Beware of poor quality castings on Quickfuel carbs

[A98Coupe]

Dear All,

I thought it about time to share my frustration & problems with a pair of custom built Quick Fuel carburetors which were provided by my engine builder on my new Shelby 468 FE with 2x4 medium rise set up.

When I first got this motor running, it suffered from bad surging at light load cruise and also ran on after ignition off. I got big improvements by changing to an MSD programmable ignition unit and was able to run with much more advance at lower rpm/ higher manifold vacuum. I also installed an Innovate LM2 O2 sensor & data logger so that I could record AFR, MAP & RPM so that I could get the carbs dialled in.

However, I could never get a stable AFR at cruise. Sometimes it would run OK in the high 13s / low 14s, but then it would suddenly go rich into the low 11s and the rear of the car would be covered in black soot. I stripped the carbs several times, checking pwr valve function and replacing gaskets and this is the list of issues I found. Maybe this is useful information for you?

1. Power valve blow-out protector ball on front carb was sealing against throttle plate so vacuum signal was not getting to pwr valve and this was the cause of the engine running rich intermittently at cruise & light acceleration. I solved this by drilling a 1mm hole into the check ball chamber in the throttle plate to allow air the pass freely out of the pwr valve chamber.

2. When checking metering block gaskets to ensure all ports were aligned properly, I found that the timed spark port gallery (which is not used on these carbs), was completely open from the side of the metering block (which isn’t tapped for a fitting), through the metering block and into the main block exiting above the primary throttle plates. This allowed unfiltered air to pass into the engine when the primary throttle plates were open, not a big issue, but poor attention to detail from Quick Fuel! I filled the main block drilling with metal epoxy so no problem now.

3. Front carb primary fuel bowl had a hole into one of the accel diaphragm screw tapping which allowed fuel to leak out. I had to repair that with metal epoxy. Again, poor quality from Quick Fuel.

4. Fuel always seemed to drain down quickly in the fuel bowls after stopping the engine, some throttle plate always appeared wet and fuel would leak out through the throttle shaft onto the inlet manifold as well as into the engine! The manifold floor was always wet and No.8 cylinder always had fuel sitting in it! I stumbled across the root cause after removing the primary metering block to increase the size of the IFRs so I could start to tune out the surging. However, both power valve chambers had fuel in them - again. I checked the power valves again with a MityVac and they were both good – no rupture – so where was the fuel coming from? I found the problem whilst blowing out the main block with compressed air. I found that the main block drilling which connects the power valve chamber to the throttle plate, is also connected to the accelerator pump gallery! I’ve attached at YouTube link below to show you what happened when I pressurised the accelerator pump gallery with air after filling the pwr valve chamber with WD40. Bubbles escape! This is how fuel is getting into the power valve chamber and of course into the engine when it shouldn’t. This will also reduce the vacuum in the power valve chamber and cause it to open sooner than it should, or even stop the valve from completely closing.

It would appear that the Quickfuel/Proform blocks on both carbs have bad porosity from poor casting process are they are both now junk.

Over the past 6 months I've tried to find some replacement carb main bodies through my engine builder but I've now given up chasing him and so I'm asking if anyone out there knows who I can contact to get some replacement good quality main bodies so that I can re-build these carbs. For reference, both carbs are model 4160, 600CFM vac seconday, downleg boosters, adjustable air bleeds & IFRs.

Thank you

https://youtu.be/SvssRUCEgZI