Fouling Plugs

[EClass20]

Over the past 4-5 weeks and with all your guys help, I have brought back to life a 1985 Merc 3.0 140. Running sweet this past Sat. hitting 40mph pulling a tube and with 3 other people on board. After going balls to the wall for 40 mins sputtered and died. Couldn't start it back up enough to run for more then 30 seconds. Had to limp it back to dock.

Started my diagnostic and found that I burnt up 4 new plugs. Caked in carbon. Put in new plugs, tuned carb to run a little richer and on Sunday we were at it again. WOT, 40mph plus, pulling tube, 40 minutes sputter and dead. I let cool and pulled plugs and they looked pretty clean still. Rainbow from heat but not to black compared to the other plugs. Put them back, but would not start. I brought a set of new plugs with me in case and put those in. Whammo! Running great but went to dock and brought home.

I did notice that when I puled the plugs I had a lot of Air hiss out of #4. After it didn't start and I pulled them again to swap with new I had air hiss out of #2. Never had that happen before.

Running AC MR43T

What gives?

Resume:

New points, upgraded wires and obviously new plugs.
Complete carb rebuild. Cleaned real good.
Re-built fuel pump. New filter. Have great pressure.
New rocker cover gasket
New Valve cover gasket
Dwell set @ 34d
Timing marks spot on.
Comp. 120, 120, 120, 130
Oil / filter changed Aug.

Thanks for your help as always.

 

[Gary H NC]

Re: Fouling Plugs

Post up some picks of the plugs...

The hissing could just have been that cylinder was on the compression stroke when you pulled the spark plug.

 

[EClass20]

Re: Fouling Plugs

Post up some picks of the plugs...

The hissing could just have been that cylinder was on the compression stroke when you pulled the spark plug.

I figured that. As for pics, I will take one of the plugs I took out Sunday. Sat. plugs are gone. They were solid, solid black with carbon. I even tried to clean them with a torch but they were to gone.

Forgot to mention I run 93o gas. Added a little Seafoam last fill up.

This is the cleaner plug. Sat. plugs were 3X worse.

 

[coheej]

Re: Fouling Plugs

Forgot to mention I run 93o gas.

I don't know your setup, but if the motor is setup to run on 87 octane, then you're just slowing down the combustion rate by using 93. It might keep the plugs cleaner on 87.

 

[EClass20]

Re: Fouling Plugs

I don't know your setup, but if the motor is setup to run on 87 octane, then you're just slowing down the combustion rate by using 93. It might keep the plugs cleaner on 87.

I have been told by many people to only run 93o gas. My Dwell and timing was adjusted running on 93. If I switch back to 87 I would have to re-time it correct?

 

[Brew411]

Re: Fouling Plugs

Possible causes: Rich fuel mixture. If the fuel mixture is rich, a complete burn is not achieved, which can result in carbon being deposited on the insulator nose. Due to this excess fuel, the insulator nose is kept at a lower temperature, so the carbon has less chance of being burnt off. Adjust fuel mixture and try again.

 

[EClass20]

Re: Fouling Plugs

Possible causes: Rich fuel mixture. If the fuel mixture is rich, a complete burn is not achieved, which can result in carbon being deposited on the insulator nose. Due to this excess fuel, the insulator nose is kept at a lower temperature, so the carbon has less chance of being burnt off. Adjust fuel mixture and try again.

I adjusted to make richer thinking that would run them hotter to burn off excess carbon. I will play with fuel mixture some more.

Another question I have from tinkering with it some today. It has the original coil in it still. Voltage tests out OK but when I hook up the Dwell it makes the engine want to stall. Could the coil be going out where the plugs would misfire after running for a bit / not hot enough? Coil not recharging itself quick enough?

 

[EClass20]

Re: Fouling Plugs

Possible causes: Rich fuel mixture. If the fuel mixture is rich, a complete burn is not achieved, which can result in carbon being deposited on the insulator nose. Due to this excess fuel, the insulator nose is kept at a lower temperature, so the carbon has less chance of being burnt off. Adjust fuel mixture and try again.

I leaned up the mixture and took her out. I had a 1/4 tank of gas left so I topped it off with 87o. New plugs. I hit 43 on the speedo since I was by myself. Best it has sounded yet. Again @ 35 min mark sputters, dies and has a hard time starting. Waited about 15 mins., drowned a worm and tried again. Wouldn't even try to fire and idle. Put in new plugs and it fires right up. 40 mph to the dock and I am back home. Plugs I took out visually seemed OK as on Sunday. $32 in plugs in a week. I have tunnel vision on the plugs but it has to be something else that is the problem. Back to my Coil question? Going bad? I am truly stumped.

 

[sqbtr]

Re: Fouling Plugs

How are you adjusting your fuel mix?

Did you check for spark when it died?

 

[Gary H NC]

Re: Fouling Plugs

I'm leaning more to a carb problem...I never had one go through spark plugs every 30 minutes...Very Strange!

 

[Aloysius]

Re: Fouling Plugs

I'm wondering HOW the high speed fuel mix is being "adjusted"? Frankly, sounds/looks like the center insulator is getting "glazed" from running the little bugger too hard, especially with plugs that may be too hot for sustained full throttle operation.

 

[antsmands]

Re: Fouling Plugs

Is it burning oil? What weight are you running?

 

[nola mike]

Re: Fouling Plugs

How about throwing the old plugs in before next run and see if it starts then. I'm wondering if you have another fuel/spark issue that resolves when the engine cools a bit.

 

[EClass20]

Re: Fouling Plugs

How are you adjusting your fuel mix?

Did you check for spark when it died?

Needle, carb choke. I did not check for spark after it died.

Is it burning oil? What weight are you running?

Not burning oil. sae40.

I'm leaning more to a carb problem...I never had one go through spark plugs every 30 minutes...Very Strange!

This is a new one for me. I agree, strange right?

How about throwing the old plugs in before next run and see if it starts then. I'm wondering if you have another fuel/spark issue that resolves when the engine cools a bit.

I will try this tomorrow. This is exactly what I am wondering too but don't have an answer for. Could it be a coil issue? My local marina has one in stock. Just don't want to spend the money if that isn't cause.

Thanks for the help.

 

[bruceb58]

Re: Fouling Plugs

Needle, carb choke.

Hopefully, you are adjusting your choke so it is wide open when it is warmed up. As far as the mixture is concerned, there is no adjusting you are able to do except for idle mixture. The running mixture is changed by replacing jets.

 

[airshot]

Re: Fouling Plugs

Just a couple things come to mind, at what RPM are you turning while going 40 mph for a half hour? If you run a cruising speed (3000 rpm) does it do the same thing? When you rebuilt the carb, did you remove the jets, are you sure they were tight when installing them? If you have spark and the jets are properly seated, float is properly adjusted and the choke is not stuck on, you may have a bad or loose power valve inside the carb. Coil failing under heat is also a possibility, check for spark next time. Hope some of this helps.

Airshot

 

[EClass20]

Re: Fouling Plugs

Just a couple things come to mind, at what RPM are you turning while going 40 mph for a half hour? If you run a cruising speed (3000 rpm) does it do the same thing? When you rebuilt the carb, did you remove the jets, are you sure they were tight when installing them? If you have spark and the jets are properly seated, float is properly adjusted and the choke is not stuck on, you may have a bad or loose power valve inside the carb. Coil failing under heat is also a possibility, check for spark next time. Hope some of this helps.

Airshot

Thanks. I am not running 40mph for 40 minutes but I am while pulling tube. Prob, 15-20 minutes on / off was 40mph. Otherwise it is normal usage. 4200-4400 rpm. I will def. check spark next trip out.

Note: Coil is very hot as I have bumped the back of my hand on it removing plugs. Not 2nd degree burn hot but to hot to touch / hold.

Carb jets were clean when I put back together. They have good flow when you prime gas into it. I did notice once when my wife was driving and I was tweaking that it looked like more gas was coming out of one of the jets WOT. It was hard to tell hitting wakes tough.

 

[bruceb58]

Re: Fouling Plugs

They have good flow when you prime gas into it.

Those are not the jets you see the gas squirting out of when you do that. That is the accelerator pump pumping gas.

 

[JimS123]

Re: Fouling Plugs

First of all, 88 octane is specified for that engine. Use 89, not 87 or 93.

Is your distributor the original points model, or have you installed an electronic ignition? I had a similar problem before I upgraded and it turned out to be a faulty condenser.

 

[Aloysius]

Re: Fouling Plugs

The threaded screw "mixture adjustment" is only IDLE mixture, and has essentially no affect on fuel delivery over about 1500 rpms. The idle holes/slots are adjacent to the throttle plate, and become inactive when the throttle is opened. As the throttle opens, air flow through the venturis sucks fuel from the main metering circuit.

We know the engine is being run outside the parameters of its design. The heat range on the plugs is for normal operation, not full throttle sustained use. The flame arrestor may be restricting airflow, choking the motor. Is the running angle of the engine steep enough to affect float level? What altitude are you running this engine? Ethanol tainted fuel?