The Ford Barn - View Single Post - Confessions of a Model A Modifier

30Tudor · 06-29-2010, 11:13 PM

Here is an excerpt from of an article on carburetors I did for our club magazine. This article was written to discuss some of the things I learned by trial and error and through fellow club members that weren’t mentioned in the reference materials.

Jets

I had been warned that sometimes the jets supplied from the part’s suppliers are not the same as the originals, particularly the jet orifice sizes. The jets I received from Bratton’s were the same dimensions as the originals, however, the orifice sizes were outside the drill size references that Les Andrews recommends in his book (Main Jet: #63 or #64, Cap Jet: #62 or #63, Idle Jet: #75, Compensator Jet #65) – see http://en.wikipedia.org/wiki/Drill_bit_sizes for conversion to thousands. My first stop was to Princess Auto (a store up here in Canada) where I picked up a set of micro drill bits for about $3.00. I soldered the oversized jet orifices shut with 50/50 solder and re-drilled the orifices to the above specifications using a pin vice. When soldering, use an electric soldering iron or small torch and be careful not to use two much solder as it will easily flow down into the jet creating a lot more work. You also may have to use a larger drill bit and a small file to recreate the dimple at the top of each jet.

Despite the orifice re-sizing my engine was still running rich which I confirmed by having a look at the spark plugs after a test drive. Ultimately I ended up sizing the main, cap and compensator jet orifices two sizes less than recommended in Andrew’s book to get the best results. Make sure the compensator jet is sized larger than the cap jet. With these smaller orifice sizes, I now can use the GAV to optimize the mixture. My recommendation would be to start with the cap, compensator and main jet orifices 2 sizes smaller than Les Andrew’s recommendations as you can always easily remove them and drill them out to optimize performance. You only have to remove the lower carburetor casting to access these jets so they are easy to remove and change. Make sure both your cap and main jet heights are even and set at a point which coincides with the narrowest point in the venturi. This may require playing around with the jet gaskets to achieve this. Make sure that any old jet gaskets are removed from the casting particularly with the main jet. A little thread sealer on the gaskets is also recommended.

06-29-2010, 11:13 PM	#36
30Tudor Senior Member Join Date: May 2010 Location: Calgary, Alberta, Canada Posts: 227	Re: Confessions of a Model A Modifier – Model A Upgrades? Here is an excerpt from of an article on carburetors I did for our club magazine. This article was written to discuss some of the things I learned by trial and error and through fellow club members that weren’t mentioned in the reference materials. Jets I had been warned that sometimes the jets supplied from the part’s suppliers are not the same as the originals, particularly the jet orifice sizes. The jets I received from Bratton’s were the same dimensions as the originals, however, the orifice sizes were outside the drill size references that Les Andrews recommends in his book (Main Jet: #63 or #64, Cap Jet: #62 or #63, Idle Jet: #75, Compensator Jet #65) – see http://en.wikipedia.org/wiki/Drill_bit_sizes for conversion to thousands. My first stop was to Princess Auto (a store up here in Canada) where I picked up a set of micro drill bits for about $3.00. I soldered the oversized jet orifices shut with 50/50 solder and re-drilled the orifices to the above specifications using a pin vice. When soldering, use an electric soldering iron or small torch and be careful not to use two much solder as it will easily flow down into the jet creating a lot more work. You also may have to use a larger drill bit and a small file to recreate the dimple at the top of each jet. Despite the orifice re-sizing my engine was still running rich which I confirmed by having a look at the spark plugs after a test drive. Ultimately I ended up sizing the main, cap and compensator jet orifices two sizes less than recommended in Andrew’s book to get the best results. Make sure the compensator jet is sized larger than the cap jet. With these smaller orifice sizes, I now can use the GAV to optimize the mixture. My recommendation would be to start with the cap, compensator and main jet orifices 2 sizes smaller than Les Andrew’s recommendations as you can always easily remove them and drill them out to optimize performance. You only have to remove the lower carburetor casting to access these jets so they are easy to remove and change. Make sure both your cap and main jet heights are even and set at a point which coincides with the narrowest point in the venturi. This may require playing around with the jet gaskets to achieve this. Make sure that any old jet gaskets are removed from the casting particularly with the main jet. A little thread sealer on the gaskets is also recommended.