Journal: Scot Abbott's Journal

Legal Disclaimer: The following is my opinion for which I hold no liability. Use this information at your own risk. It involves using pressurized flammable material which can cause injury to property, person, or death. You read this at your own risk and agree to not sue me for any information provided here or its influence on you.

Modifying the 1.5 SU’s for “Bottom Up” EFI

The 1.5 inch SU’s can retain a “stock look” and serve as throttle body injection devices when they are irreversibly modified. It greatly improves the air flow through the carburetor body. I have done this with 2 sets of mine. It can be done with carbs in very poor condition (e.g. sticky/stuck risers, loose throttle shafts, leaky floats, etc.). This is what I did:

1. I first removed the ‘dome’, bottom plate and the contents of the float bowl. I also removed all the other stuff from the outside of the carb. I left the throttle plate in place.

2. I drilled several holes through the floor of the air passage so that this metal structure could be removed. I carefully removed it by using a small saw to connect the holes together. I used a pneumatic muffler saw, and took care not break the carb body or saw through regions I wished to retain. (That can be repaired with JB Weld or Lab Metal, but I’d rather not have to do that). This is a picture looking down through the carb body.

Figure 1

Once the floor of the air passage is removed, this is what the view is from the top side of the carb body. The remnants of the drilled circular outline can still be seen. From the top here.

Figure 2


I next smoothed the opening in the areas of the air passage which remained after I had removed the floor of the carb body. This work was done from the other side (bottom) of the carb body.

Figure 3


The picture below is a view from the bottom of the carb body. The goal here is to open the structure enough for the injector(s) and holder to fit in place, but not remove so much material that there is no material left. I used a rotary tool called a die grinder with coarse bits for use with aluminum. (If a single injector of adequate flow is used, there is no need to widen the passage)

Figure 4

I then removed a portion of the back of the float bowl structure, sawing with the muffler saw. I did this using a number of cuts so I could remove material and keep the structure I wished to retain. The picture below shows the material which comprise the back of the float bowl casting removed. I returned to finish this area later, using a saw, die grinder and a file. I had to trim a small amount of this cross piece away for the injector holders to fit inside the carb body.

Figure 5

(This following step can be done later)
I then reshaped the bottom float bowl covers to make way for the injector feed lines. These covers serve to retain the injector blocks and any other material inside the bowl cavity. First I drilled holes in the covers, then ‘connected the dots’ to remove the center part of the cover, and finally smoothed the resultant opening with a file and die grinder. This material becomes more fragile once the holes are drilled, so I took care not to grip it too tightly in the vice, etc.

During this process, portions of the sides of the carb bodies were removed to make way for the injector connectors. (This may not be necessary if a single injector is used.). These openings were smoothed and finished later.


At this point I started to check the fit of the injectors (with the connectors on them) in the prepared space. The goal is to have a small lip on to support the nozzle end of the injectors, and have it wide enough for both of them, but not have all the material removed from the carb body. With these injectors, the connectors have to point slightly outward because the injectors are too wide to stand upright side by side.

Figure 7

Once the carb body was mostly prepared for the injectors and holders, a set of holders was prepared so the final assembly and fitting could be done.

Figure 8


About holding the injectors in place:

I chose to use two injectors in each carb. I made a holder for them in the following steps:

Used 2 brass elbows (either Home Depot or Fastenal )
Drilled out one of the pipe thread holes in each to 17/32”
(first used ½” drill, then 17/32) Feed Drill slowly
Fitted barbed tubing fittings into each, using tinning flux
Soldered the two elbows together and to a small piece of sheet copper
( I used a large sheet so I could support the elbows during soldering)




Later, I Trimmed away most of the copper sheet and left a tab for attaching to the carb body. (shown in the ‘final ‘picture below)




Figure 11

Once this was finished, I trial assembled the injectors in the holders and the bottom covers. I trimmed small amounts of material away from the cross bar of the carb body and from the back of the holders to make the fit better. During the trial fittings, it was clear that placement of the holder is important. If the holder is placed too far inside the bowl structure, the other end of the injectors can interfere with the throttle plate. This is easily avoided by bending the tab appropriately.

Below is a carb set with the left one converted and the right one left stock. This shows how hidden the conversion is.