Autolite 2 barrel tuning
By John Logan
Published first in Rootes Review, 2006
The following holds true for any stock Ford carburetor, 2 barrel or 4 barrel.
To identify your carb you need the Ford tag number which is stamped into a triangular shaped aluminum tag under one of the fuel bowl screws. The numbers on that tag are the carb identification number, the design code and the production date code. The identification number is also stamped into the carb base plate in front of the throttle lever. You should compare these 2 numbers; they should be the same, if not, you need to go by the number on the base plate. Another number you want to record is the venturi diameter; this number is cast into the fuel bowl on the left front side above the identifying number.
The stock 2 barrel carb on any 260 engine has a 17/16″ bore diameter with a 1.01″ venturi diameter; this is the only size listed for the 260. The 289 carb had venturi diameters of either 1.02″, 1.08″ and 1.14″; the 302 had 1.08″only.
If you are rebuilding your carb, that identifying number should be listed on the rebuild sheet along with some of the settings. I prefer to use Mustang and Ford Small Block V8, by Bob Mannel, as my reference source. It is an excellent source of information on all Ford Small Block part numbers and there is a separate section on Ford carbs with a complete listing of the stock setup, specifics, settings and adjustments listed by carburetor identifying number.
I have one 2 barrel carburetor that has no identifying tag; the number on the base plate is C3AF BE with venturi diameter of 1.01. This is a 1963 carb for a 260 engine. The 2 barrel carb on my Tiger has a tag # C7DF R and B 7CB with venturi diameter of 1.14″.
C7DF R identifies this as a 1967 carburetor. B 7CB = design B; Production date – 7 = 1967, C = March, B = 2nd week. This is a replacement carburetor that someone installed on the engine.
Stock Tiger 2 barrel carburetors as listed in the Rootes Parts Manual:
|C4DZ-9510-A||up to B9470592|
|Tag #||Main Jet Size||Tag #||Main Jet Size|
I have very little information about the Stock MKII 2bbl carb. Tag number C5ZF-H is listed in the Mk II parts manual, and #C6DF-A is listed in the Shop Manual.
|Engine||Carb Bore||Venturi Dia||CFM – My guess
for all data inside ()
I found very little data CFM volumes for 2 barrel carbs, so I made some best guesses as noted for this chart.
Basic carburetor tuning
To get the best fuel mileage out of your carb it needs to be in good working shape. The moving parts must be free to move. All the gaskets need to be in good shape with no deterioration. Your goal is no fuel leaks, no wet spots and no vacuum leaks.
Refer to the Tiger Shop Manual for a general description, set points and adjustments for your original carb; if you do not have an original carb then you need a spec sheet for your carb. The Shop Manual is a good starting point if it is the only reference information you have available.
Start off with the Engine cold. Examine your carb and check all the linkage for ease of movement; lubricate as needed. Check all the carb gaskets for wet areas and deterioration, especially the base plate and the carb top cover gasket. Replace gaskets as needed.
Run just the fuel pump to fill the fuel bowl; listen to the fuel pump; make sure it stops pumping after a few seconds. If the pump does not stop; shut it off and look for the leak; fix any leaks first.
Look down the carb; it should be dry down the bores; if it is wet, most likely the fuel level is too high or the fuel pressure is too high. Check the fuel pressure with a fuel pressure gage.
The stock Ford fuel pressure range for the Autolite 2bbl is 3.5 to 5.5 psi. I believe, but have not confirmed, that the Tiger electric fuel pump’s range is 2 psi. This is OK if you have no highway speed problems.
Remove the carb top cover and check the wet fuel level with a metal rule; many 2bbls are around 3/4” wet. This is where you need a spec sheet to match up with your carb ID number.
If the float has sunk; replace it. Brass floats can be repaired with solder but be very careful about getting all the gasoline and fumes out of the float before applying high heat or flame. Safety first.
Set the correct fuel level if needed. Fuel level set just a little low is better than too high.
Put the carb top cover back on and pull the throttle lever while looking down the carb; check that the accelerator pump squirts fuel equally into each bore.
Choke cover – many are set at 2 notches rich. For fuel economy the choke must be fully open when the engine reaches operating temperature; adjust your choke according to your specific location.
Set the choke and start the engine. Fast idle – 1200 to 1300 rpm I think 1200 rpm is fine.
Engine operating temperature is 180 degrees F. You are using a 180 degree thermostat, right? Watch the choke plate as the engine warms up; make sure it opens all the way.
Move the throttle a little; it should come off Fast Idle, down to idle speed. If not; check and fix any binding linkage.
For an initial set point, lightly seat each needle screw, then back each out 1 ½ turns. You can use a separate rpm meter to set Idle Mixture or do it by ear. Slowly turn one idle mixture screw out until the rpm drops off; then turn the screw in until the rpm drops; then turn the screw back out to just the maximum rpm point. Do the same with the other idle mixture screw. You are looking for the best rpm with the idle mixture screws turned out the least amount.
The shop manual states that idle speed should be from 575-600 RPM. The lowest I can get with my engine setup is about 800 rpm. Use your judgment here to set your idle speed.
Carb Rebuilding Tips
The following information should be recorded for future reference:
- Tag number on the aluminum tag
- Carb number on base plate
- Venturi diameter
- Jet size number
- Power valve number – if it is marked; some are color coded as listed in the Shop Manual
Disassemble, inspect and clean all parts thoroughly; replace any worn parts. Check the base plate gasket surface for flatness. Also check the flatness of the spacer plate under the carb, the accelerator cover and the power valve cover.
It is very easy to over torque the screws for these parts and warp them. If any of them are warped; you need to make them flat again; here are some options:
Put the cover in a press and use just a little pressure to get them flat or just a bit flatter.
Put fine sandpaper on a flat surface plate and gently wet sand with light oil until the surface is flat.
I use 400 or 600 wet/dry sandpaper.
Buy new parts
Do not over torque any of the cover screws. Your goal is no fuel leaks, no wet areas and no vacuum leaks.
Higher Tech Tuning
This requires a wide range O2 sensor and a means to record the numbers and other variables while driving; or a chassis dynamometer would be helpful. This information will give you when and under what conditions the engine is running rich or lean. This is where you can determine with certainty if the jets are the wrong size or the power valve is opening too early or too late.
- Mustang and Ford Small Block V8 by Bob Mannel
- Sunbeam Workshop Manual
- Rootes Parts List Publication No. 6601131 with Ford part numbers
- Rootes Parts List Publication No. 6601334 with 289 parts list
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Technical articles, shop manual, Rootes Parts Manual