Sooty Plugs

[Unregistered Guest]

Just wanted to see if I could get some input with a re-occuring problem. The spark plugs are fouling and are right black after about 6 hours of running? Any suggestion would be greatly appreciated.

[Don Moyer]

If all of your plugs become covered with a dry black sooty residue, but generally keep on firing, the most common explanations are:

1) Engine not being run often enough or hard enough. In their latest owner's manuals (circa 1975), Universal recommended a continuous cruising power setting of approximately 2000 RPM, or 80% of maximum available power.

2) Engine temperature too cool (below 150 degrees).

3) Engine might be running too rich. This condition would be indicated if there is a lot of dark blue smoke coming out with the exhaust. There are a lot of late model carburetors from the early to mid-seventies that tend to run on the rich side, and there is no adjustment on the main jet to correct the problem. If this is your situation, you could consider installing an adjustable main jet (which were standard on early model carburetors. In many cases, an adjustable main jet will enable you to lean out the fuel mixture enough to clean up sooty spark plugs, particularly in engines that have good compression (90 psi or above) are otherwise performing well. You can check out adjustable main jets in our online catalog at moyermarine.com.



The orifice in the "fixed" part of the adjustable main jet assembly is almost twice the diameter of the orifice in the current fixed (non-adjustable) main jet in standard late model carburetors. This gives you control over the richness of the mixture, from total shut-off (needle part of the assembly all the way in against the orifice of the fixed part of the assembly) to extremely rich, as the needle is withdrawn from the orifice of the fixed jet.

4) Ignition timing not being optimum. The procedure for timing an Atomic 4 that Universal recommended (after the ignition had been initially set statically at TDC) was called "power timing". Power timing is nothing more than fine-tuning the ignition advance at your favorite cruising RPM (engine fully warmed up) by rotating the distributor a slight amount in each direction to find the location of highest RPM.

5) Partial restriction within the exhaust system. While it's very difficult to measure, exhaust back pressure above 3 psi will usually result in sooty plugs. After checking all of the above reasons for sooty plugs, you might want to make a winter project out of removing and checking (or replacing) the hot section of your exhaust system. This is the section between the exhaust manifold and the inlet to the water lift muffler. The most common place for restrictions to occur are in the location where engine cooling water splashes into the hot section.

If the plugs in only one or two cylinders become sooty, you would have to look for a condition that can affect only one or two cylinders. These conditions include:

1) Low compression. While we consider 80 psi to be at the low end of normal for an Atomic 4, compression can frequently get as low as 60 psi (or so) or below before spark plug sootiness is noticed.

2) A bad spark plug or spark plug lead.

3) A hairline crack in the distributor cap.

If your plugs are becoming covered with a wet oily residue, it would be an indication of worn or broken oil rings.

[Fred]

Idle Valve adjustments.

I have a very unusual situation where my low idle mixture needle valve has a tendency to tighten itself down. The way we found this was, the engine would idle fine, but when under power we would constantly be pushing the accelerator for more power, it would respond and then would die out.

This caused a strong smell in the cabin and a build up of carbon on the spark plugs.

We adjusted the valve two years ago, but at the end of last season we were getting similar symptoms, of course on the day we were hauling out.

I'm planning on cleaning the carb this season prior to launch, and installing an adjustable main jet.

Have you heard of similar cases? Any other suggestions?

Fred N.

[Don Moyer]

Fred,

I can't say that I've heard of anything similar. Please let us know if cleaning your carburetor and installing an adjustable main jet helps. I suspect that it will.

Don