Re: Metering rod for Carter WDO 531 S

Posted by DavidPackard On 2021/5/22 19:33:36

Hi Josef;

For the past 14 months I’ve been hiding in the house, and only driving my Packard on solo adventures to keep everything wet and working. Some good news came in last week, in that the hospital where I previously volunteered will begin letting us back in the building providing limited and highly structured service. My first day back is next Wednesday, for some mandatory training on the new rules driven by COVID-19, and then my regular weekly shifts, unless of course another wave hits us.

From what I can tell you have the correct carburetor parts, but there seems to be a story about the bypass bleed. Early in the production of the 531S the plug was 11B-159, and then so some reason was changed to 11B-168, but the 643S carburetor (logical successor to the 531S) reverted back to the 11B-159 configuration. We are talking about starting with a #52 drill size, then changing to a #53 drill only to revert back to a #52 drill again? Since this by-pass and the basic idle mixture screw are the means to control/alter the vacuum being applied to the low speed jet I would suspect the business of the #52-#53-#52 by-pass intrigue has nothing to do with the observed lean condition at idle. I think the by-pass is there to begin the atomization process, and to a lesser extent define the nominal idle mixture screw adjustment. At this point in time there is insufficient evidence to change the idle fuel jet.

So far I’m not convinced the carburetor is the root cause of the lean condition you have observed . . . it may be, but I’m just not convinced yet. If your car has an Electromatic Clutch then you must make sure that item is not creating a vacuum leak along with the windshield wiper system. Once the known suspects have been checked we can resume with a discussion about the carburetor . . . the good news is you have the correct parts.

Now for a story about a WDO 644S with the almost the same symptom as your 531S. I was setting-up a replacement carburetor for my ’48 288, but really hoping I would never need it. When I was finished fussing with the ‘refresh’ of the back-up carburetor I put it on the engine, but I could not get the engine to idle correctly. It did however try to run better when I backed-out the mixture screws. The engine had a steady, repeating misfire, and the carburetor made a whistling sound. Once I switched back to the original carburetor all of those symptoms went away. I’m still looking for the root cause for the whistling because that’s likely a source of a vacuum leak, which depending on where it is could have also disrupted the idle circuit. The point of this story is twofold, a.) a vacuum leak can push the engine lean, and no amount of carburetor adjustment seems to help, and b.) the vacuum leak could be the carburetor itself.

Let’s loop back to item a.) above. While I’m sitting here writing this it dawned on me whenever I adjusted a carburetor I always have adjusted both screws equally, meaning if I turned one screw out 1/4 turn I would dutifully adjust the other a 1/4 turn out, and as long as the engine speed hadn’t decreased go back to the first screw for some more adjustment. The intake manifold of the Packard is dual plane meaning one barrel of the carburetor is feeding a 4 cylinder engine, while the other barrel is feeding another 4 cylinder engine, so I could have started with 4 cylinders running lean, and the other 4 cylinders running OK, but by the time I got done with it I hadn’t completely enrichened the lean cylinders, but wildly over enrichened the other 4 cylinders. You might try to investigate whether adjusting only one idle mixture screw helps the lean condition . . . and then start all over and try the other mixture screw. This is a diagnostic procedure just to determine if ‘one side’ of the carburetor is faulty . . . I have never read any Carter advice on how much the idle mixture screws could be misbalanced. I’ve always thought the answer was ‘not much’.

dp