6 replies to this topic

[dklawson]

I'll try and keep this short and still let you have as many facts as possible.

Things to know about the situation:
It was warm and dry, late in the afternoon so not likely to be condensation in/on the dizzy or leads.
The spark plug leads are new and in good shape.
The cap and rotor are in decent shape but I'll probably replace them anyway.
The ignition is electronic so it's not a points/condenser issue.
The firing order wasn't wrong.
The head is a rebuilt 12G940 with the engine (and head) having less than about 3000 miles on them.

I started my drive home yesterday and got less than 1/4 mile from my parking spot when the engine stumbled, coughed, and lost power. It was clearly running on less than four cylinders. I pushed the choke in the rest of the way thinking I'd fouled a plug. No improvement. I decided to take (as much of) a 1 mile run as I could to see if it would clear up. It didn't so I limped back to the office and installed my set of backup spark plugs. The old plugs appeared fine... no cracks, carbon tracks, etc. While the car was stumbling, the tach was a little bouncy but did not fluctuate too wildly... so the ignition appeared to be working normally.

When I installed the new plugs the car did not INITIALLY run any better. I was preparing to call for a tow and decided to give it one more try. The car restarted and idled fine... no problems at all. It then ran fine for a test drive and subsequently ran fine for my entire 25 mile drive home.

Could this have been a stuck valve that freed itself? What are the symptoms of a stuck valve?

Alternatively, could it have been one or more plugs the fouled (and I just couldn't see the problem)? On restart... perhaps I needed to clear out a mess of unburned fuel in the cylinders?

[Dan]

Could have been dirt in the fuel partially blocking the jet or float valve and then clearing itself I suppose.

[dklawson]

A couple of people suggested fuel... but I have a coarse filter before the pump and a fine one before the carbs, both of which are clean. The needles are very new "standard - M" needles installed in January so there shouldn't have been scuzz on them. I appreciate the suggestion though.

I'd like to have a list of things to look at/for when and if this happens again so as you think of other possible causes, please post them here.

[Udo]

when valves stuck on mine recently, and yes valve'S, they both 'touched' the tops of the pistons. If you look into the cylinders thru plug hole you should be able to see with aid of a torch

mine had a very large drop in power when it happen, as in stuck then freed up

[Bass Man]

A compression test should give you some indication. The head on my daughters Mini was bunged up with a black treacle like substance than caused a valve to close slowly and stopped it firing on that cylinder. I didn't see the problem initially because when I took the head off it had closed up and had a good seal.

[dklawson]

Thanks all.

I don't anticipate a compression or leak down test will reveal anything unless I happen to have those tools at the ready the next time the problem happens. As mentioned, the third or fourth time I stared the engine the car ran like a top with no indications of a problem.

The car has a manifold vacuum gauge and when the engine is running normally... the vacuum readings are high and steady. If I had a valve that was sticking regularly, the vacuum readings would indicate this.

For the time being I'm going to adjust my use of the car... limiting it for short runs around the local area and allowing it a couple of minutes to warm up before I drive away.

[dklawson]

I may have the answer to my problem.

I checked the leak-down values and was going through the ignition checks without finding any problems.

When my problem first started I checked to make sure I was getting fuel BUT I only looked at the level of the fuel in the "back" carb's bowl. I don't know what possessed me to check today but I took the cover off the front bowl and discovered it was empty. A little bit of work to free the needle valve and the car fired up normally and immediately settled in to its normal idle.

So... I had a valve problem... just not in the cylinder head. The pi$$er about this is that I had recently replaced those fancy Grose Jets with regular rubber tipped needle valves because my Grose Jets appeared to be leaking. Darned if you do, darned if you don't. Too much fuel, then not enough.