Rochester 2 Jet Fuel Percolation

Throttle Body Venting

I ran across some information about some of the 2 jets that deals with the percolation of gas after shutting off the vehicle. We hear about this problem a lot these days because gas now has a lower boiling point.

Percolation means that the gas is boiling resulting in a very strong fuel mixture. This can make a hard to start situation.

I’m not sure if this was done on every 2 Jet, but it wouldn’t be hard to figure out if you have this feature just by looking at the throttle body (float bowl side).

The purpose of throttle body venting is to give quicker hot engine starting after the engine had been shut down for a short period.

During extreme hot engine operation the fuel in the carburetor tends to boil and vaporize due to engine heat. I said extreme, but gas now has a lower boiling point and it doesn’t take a lot of heat for percolation to happen. Some of the fuel vapor tends to reach the carburetor bores and condense on the throttle valves and seep into the engine manifold. By venting the area just above the throttle valves, hot engine starting time can be reduced to a minimum, on applications where the carburetor is exposed to extreme engine heat.

 throttle vent

There are 2 methods used in venting the throttle bore area.

1. A special throttle body to bowl gasket is used. See figure A. This gasket has cut-out areas which vent fuel vapors from the carburetor bores just above the throttle valves.

2. The other type of venting is accomplished by drilled holes through the throttle body casting just above the throttle valves. See figure B. They serve the same purpose as the vented gasket.

The location of the vent holes are such that they will not disrupt engine idle or off idle operation. They are located above the throttle valves on the side opposite the mixture screws, in an area where the transfer from idle to main metering will not be affected.

Now don’t go out and cut holes to create the vent holes if you don’t now have them. That isn’t going to work. You will most likely create a vacuum leak.

Car Stalls When Putting in Gear

I was recently asked about a Thunderbird that dies when putting the transmission in drive.

This could be caused by a vacuum leak, or possibly it is starving for fuel. Knowing that this particular vehicle has a multitude of vacuum hoses going to the carburetor, I would go with a vacuum leak 1st.

Disconnect all of the vacuum lines from the carburetor and plug off the vacuum ports on the carburetor. If the problem goes away you know it’s a vacuum leak causing the problem. Connect the hoses, one at a time until the problem returns. Obviously if the problem returns when connecting one of the lines, you have found your problem. The hose may have a hole, or something it connects to is leaking.

The carburetor itself could be leaking vacuum. You can spray carburetor cleaner around the mounting plate and the throttle body. If the idle changes, or smooths out, then you found the problem.

If you rebuilt the carburetor check to make sure you installed all of the gaskets correctly. The wrong gasket could leave a passage open to air causing a vacuum leak.

If it is starving for fuel, then you have all sorts of things to look for. 1st, if the carburetor hasn’t been rebuilt, then it may just be dirty, clogging up a passage. The float valve could be sticking, not allowing enough fuel to flow in.

The fuel pump pressure could be too low. Test the fuel pump pressure with a fuel pump pressure tester. On a Thunderbird it is probably around 5-7 lbs, but always check your motors manual for the correct specification.

The float valve could be sticking closed not allowing enough fuel to enter.

The float could be adjusted incorrectly. Check the float level.

I’m sure there are several things I haven’t even thought about. Let me know if you have any suggestions. I would appreciate it.

Visit our technical section for more carburetor help.

Carter AFB Flooding Problem

Gas is leaking out of the main throttle shaft, or gas is coming out of the top vents, or you get black smoke while idling. All of these are indications of too much gas, or flooding.

Here is a list of possible causes in no order of importance.

afb needle & seat
Most float valves (needle) have a black Viton rubber tip on the end. The Viton tip needle may have been damaged when installing. Be sure not to put pressure on the needle when adjusting the float. A damaged Viton tip will allow too much fuel to enter the float bowl. Sometimes wiping off the Viton with mineral spirits to take any residue off will help.
Did you forget the gasket that goes behind the seat. Also make sure the old gasket was completely removed. See #22 in the illustration.
Check for cracks around the seat area. This would allow the fuel to bypass the needle & seat, so the fuel would never get shut off.
The float may be leaking causing it to sink. Heat up some water just prior to boiling and immerse the float. There should be no bubbles.
Gently move the float up and down. You should not feel any resistance or catching. A worn float pin, or improper installation of the float might cause this. If there is a metal clip that attaches the float to the needle, be sure it pulls the needle straight out. Move the clip around until it does. The float could be pulling the needle at an angle and it might cause it not to seal.

Check the venturi gasketmain discharges to make sure they are sitting flat and there there is no old gasket residue left under the new gaskets. See number 32 in the illustration.

Ethanol will leave residue behind and the small orifices of the venturi are subject to clogging. Use thin wire to clean out the small passages. In the past we would have said not to do this because you might make the openings bigger. This is still a possibility, but there isn’t any other way to get the passage cleaned. Carburetor cleaner and air pressure will not remove ethanol residue, so the wire is necessary. Just be careful not to enlarge the openings.

The fuel pump could be putting out too much pressure. New pumps are especially suspect. Test your fuel pump and compare with the specification in your motors manual. It would be somewhere between 4 & 7.

That should cover most problems.

If you had an experience with a flooding carburetor, it would be nice to hear what your solution was.