Plug fouling on a motor with proper carbs and compression:.........

-Either you have a heavy vacuum leak in the intake manifold somewhere.
(spray carb cleaner on all gasket joints to find it, the revs will increase when the spray hits it).

or

-your points ignition is old and not working properly.

I'm inclined to think it's the second one, especially as it's random. A mixture problem woud be more consistent.

-The easiest solution is to replace it with a 280ZX distributor.
The procedure is at http://www.geocities.com/zgarage2001/engine.html

Bryan

 

I have a pertronix ignition to replace the points and condenser, so the points can't be causing te problem. I could put points on there again to see if it changes... but I'll have to get back to you all in about 2 months or so to let you know what happened. The cap and rotor were replaced with the addition of the ignitor about 4-5 months ago. Unless I just got crap, my electrical system is virtually brand new... so I don't think there is a problem there.

As for the vacuum leak, the balance tube has probably never been removed since the bolts are locked in tight enough even a strong, car-buff type friend couldn't even break them loose (we didn't try the air impact or long-handled wrenchs because it is aluminum and if it doesn't want to come off, I ain't gonna break it off). So the leak probably isn't there. Now I'm down to just the hoses for the master vac, vacuum advance, vacuum control thingy (that is ment to keep the carbs from dropping to idle too quickly), and the PCV valve... everything else has a bolt through it. One hose I know needs replacing, but shouldn't be leaking. The rest are pretty much brand new.

Just a question, I know that when running a lean mixture, the car tends to run hot. That is a problem that I am having, but would running a lean mixture foul plugs? That just stupped me because I thought that the lighter the plug was, the leaner the mixture was, and darker for richer. Although, if the PCV valve was stuck in an open position, would that be considered a vacuum leak? and would that foul my plugs?

I'm shooting blanks here because I'm completely and utterly stumped.

Thanks again

Jim Parker
Fighten' "to the bloody end" Texas Aggie class of 2002. WHOOP!!!!!!!!!

 

If you are using the SM needles in the carbs you could be running too rich. I experienced that situation. What heat range spark plugs are you using? The proper Autolite is a 64.

 

Follow up. When the engine is running too rich you will see a great difference in plug color between cylinders. This will be corrected when the mixture is correct.

Check your engine temp with a meat thermometer in top of the radiator. It could be an indicator problem. Never believe the vehicle guages without double checking with a known accurate guage.

I think that your fouling problem is mixture strength related.

Usually if the mixture is running too rich some of the spark plugs will look white on the exposed portion of the insulator in the combustion chamber but the insulator will look black when looking down inside the plug.

 

I had a similar experience with my '71 Z. I went completely through the timing components and tuned the carbs the way the book and the video said to. I have ZTherapy carbs that are two years old so I knew that the problem probably wasn't in the carbs but I went through them anyways and cleaned them up and put new oil in them.

To make a month-long story short...My problem was an intake leak due to a warped manifold and loose bolts. I took manifold off, put gasket sealer on, torked bolts down and solved the problem.

I didn't suspect an intake leak because I had just switched out engines and had a new gasket on and figured even if there was a leak it wouldn't cause the drastic problems I was having.

My problems were only in the rear three cylinders (where the leak was). I tested spark by pulling plug wires off and holding the wire close to the plug while running to see if spark was jumping from wire to plug. I found that spark was getting to the plugs but when I pulled the wires from the rear three cylinders there was little to no change in engine sound. So I was running only on the front three cylinders! This made me think either no fuel was getting to rear carb or the carb needed a rebuild.

Anyways, some tips that could help.

Screw idle mixture nut all the way up on both carbs then back them off 2 1/2 turns each - this is a good mixture not exact but you shouldn't have to adjust more than one turn from this point to get the right mixture. If you don't have a air flow meter its a good idea to get one so you can be sure there is equal air flow to carbs.

Once you have carbs tuned as good as you can, if car problems still persist you probably have an intake leak or carbs could need a good rebuild.

What have you tried other than what has been mentioned in your other posted messages? Try holding your hand in front of the carbs, one at a time, blocking the air flow for a second. How does the engine react? Mine sped up when air flow was stopped for a second to the rear carb. Normally the engine should slow and want to die if hand is held for more than a second.

I would like to help with any suggestions so if you need more info e-mail or post another message.

Good luck!!