Read This Plug

[Dan Kalinski]

#3.

Rebuilt the carbs a couple of months ago for the '59. Engine ran nicely with lots of power. Drove it a couple of weeks ago and noted a little popping while cold (chuffing) like it was too lean on one of the cylinders, but went away by the time I hit the end of my driveway (900'). About a half hour later and very close to home, the engine started to "pop" quite a bit on a pull up a hill and once I hit the top of the hill, was able to idle down the hill and driveway and into my shop.

Looks like super lean fuel delivery. Hope this is not metal spray on the plug. Have not done a compression test yet or leak-down. Once that is done, if good results there, will pull the carb and look for obstruction in main/idle jets. Possible intake leak? Zeniths.

What do the experts think?

Rest of the plugs look wonderful.

TIA.

[Dick Weiss]

If that's the only plug and it's kinda white, it's a lean condition--check the idle air jet (on top); Otherwise, drain the float-bowl
for access to the idle fuel jet. Also, check the float level after refilling it. Final checks involve the mixture adjustment screw(s).

Dick

[Martin Benade]

I would take the jet cover off the side and check the main jet as well, it could be partially blocked.

[John Brooks]

Dan.
That looks like an air leak, if it's only on one cylinder, take some carb cleaner with the spray tube on it, at steady idle, spray a little around the manifold and head, then under the carb base and look for an RPM jump.
You could have a plugged jet, but you just did the carbs.
I have an antique "colurtune" glass spark plug if you want to look at the flame color.

[C J Murray]

What do the other plugs look like?

[Jim Nelson]

I'd also do the leakdown to see if there were any other issues that accompanied the funky plug reading.

[Steve Douglas]

I had that problem, one of the intake valves stuck partially open, allowing some combustion to back flow into the carb and Partially melted the diffuser ring in the Zenith carb, hence the traces of pop metal on the plug. Harry Pellow was there and helped me out. His thought was that gas had leaked a little bit and evaporated leaving the valve stem with varnish which when driven a little heated up enough to stick the valve. We sprayed the top of the valve with some WD40 and let it sit then hit the rocker with a hammer, before driving it, everything worked ok to get home. The spark plug has some pot metal melted to it so cleaned it off, and replaced the diffuser ring and things have been good since. Harry thought that since the heads had new guides that they were a bit too tight not allowing enough oil to lubricate the stems, running a bit of Marvel Mystery Oil in the gas for a while might have helped till the guides wore in.

[Dan Kalinski]

Thanks everyone for your help and advice.

First off, valves were checked and they all measured to specification.

As per CJ's request, here is the shot of all the plugs and #1/2 look pretty good to me. #3 is the culprit with #4 looking (to me) a bit on the lean side.

Compression is over 100lbs, ~104lbs and stable at that rate for a minute or so. BTW, there are about 13K+ miles on this rebuild from August of 1989.

Think the valves are good as are the rings. Has to be crap in the idle/main jets or some nasty vacuum leak at manifold or at the head.

Remove carb 1st? Or put it back, start and squirt carb cleaner as John suggests?

Thanks again.

[John Brooks]

If you have not pulled the carb, ether or cleaner will cause the rpm to increase with a leak.

If it's already off, put a sheet to 400 grit on a mirror or sheet of glass. Sit the carburator base on the sand paper and slowly, lightly in a figure 8 pattern rub the base of the carb on the paper and insure the base is flat. High spots will be bright, it does not take much warping to leak.

[Wes Bender]

He indicated he has Zeniths. They have an iron base and shouldn't be warped.

[Vic Skirmants]

He indicated he has Zeniths. They have an iron base and shouldn't be warped.

+1

[C J Murray]

3 good plugs and one bad lean one and all equal compression means either an air leak or a blocked fuel jet at only the number 3 carburetor barrel. My guess would be dirt in the idle jet unless you are driving very aggressively and there is dirt in the main jet or, unlikely, your air correction jet has fallen out. My bet would be on a clogged idle jet. The hole in that jet is very tiny.

[Dan Kalinski]

Debris in float bowl. For sure the same crap in the jets/elsewhere. Took the carb apart and cleaned with the pressurized can of cleaner, especially all the ports that were accessible. Note too, there was a bit of the old gasket on the top of the manifold, so my bad there. Need to sync now and fine tune. Time for that tomorrow. Time for "Happy Hour" now.

Thanks to everyone that contributed to this thread.

KTF!

https://youtu.be/8QfVRqNPqYk

[Ron LaDow]

Dan, great you found it, but in the future, you can avoid a lot of the guess-work (and wasted effort) by swapping the carbs, side-to-side. If the problem moves with the carb, you can forget that 'plugged idle jet in the distributor' for example.

[C J Murray]

Debris in float bowl. For sure the same crap in the jets/elsewhere.

Please describe the debris. Is it a fine silt with a slight rust color tint? This is usually the case and the only fix is to inspect the gas tank for rust and repair the problem aggressively. Filters do not stop rust from passing, that always seems to be handled by the idle jet!

Zenith issue: the accelerator pump diaphragm is leather. When you rebuild the carbs there can be the possibility of tiny "dangly bits" of leather where it was cut to shape. These little bits usually decide to clog the idle jet when they pull away from the diaphragm.