What would cause spark plug gap to change?

[johnnyzoom]

My 200's performance had been erratic past 2 weeks, increasing intermittent light thump (miss?) at idle then at acceleration (worse at around 40-45 mph, nonexistent above that). No fuel or apparent exhaust blockage, played with carb a little, no change. Timing is correct at 12, TDC verified about 9 months ago. Manual choke seems to be handling weather change OK.

So I took the oft given advice here to look at ignition when carb is suspect and checked spark plugs. All were in very good shape, except #3 which had a little black gunk, but the gap had closed all the way, metal to metal. Cleaned and regapped to .025, all seems to be well now.

Plugs, wires, coil, and diz are about 9 months old and I'm using points. I'm wondering what some possible causes for the spark plug gap closing up like that are, and what this may be a sign of. I got a feeling nothing good!

Thanks as always-

Johnny

 

[wallaka]

Dunno, maybe a foreign object was ingested, and it hit the spark plug? This is a strange issue.

 

[Anonymous]

That'd be my guess too.
Number 3 is right of the plenum in the intake. Maybe a screw off the choke plate, or a lock nut on bottom of the air cleaner post, or something like that. Would have to be pretty small to get past the valve, but something solid.
Just a guess though.
Also, don't forget, if it had any weight to it, whatever it was could have been laying in there for a LONG time, and finally bumped around enough to end up in the runner.

I remember back when I was a teenager my brother did a set of heads, new cam, lifters and pushrods on a 318 Dodge in an old 68 D200 we had. Ran great for about 6 months, and then one day he went to start it and it locked up solid before it even fired up.
Checked everything, starter OK, battery OK, but wouldn't turn over, and couldn't crank by hand either. Took the oil pan off, and lo-and-behold there was that pushrod he never accounted for, wrapped around a con-rod. Teased him about it for years...

 

[mjpchief]

Cousin of mine had a '55 Chevy once with a 283. We double dated one night and went to the show here. When he went to start it after the show it made a hell of a racket. Shut it off and towed it home.

Next morning we pulled the heads to find that someone had poured a whole tube of BB's down the carb. Made short of the 283.

 

[mugsy]

Had the same issue when I drove my '65 from NC to my house in Toledo . To make a long storry short the top land of #6 piston blew apart . When I finally opened the engine up and took the piston out there was only two small chunks of the top or compression ring left. So it was either the piston material or the ring that hit the SP electrode. Wish I had a camera and could post a picture. It explained why all the gaskets on the block blew out, and why there was a puff of smoke coming out of the oil breather cap . Anyway... do a compression check on that cylinder and the ones next to it. Hopefully it was something foreign, and not part of the piston/ring. The compression pressure should be plus/minus 20 psig between cylinders I think.

tanx,
Mugsy 8)

 

[jamyers]

If the head hasn't been apart for some time, you could have had "just" a piece of carbon come loose from somewhere like the backside of a valve, get in the chamber, and bang the sparkplug electrode closed.

You said it had some black gunk on it - I'd watch that cylinder and see if that plug likes to foul more than the others.

At any rate I'd do a compression check just to see what's going on.

 

[Anonymous]

We double dated one night and went to the show here......someone had poured a whole tube of BB's down the carb.

This story would be hilarious if you had been at a drive in theater, and the "double date" was going so well no one ever noticed the hood being opened and closed!

you could have had "just" a piece of carbon come loose from somewhere like the backside of a valve, get in the chamber, and bang the sparkplug electrode closed.

carbon was the reason my 1968 Buick 430 would push the gaps closed!

 

[Anonymous]

could it have been in too far (too long a plug) and the piston hit it?


when I got a hell of a racket, it was a cracked piston...but since your went away, your golden

 

[mjpchief]

Linc, I did say it was a Chevy and nobody would have noticed anything out of the ordinary if the hood was up. No it wasn't a drive-in.

 

[johnnyzoom]

Thanks for all the info. Will do compression check and keep an eye on all the plugs. Head was off beginning of year for exhaust manifold change. Car has 62k miles on it, I've put about the last 2k on it in past year. I anticipate rebuilding it down the line when skill level improves. Pouring BB's in someone's carb is pretty low!

 

[mjpchief]

Think cousin's girl friend's ex boyfriend might have had something to do with the BB's. Never could prove it but back then we didn't need much proof, just took care of business.

 

[matt1967]

my '51 Chevy 235 pushed the gap closed on the #5 cyl 1 time, never figured it out, re gapped and it hasn't happened since