Classic/Antique Car Repair: vacuum advance, 55 ford, vacuum fitting, manifold vacuum

Question
Had after mkt 4 bbl carb on car (1955 ford). Changing to 2 bbl carb made specifically for my 292 ci engine. Only place to hook up vacuum advance on carb is vacuum hole near bottom of carb. This port seems to be straight manifold vacuum. Is this correct? My 4 bbl aftermarket carb. seemed to have almost no vacuum until accelerated rpm.
I hope my question makes sense. Thank you.

Answer
I assume you changed the intake manifold also, or else the aftermarket carb used an adapter to the 2 bbl manifold?

Anyway, the right carburetor for your car is the B5A-9510P, and the right distributor is B5A-12127B.

The aftermarket carb had a "ported vacuum" fitting, which most cars use for the distributor vacuum advance.  This fitting comes from a place just above the throttle butterfly, so when the butterfly is closed (at idle) there is no vacuum there.  When the throttle starts to open, the intake manifold vacuum starts to appear at the ported vacuum fitting, reaching a peak vacuum at low engine loads with partial throttle opening - say at 30 MPH in high gear, with moderate acceleration.  When the thottle is wide open, the vacuum actually drops because there is so much load on the engine that the pressure inside the intake manifold approaches atmospheric (the throttle wide open means there is minimum restriction in the air flow).  The distributor advance unit is calibrated to do the right thing at the right condition to keep the spark timing where the designer wanted it for various conditions.

Some Fords in the 55-57 era had a "better idea" and used a very complicated distributor with both vacuum retard and vacuum advance, so there were two vacuum hoses, one from ported vacuum and one from straight intake manifold vacuum (which would come from some other fitting on the intake manifold).   I recently helped a friend's widow get his 56 Mercury going as a last favor to him, and the main problem was that these hoses were all screwed up by some other "helper" when they were trying to fit an aftermarket performance carburetor to the engine.   

This is all background information for you - lets get to your problem.  If you have the right distributor and carburetor for your car (look at the tags), you can dope out what to do from the above info.  If the distributor has only one vaccum advance pot on it, that line should go to ported vacuum, and if you have the right carburetor, it should have a ported vacuum fitting.  The vacuum fitting you found that gives you intake manifold vacuum (I assume you've checked and there is strong vacuum there even with the car at idle?) is not the one you want - there should be another fitting on the carburetor somewhere. It may or may not be higher up on the body of the carb -they sometimes route the vacuum passages internally, so you can't always predict where the fitting will be, but look for one that has no or almost no vacuum until you blip the throttle.

I guess step one is to see if you can find the ported vacuum fitting on your carb.  If you can't find it, see if you can find the number on the carb. It should be on a metal tag, but it is also probably stamped on the base somewhere also.   

If you have the double vacuum advance distributor on the car, that is probably not the right one for a 2 bbl carb.   

Let me know what you can see and we'll try to get you going right here.  I have to tell you I am not a Ford expert, and we may need to get some more competent help, but at least let's see if you've got the right parts on the car.  

One thing I can tell you for sure is that you will not damage anything by disconnecting and capping the vacuum fitting you used, and just letting the distributor go along with no vacuum advance hooked up at all.  It will affect your power and gas mileage a little, but not all that much and you can certianly drive the car that way until we figure out what to do next.  Just set the timing so you get only a tiny bit of "ping" on light acceleration at 30 MPH, and you'll be very close to optimum even with no vacuum advance working.   Probably 90% of old distributor's vacuum advance don't work anyway!

Let me know what you find, and we'll stay on this until we figure it all out.

Dick.