sooty spark plugs

BILLF/TRF

BILLF/TRF

Senior Member
I have been plagued by what I call "sooty" spark plugs in my '31 tudor sedan and can't seem to find out why. They got so bad after a parade that I barely made it home (30 miles). I replaced my Autolite spark plugs with Champion W16Y (supposedly hotter), set the timing with new points, carburetor has been gone through, a new alternator (6 volt with the proper changes with negative ground), and still no change in the carbon deposits on the plugs! About the only things I can think yet to try is to replace the coil and/or condenser. Is there anything else I could try?
 
Your carb may be too rich even with the GAV closed. Flow test the main & comp. jets, or try a different carb. I assume (:eek:) you have the spark lever properly advanced when driving, and are not running an air cleaner on an externally vented A carb.
 
Also in addition, check the comp jet (the brass one inside the float bowl. If it is 3 times the size of the main and cap jets then someone may have swapped the comp jet with the GAV seat ... threads are same same size.

This will make for a very rich condition.

Also if the GAV rod is turned down very tight it is possible to unscrew the the GAV brass seat if it gets stuck to the needle valve.

Note later carbs mid 30 (if I remember correctly) and later do not have a brass GAV seat. But the GAV seat is included in the kits.

If the comp jet, main jet or cap jets are loose or have damaged gaskets the leaks will cause a rich condition also.
 
Last edited:
All of what has been said plus W16Y plugs are too cold for slow parade speeds. I run Champion W 18 in some of my cars others I run Champion C4 that is no longer produced that I bought new old stock. W16Y is 6 on the heat chart W18 is 7, just a little hotter but I like them better ,especially for stop and go driving and short trips like most probably drive.
 
I see many "518 W18" available on eBay. Are these correct for the A, are they 7/8"- 18?
Thanks.
 
First, is it 'wet' sooty or 'dry' sooty? If the plugs look wet and black, it's oil. At idle, intake manifold vacuum is high, and if the rings are worn or not properly seated, oil is pulled up past the rings and burned. At idle not much fuel is burned so cylinder and spark plug temperatures are lower and unburned/partly burned oil collects on the plugs. If after a parade as you accelerate up to speed, a momentary blue cloud is a give-away.
If it's dry fluffy soot, the carb is too rich. With a properly set-up carb, you should be able to adjust it too lean if you want. At idle, it should die before the idle mixture screw is turned all the way in. Driving sown the road, turning the gas/air valve in should make the engine begin to struggle and lose power. If not, the carb needs work! A hotter plug is just a 'Band-Aid'.
 
I have gotten 2 sets of repro 3X plugs because they were "bad" they got sooty black ---but in my car they change to white.

As 40 Deluxe says, a hotter plug is just a band-aid, fix your mixture problem.

I used to play different plugs and "saved" the 3x for show, then after a show some 30+ years ago I left them in, after several long trips I decided the car liked them better, when amortized over 35 years they are cheap.

When you say the carb has been "gone through" what do you mean --all repro parts??, I like original jets, and now with the ethanol in the gas my jets that were slightly rich (couldn't make it too lean with GAV closed) now are perfect, with GAV shut the plugs burn off everything, pure white, can feel the loss of power, have a lean surge.

Many of the repro jets are not shaped properly --there is more to a proper operating jet than just the hole size, ---just like with a garden hose nozzel, one shape will give a concentrated stream ,another shape will give a cone shaped fog.
 
Length of jets also make a big difference in that if the main jet is too short or has more than one gasket under it will cause a drip out the back of the carburetor that nothing will stop. A lot of guys will buy a carb kit with repro jets that are way off . I remember the kits that contained the bullet shaped end to some of the jets. These jets caused a lot of Zenith carbs to be taken off and replaced with a Tillitson. I remember buying two Tillies from Whitney back arround 69 or 1970 for 12.95 Ahh
the good old days. In later years when I found out what the problem was I fitted my Zenith out with good original jets . By that time both of the tillies had warped so I put them on the shelf and went back to the zenith carbs.

Now back to the subject of the thread . Black oily spark plugs are obviously caused by a worn engine and doesn't apply to a too rich condition. Black Sooty spark plugs are very common for the average model A that is driven slow like parades and short drives with lots of stop and go driving. I think that this is common for most. If you really want to test if the carb is running rich or the plugs are too cold, take the A out for a several mile trip at a steady speed of 45-50 mph, look for a good out of the way spot to pull off the road. Push in the clutch, cut off the ignition and coast to a stop, off the road and remove the spark plugs and inspect, if they are sooty this will indicate a too rich carb or a too cold plug. Usually if i'm going on a trip or long drive especially in the tortuous Alabama summer heat I prefer the air condition comfort of the tahoe and leave the model A's out in my little museum. With all of this said I run the Champion W18 plugs that are hotter than the Motorcraft or Champion W16Y plugs that are commonly offered by the model A vendors, not because my carburetor is running too rich and I don't know how to fix it but because a hotter spark plug better suits most model A driving. The W18 is a much less expensive spark plug than the 3X and I believe hotter.
 
MikeK said:
Your carb may be too rich even with the GAV closed. Flow test the main & comp. jets, or try a different carb. I assume (:eek:) you have the spark lever properly advanced when driving, and are not running an air cleaner on an externally vented A carb.
Click to expand...

It looks like MikeK gets the prize! I took off my Air Maze Filter with paper filter and after running about 10 miles at 50-55 mph with no filter, I checked the plugs and they were cleaner than before the trip! Does this mean that I should not run a filter or just frequently replace the paper filter? Thanks for all who had input on my question!
 
BILLF/TRF said:
It looks like MikeK gets the prize! I took off my Air Maze Filter with paper filter and after running about 10 miles at 50-55 mph with no filter, I checked the plugs and they were cleaner than before the trip! Does this mean that I should not run a filter or just frequently replace the paper filter? Thanks for all who had input on my question!
Click to expand...

Your going to get many different opinions very soon, but my opinion is to mount the commonly used large K&N filter on the airmaze base. The paper element you are using is way too small and causes quite a restriction.
 
Unless you drive a lot of unpaved roads, an air filter on an A is more of a liability than an asset. MrTube is correct, those little paper filters are way too restrictive, but replacing it with a K&N fabric filter does nothing to address the extreme (my opinion) fire hazard that exists when a parked carb leaks some gas backwards into the filter that is lower than the intake & next to the very hot exhaust pipe. If you do run a larger K&N I would do TWO things: 1)mount it so the filter faces up, not down & make sure the tiny drain hole at the bottom of the carb intake is open, and 2)convert the carb to internal vent. This can be done a few different ways and is easy to do. Perhaps someone here can post pix and/or a link with details.
 
You must log in or register to reply here.
Back
Top