Need engine misfiring advice Options

[oakdalecurtis]

Here’s a question for the braintrust.
I have a 1974 stock FI original motor, 130,000 miles. It has a Crane Fireball ignition and Novolny plug wires. It has always run flawlessly from startup. Lately, upon cold start, it has been badly and randomly “missing” for the first 2 minutes, especially on acceleration. It is not a constant steady miss on one cylinder. I can’t say that is isn’t one cyclinder doing the missfiring, but the miss is random, 2 seconds, 4 seconds, 3 seconds apart. Then it runs flawlessly after the first 2 minutes.. The AAR valve works fine. The MAPS board has been replaced with a new one a while back. I have checked that all the plug and coil wires are tight. I thought maybe some long term water concensation in the gas tank might settle and cause an issue. I added a fuel additive to prevent that, made no difference.
What do these clues indicate on where to start looking for the this inital start random misfiriing?

[VaccaRabite]

I'd start with the basics. Check valves and do a compression test. Look at the spark plugs and see if one looks different.

My guess is that you have fouling forming on your plugs before the engine warms up, and as the engine warms the fouling burns away. Slight fouling could be worn guides or rings. Or just overfueling during warmup.

Zach

[emerygt350]

What he said. I assume that ignition system replaced the points? Rest of the dizzy is stock?

[oakdalecurtis]

Correct, Crane replaced the point only, rest stock. Could a fouled sparkplug only causes missing for the first 2 minutes and then become perfect after that. If you say yes, I will pull and check them first and report back…

[emerygt350]

It's a possibility. If it were the coil I would expect the reverse, but a weak connection could be a thing. Another testing trick is to pull spark plug wires while it is in missing mode. That will help you identify if it is cylinder specific or system wide. These engines idle fine on 3 cylinders.

[VaccaRabite]

Yes.

Think about it this way. Something is happening that is keeping a plug from firing until the engine gets warm. So a great first step is looking at plugs. Make sure you know what plug comes out of what cylinder.

While you have the plugs out anyway, do a compression test. (engine cold, since you want to replicate the issue you are having with a cold engine - though if you get a dodgy result, do it engine warm too.)

And since you already have plugs out ant the engine is cold, do a valve adjustment. Hopefully you find everything more or less to spec. But if you find something badly out of adjustment it could be an issue you are able to nip in the bud before it becomes larger.

Since its missing badly, one of these tests is likely going to show you something what goes away as the engine comes to temp.

Zach

[oakdalecurtis]

Thank you all for your suggestions. I found the problem. I’m embarrassed to admit this, but I will so as to maybe help someone else sometime. I opened the air cleaner and the K&N filter was really dirty and plugged up. After cleaning, the car runs perfectly! Here’s what was happening. When the car and the K&N filter was cold, the thickened K&N filter oil and gunk restricted the air flow so much that upon acceleration, the engine could not get enough air to fire consistently. As soon as the filter oil warmed and thinned slightly, it allowed enough air to pass through to fire the cylinders properly. I feel like an idiot for not keeping the K&N filter cleaner, but maybe my experience will help someone else solve the same problem….

[Geezer914]

Glad it was something simple. I would suggest changing back to a Wix paper filter. K&N will allow some fine particles to get by. You may loose a little air flow but it will be cleaner.