Right so I finally got around to doing this. At the same time did a full service, gasket change, checked everything over.
Originally I had a 65F9 idle jet in the car. Since drilling a 0.8mm hole I’ve managed to reduce the size down to a 60F8. This is about 3 sizes smaller, which on the surface may not seem worth it, but I can assure you that it was. The reason my idle jet is still reasonably rich is because of how I have the main circuit set up. Ive tried going leaner to 55F8 and it opens a hole if you “aggressively” apply power. To some degree this can be filled with a change in accelerator pump jet, but in all my experimenting (including going to quite large accelerator pump jets) and I’ve actually swapped the accelerator pump spring over to a medium set up, I’ve found this set up works well for my driving.
The main idea behind this project was to make it more drivable when cruising and not have the throttle plate sitting before a progression hole and making it lean, causing the engine to stumble but be ok with the throttle plate either further open or closed. This has cured the problem completely and I would do it again!!!
Idle is still normal and can be adjusted as low as about 700rpm but it’s more comfortable (with the 72 degree cam) at around 1000rpm.
To do this, I used a piece of laminated plastic, to stick pinholes through into the current progression holes. Then pulled this piece of plastic out and laid it onto some 1mm x 1mm graph paper. This took me 4 attempts at marking it correctly on the graph paper before I was happy. I then glued the graph paper to a thin piece of metal and used a dremel tool with drill press to accurately drill these holes.
Once the 4th (new hole) was drilled in the metal, I installed the tiny piece of metal into the progression hole case, used needles through the old holes to line everything up then used a pin punch to mark the new hole to be drilled. Then opened the throttle plate, and used some wire to hold it open (you could probably wedge a rag in but you need it out of the way to drill. Then again used the dremel and 0.8mm drill bit (and a lot of patience to make sure it’s lined up perfectly) and finally drilled the hole. The piece of metal will work both sides of the carburettor you just need to flip it over when doing the other barrel of the carburettor.
Any questions or additions please feel free to comment and I can try and answer any questions as best I can.
As a side note, if you’re throttle plates aren’t closing all the way I also discovered that my accelerator pump rod had a small nick in it which was catching on the retainer preventing it from allowing the cam (tied into the throttle shaft) to return, therefore keeping the throttle plate slightly open.