Black plugs

marty in Ohio

marty in Ohio

Well-known member
An "A" friend, without computer access,was having a miss when driving his car. We set the points, timed it, and changed the condenser driving it between each step. No change. Then we thought it was a fuel problem and put a spare carb on it. Still missed. The last thing we did was check the spark plugs and found that they were black with carbon. We put in a spare set and she ran like a champ. This engine was recently rebuilt and has only about 450 miles on it.
What would cause the plugs to be so dirty in such a short period on time?
 
Do you have an air cleaner on it?

When I bought my Town Sedan ~2 months ago it had an Air Maze on it. After reading a thread here on The Barn about air cleaners (or not) I decided to try it without. Runs much cleaner. Still on the rich side tho.

I have since ordered some 'warmer' plugs to see if that'll help.

Which brings up a question I have asked before but got no response... Are different size jets available for a Zenith? Be nice to be able to rejet and fine tune the carb. I saw a list showing jet drill sizes, has anyone soldered and redrilled Zenith jets to a smaller size?
 
I have a few sets of old original jets that have the numbers on them ; can change them around for best results. The new ones can be anything & need sizing if you can do it.
Ever since unleaded fuel returned [ it was unleaded when Model A's were new too] I have found the plugs can soot up unless the engine gets hot on a long run.
Going to a hotter plug may be the way to go depending on what use your car gets.
 
I recently serviced my carb and checked all jets flow through (I made a simple test setup to measure cc/min) and I found my main jet was too big. This was consistent with my engine "stuttering" at high rpms, so I soldered the main jet closed and drilled it out. It took 3 tries to get the flow right but she runs like a champ now!
 
I recently had a similar problem, and like others immediately thought it was too rich. Turned out to be poor spark. Too much play in the distributer plate, and body. Could not keep consistant rotor gap, or points gap. Ran ok until it built just a bit of carbon and then would start to give problems. Because changing the carb did not help, this might be something to check. Rebuilt the distributor, and it runs like a champ with no more Carbon build up.
 
Badaboom said:
I recently serviced my carb and checked all jets flow through (I made a simple test setup to measure cc/min) and I found my main jet was too big. This was consistent with my engine "stuttering" at high rpms, so I soldered the main jet closed and drilled it out. It took 3 tries to get the flow right but she runs like a champ now!
Click to expand...

Do you recall what size drills you were using. I may want to experiment. It seems if I get the lower end mixture correct it is to rich at WFO!!:eek::eek:

KGBnut said:
I've never heard of a source for different sized jets, but my guess would be that IF they are available, this would be the guy who would know:

http://rennerscorner.com/carburetor.html

Good luck,

Ken
Click to expand...

Thank's for the link Ken.
 
It can be any of the things already mentioned. Most will blame the carburetor but It usually is weak ignition, short drives or both. Bill makes some good points in post # 7 . As for rotor air gap, I set all of mine at .035 , this gives a better spark than .025 but will require some filing either the rotor tip or contacts in the cap. I had the same problems with Motorcraft TT10 and Champion W 16 Y spark plugs being to cold for stop and go driving . I use Champion W 18 plugs and the run clean for the way most model A's are driven. Many times a weak oil filled coil or reversed connections at the coil will cause weak ignition. I never run an air filter because they are one of the number one causes of black sooty spark plugs and rich operation. I would check these possibilities before fooling with the carburetor . Usually installing one of the kits from some suppliers will only make matters worse because of wrong length and size of jets. I always try and use original numbered jets but the carburetor would be the last resort after checking all other possibilities.
 
I used a 0,8 mm drill (smallest I had) but after testing I enlarged the hole a little bit with this same drill as the flow rate was too low.
With the correct flow rate (I think I used 160cc/min but I have to double check) the hole is still less than 1 mm.
 
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