Fouled plugs

[Enigmalk2]

I am having a great deal of trouble keeping spark plugs in my A-4 from getting fouled. I can only use a set from two to four weeks before the engine runs rough or only on two or three cylinders or quits altogether. When I replace the plugs the motor runs fine again !

Thus far I have replaced the points with a set if Ignito's from MMI, replaced the Distributor cap, replaced the plug wires, replaced the coil with IR type that I have always used, and had the carbuerator professionally rebuilt by Vosbury. The gas line has two filters, the latter of which is the polishing filter from MMI.

I have checked the compression on all cylinders and they show 115-120 psi. across the entire four cylinders !!!!. Three separate people , including a pro have reset the timing, and none has had to move it more than a degree.

Oil pressure is 45-50 psi at startup, and starting is very good, and drops to about 30-35 under way when warm. Maximum RPM is no more than 1800 at top speed and the wiring has been checked and rechecked to assure us that the wires are not crossed or missplaced.

This has been a rather verbiose description of the malady but frankly I'm at my wits end ! Anyone have any other suggestions, or thoughts that we or I haven't had as to the cause of the fouling.?

As far as backpressure from the exhaust, a water-lift system was recently installed by Conklin marine but the problem preceded that work. Also I believe that the airgap in the water line to the exhaust should negate any backpressure interference.

[Don Moyer]

When plugs foul as quickly as yours, there is usually some evidence of discoloration within the exhaust. It would be helpful to know if your exhaust is darker than usual (indicating a rich mixture) or light colored (indicating that oil is passing by the oil rings).

Don

[Enigmalk2]

Don..the exhaust does not seem to be either darker or lighter than what might be expected of a normally running engine. *I have had a dirty carburator and am therefore familiar with the exhaust being dark. there does not seem to be any loss of oil in the crankcase over several months operation as a normal amount of oil is obtained upon changing . The dipstick also shows no depletion. From the amount of soot on the plugs I would expect some oil to be lost.if indeed thre problem stemmed from a blowby situation.

[Don Moyer]

Since the exhaust is not indicative of either a rich mixture or oil passing through the oil rings and all plugs are affected somewhat evenly, we're sort of left to guess that the spark might be weak. Would you please remove the coil lead from the center of the distributor cap and hold it close to the head as someone cranks the engine with the starter? A healthy secondary ignition circuit will yield a spark of at least 3/8" to 3/4".

If the secondary discharge is healthy, we're sort of at an impasse. When an engine has good compression, no abnormal discharge from the exhaust, good ignition (including proper timing), no restrictions in the exhaust system and still develops badly fouled plugs in less than 10 hours, it's time to go back and review our observations because we're obviously missing something in the symptoms.

Regards,

Don

[ej14]

Last year we had a very serious problem with plug fouling. We ultimately found a blown head gasket.

A possibility for you?

[David Masury]

If you have a good flow through the exhaust and the carburator is good, that leaves a few other choices... as suggested perhaps a head gasket and a compression check will help their. That would also help if you may have either weak valve springs and they are not seating all the way, thuu you are not getting full compression.

I would also look at the plug wires...how old are they/ They will deteriate over time and compound other issues.

The one thing that I find is that most A4's run cool, this contributes to fouling, especially if you do not use the engine enough to really warm it up.
Try using Autolite 437 spark plugs, they are abtter and slightly hotter plug.

If your compression is good, then follow Don's guidelines for diagnotics.

Happy Hunting

David

[msauntry]

When the timing was checked, did the mechanic rev the engine to see if the advance mechanism works? The springs under the distributor should move freely. Could explain the inability to get past 1800 rpm. (With the same prop and a clean bottom, you used to be able to get a higher rpm, I'm assuming. Otherwise, reduce your pitch and/or clean your bottom.) If you're overpropped and running cold, that might do it.

From what I've learned here, it seems exhaust back pressure can cause plug fouling. Though your entire exhaust section (including the hot pipe section from the manifold?) is new, does the design of the system somehow increase pressure with check valves, loops, long lifts, or extra mufflers?

Might be as simple as a carb adjustment, though. Running too rich...

[tenders]

Given that the original post was made in April of 2005 I'm guessing this issue was solved.