Classic/Antique Car Repair: Idle air adjustment, spark plug wires, vacuum gauge

Question
Dick:  I had quite a time finding you again, however, I am glad I did.

I installed the new distributor shaft, and it is working fine.

I now have a difficult time setting the idle air adjustments on the carb.

The car has more power than ever, since the new distributor, but at a stop, in gear, I am experiencing a "RUMBLE" or lack of smoothness at an idle. In neutral it is o.k., but in gear, I cannot seem to be able to adjust the idle air screws to eliminate the " RUMBLE".

Please advise, if you have any of your always accurate solutions.

As always, I would like to thank you in advance, and appreciate your advice.

Sincerely,

Anthony J. Vitale  Retired Police Detective/Shield # 246

P.S. Does it sound like it is time to rebuild this carb?  

Answer
Hello again, Anthony.

I think you may have run into the fact that I had gone on vacation and marked myself "unavailable" for a week from April 10th through 16th.  We take our family to a beach campout in Baja CA every spring (28 of us this year!).

Anyway, I'm glad your distributor is working OK.  I don't know why there should be any interaction between the carburetor and the change of distributor, unless perhaps the vacuum lines somehow got crossed up.  

Assuming this is not the case, your rough idle could be caused by reduced idle speed, or perhaps by a low speed miss.   If you have a vacuum gauge, you need to connect it to intake manifold vacuum (not "ported vacuum", which is the line that goes to the distributor vacuum advance unit) to see what the idle vacuum is. If you live near sea level, the vacuum reading should be around 20" Hg, plus or minus 1".  If it is below that reading, or the gauge is bouncing in time with the engine roughness, you need to start looking for a vacuum leak, or a non-firing spark plug at idle.  If you cannot find a vacuum leak, try disconnecting the spark plug wires from the plug end, one at a time, until you find the one that does not affect the idle speed.  That one will be the one that isn't firing at idle.  You might have a bad plug wire, or a bad plug, or perhaps something else wrong at that cylinder, but let's take baby steps to track this down.

I'd leave rebuilding the carburetor to the very last step.  This carburetor has not been trouble in the recent past, and I'm a firm beleiver in letting sleeping dogs lie, especially when they are carburetors!

If you have fiddled extensively with the idle mixture screws, reset them to the approximate normal position (about 2 turns out from seated) and proceed to look elsewhere for the cause of your trouble.

Let me know what you learn.

Dick