Merc 5.7 350 mag '88 - Spark plug 5 & 6 carbon build up.. Others look good

[Fastrack]

Hey.

I pulled the plugs so I could rotate the flywheel easier to measure the run-out (.004"), before installing my new coupler.

Anyway.. I noticed plugs 5 & 6 which are across from each other and 2nd from the rear, look different then the rest. There isn't a HUGE amount of carbon build up, but they are "black" where the others are cardboard brown.

Any idea? I did check the timing just after I got it and it was spot on @ idle. Black is usually oil? But it seems odd that they are in sequence in the firing order. I find it hard to believe there are 2 bad wires? The inside of the boots are clean - no arcing.

I haven't checked the gap yet, however visually they are all close.






Ben

 

[Bondo]

Ayuh,..... What carb,..?? They all look Too clean to me,.... How old are they,..?? No brown on the porcelain,...

5, 'n 6, are directly below the carb, downhill,...

If the carb is leakin' at all, they'll see the gas 1st,...

 

[Fastrack]

It's a Rochester Quadrajet. The plugs are the correct stock ones AC MR43T. As for the age of the plugs, no idea. I've put about 10 hours on it since I bought it in May. Total on the motor is 875. I'm running ethanol free gas (89 octane), not sure what the previous owner used.

I suspect it could be leaking, as if it's left for 1 week. While someone pumps the gas, I see the accelerator pump do 2 streams of gas into the carb and that's it. If I crank the motor for about 5 seconds, then do my 2 pumps it starts up every time. Otherwise it sometimes stalls as the fuel bowl is empty. I assumed this was evaporation?

The porcelain is not quite as bone white as the photo(s) depict but they aren't very dark by any means.

Ben

 

[Scott Danforth]

I suggest going thru your carb. You already have leak down issues, and your plugs look like your running lean

 

[Fastrack]

I suggest going thru your carb. You already have leak down issues, and your plugs look like your running lean

Based on that I'm assuming the fuel should not evaporate enough from a fuel bowl to see what I'm seeing? I'll have to get the model of the carb and order a rebuild kit.


Ben

 

[NHGuy]

I think one side of the carb is getting too much fuel. Since it likely is on a dual plane manifold the both of those cylinders are being fed by the same side of the carburetor. So it probably needs a redo.

I always thought the white insulators were a bad sign but I have been doing some research on the subject and found that white is good if the ring at the bottom of the threads is grayish. Check out this link...https://4strokes.com/viewtopic.php?t=24

 

[NHGuy]

Yes quadrajets do dry out while sitting, the gas evaporates after a day or two. Don't sweat it, crank a little and the fuel will arrive.

 

[Fastrack]

Yes quadrajets do dry out while sitting, the gas evaporates after a day or two. Don't sweat it, crank a little and the fuel will arrive.

Ah. That's kinda what I thought.. If it was 1 side of the carb, why would only 2 cylinders be affected ?

Ben

 

[NHGuy]

Cause they are right down below the carburetor, the other 2 are on the ends.

 

[Fastrack]

Cause they are right down below the carburetor, the other 2 are on the ends.

According to this:
Small-Block Chevy Performance 1955-1996

The firing order: 1-8-4-3-6-5-7-2

Dual plane feeds every other cylinder during firing:
= 1-4-6-7
= 8-3-5-2

So the cylinders in question (5 & 6) are in fact being supplied fuel from different sides of the carb.

Bondo is on to something.. Is it lean? Clogged jets? Wrong mixture?

Unless I'm misreading this?

Ben

 

[Scott Danforth]

if your carb is dripping fuel, the two cylinders it drips into are 5 & 6 as they are directly below the carb.

Im going with the plugs are lean

 

[NHGuy]

I had been using that same chart that Scott posted til I came upon the page in engine basics and it changed my thinking.

[SIZE=+1]Jetting:[/SIZE]​

​

Jetting = the air/fuel mixture ratio shows up on the base ring (the last thread ring, it has the strap welded to it). You want a full turn of light soot color on the base ring.​

​

​

​

 

[NHGuy]

They go on to describe heat range and preignition/detonation. But de emphasize color of the insulator which I had been considering. The thing that throws us off in boats is, these test are valid after a run at the designated engine speed with a quick engine shutdown before inspecting the plugs. No idle time or anything.

I had a weekday off last Thursday and did it in a nice empty harbor.

First I rigged my anchor line through the bow chock, ran to my upper mid rpm speed where I feel torque load. Then I did the normally ill advised throttle chop and shutoff, dropped my hook, raised the lid and looked. Still had white insulators so I added more fuel.

Then when I got home I thought of another way to search reading spark plugs and I came across many posts with this technique.

In my earlier link the plug condition was not as easily seen. This is a better page.

http://www.enginebasics.com/Engine%2...k%20Plugs.html

Whatever it is, the OP ought to break down that carburetor. 2 plugs dirty indicates unequal fuel delivery in my opinion.

I have a dual plane Chevy Mercruiser high rise on my boat, I'll have to get a look at it again regarding which tube goes where.

Mercruiser has had that carb figured out for a long time, I'd not be changing the settings as much as cleaning it up and getting it back to proper running condition.