading of vacuum will be the sweet spot. I've worked at car dealers and a shop building and tuning old school muscle cars & motors and that's how we always finished adjusting the carbs.
But that's alright anyone can adjust by ear and call it good enough.

I would have to figure out how to do that. I think the ear thing would be good for me. I just don't want the thing to smoke like it is.