Carbon Buildup on Plugs Problem Solved

E

Ed in Maine

Well-known member
A couple of weeks ago I posted that I my plugs were getting covered in carbon and the car was idling poorly. Instead of trying a hotter plug, I decided to focus on the carburetor. First, I took out the jets, soaked them over night, hosed them out and then rechecked their flow rate. I took a close look at the Main Jet because this jet is directly connected to the bowl and there are no other adjustments that can be made with this jet during operation. The published flow range for this jet is 150 to 160 ml/min. measured with a graduated cylinder. Please see the testing tool that can be built by the restorer described in the carburetor restoration manuals. My Main Jet was flowing 159 ml/min. While this jet was within spec. it was at the high end of the range. I soldered up the jet and resized it to 150 ml/min. Since then, I have put about 500 miles on the car. I took out No. 1 plug and was surprised to see that the plug was free of carbon and had a very light gray color. I did find that with a smaller Main Jet, the GAV had to be opened another 1/4-1/2 turn. I would now recommend that the range of the Main Jet should be 150 to 155 ml/min.

There is one down side! On a cold start, if you are too fast with the choke, there is a tendency to have a back fire because you created too lean of a mixture. Pull the choke for 2-3 seconds on a cold start for a richer mixture.

It has been my experience that purchased jets and even original jets are too large. If you are going to rebuild a carburetor, flow test all the jets. I would recommend the following ranges:
Main Jet: 150-155 ml/min.
Idle Jet: 44-46 ml/min
Cap Jet: 165-175 ml/min
Compensator Jet: 140-142 ml/min

With all other things in good order and a clean carburetor body and jets that have been flow tested, you can't miss. The car will run perfectly. Ed
 
I should think that other factors enter the mix: altitude, temperature, humidity, camshaft design, engine condition, to cite a only a couple. What works well for jet size in Florida might not be ideal in the Colorado mountains. That's probably why there is a 10 ml/min. leeway published for main jet sizes to accommodate so many variables where our Model A's are driven. Your information is good, however, and your recommendation to check flow rates is valid. Your diligence in solving the problem and publishing the results here are appreciated by the readership, I am sure.
Marshall
 
Ed, nice work. You get an A+ in your automotive engineering class.

In my case, with my Weber carburetor, I measured the jets with the round end of small drills. What was marked at 150 on an idle jet was actually more like 120. I drilled it out to 160 size and now the car pulls from idle much better. No hesitation or rough running.
 
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