My carburetor percolates and siphons after shutoff

Question:

I have a 1966 Thunderbird with the 390 cid V-8 and the original equipment Autolite 4100 4-barrel carburetor. It has always had a problem with fuel percolation and fuel siphoning after shutoff. At one point a re-cored “Desert Special” radiator seemed to correct the problem but now it’s back. The radiator has larger tubing and does a better job of cooling the engine in our severe summer heat but I had the problem most recently when the outside temperature was only 68 degrees.

The engine temperature gauge reads normal and there are no signs of overheating. I’m not even sure that the percolation problem and the siphoning problem are related. The carburetor was professionally rebuilt about 10 years ago and I rebuilt it again with a kit about six weeks ago. All parts are correct, none are missing and I cleaned all air and fuel passages thoroughly with carburetor cleaner, compressed air and a fine wire.

Both primary and secondary bowls are vented on top so I don’t know how fuel vapor pressure caused by percolation could force fuel through the jets and into the venturies. The vents prevent any pressure build-up.

It could be that I have two separate and independent problems. The carburetor has its original phenolic spacer which insulates it from contact with the manifold. When I last had the problem I removed the carburetor’s top plate and saw fuel literally boiling in the bowls. The bowls were hot but the fuel wasn’t so hot I couldn’t momentarily put my fingertip in it. It wasn’t as hot as boiling water because it didn’t burn like boiling water would. Fuel was running out of the venturies and pooling up on top of the throttle plates.

The original shop manual for the car has a whole chapter on the carburetor and mentions measures taken in its design to prevent fuel siphoning after engine shutdown. A page from that manual has an illustration of a close-up detail of anti-siphon air bleed holes on the back of the venturi assemblies (both primary and secondary should have them). Yet there are no anti-siphon air bleed holes on the back of my primary venturi!

My question is: Did Autolite ever manufacture a model 4100 for a different application that did not have the antisiphon air bleed holes? Of course, these venturi assemblies could have been switched before I got the car about 12 years ago. Could the anti-siphon holes have been added at some time during the carb’s production run (model 4100 went into production in 1957) to address a problem of fuel siphoning? Clearly by 1966 these carburetors had the holes because they are mentioned in the manual. Could the necessary holes be drilled? They are very small in diameter if they are like the high-speed idle holes. Too small for any drill bit I’ve ever seen.

As President of the Arizona Automobile Hobbyist Council and a member of the Central Arizona Chapter of the Vintage Thunderbird Club, nobody I’ve ever talked to with these cars has had the problem of fuel percolation or flooding caused by fuel siphoning that I experience.

My mechanic friend, whom I bought the car from, got it from a customer of his. He worked on the car for many years before he owned it. He doesn’t remember dealing with this problem on the car but has a theory about the fuel percolation. In Arizona we have a “brown cloud” air quality problem during the winter months which requires the use of oxygenated fuel during that period. He speculates that adding oxygen to the fuel lowers its boiling point, thereby making it more likely to boil. So, why aren’t other owners of vintage cars having this problem? What do you think?

Answer:

Unlike some other manufacturers, Ford/Autolite did not publish or otherwise make available detailed specifications and information about running production changes, or other intricate data about Autolite carburetors.

I don’t know whether Autolite ever manufactured a model 4100 carburetor that didn’t have anti-siphon air bleed holes on the back of the venturi assemblies and could not find anyone with that knowledge. I called some old-time carburetor experts I know and none of them had the answer. We all agreed that there probably is someone out there who does know (maybe a retired engineer or assembler who worked with the carburetors when they were new??). We also all agreed that hands-on carburetor experts are going the way of the Dodo bird! Sad but true.

Though I can’t answer your antisiphon air bleed hole question, I can offer an opinion about the fuel problem you’re experiencing. As you’ve noted, fuel percolation and siphoning is all but unheard of with Autolite 4100 carburetors. One important reason for this is the units’ design. Because the fuel bowls are sort of out there they benefit from good air flow around them, which goes a long way in keeping them from getting too hot.

A much more likely cause of the problem you’re experiencing is a leaking economizer valve. I would try replacing the valve before doing anything else. If that doesn’t cure the problem I’d recommend sending the carb to Jerry at Precision Carburetor ( ). After well over 30 years in the business, he has the expertise, equipment and patience to diagnose and repair just about any carburetor problem under the sun.