Good pick up on the 4th progression hole and some good info. I obviously never checked the video once complete! The fourth hole on the 152G is positioned closer to the engine. With the 3 progression hole model, the first hole is too far from the throttle plate and tends to "flat spot" on a number of engines. Although for most, I wouldn't recommend drilling extra progression holes unless you are totally confident in your accuracy to .05mm. I have had to repair more than I care to remember!
The hole next to the idle mixture screw is the vacuum take-off port. It's used to balance multiple carburettors and log manifold vacuum on a dynomometer. Normally they have a plug fitted.
152 can be modified by drilling the 4th 0.80mm hole in a milling machine with the hole situated directly over the throttle plate in its idle position, just be careful not to drill the hole too close to the engine side or it will interfere with the idle mixture setting, alternatively a drill jig can be used to guide the drill and use a dremel or similar high rpm drill to add the extra hole Adding another 1mm hole towards the trumpet end of the carb also helps progression on competition engines
Good video, although the extra hole that is added to the 152G is the Top hole in the picture closest to the engine side of the carbs, it is a 0.80mm hole that is situated right over the throttle plate at idle so that as soon as the throttle is pressed the extra progression hole will send more fuel into the engine, 152G is a better carb especially for competition engines with larger choke sizes which make the progression phase more critical than when using smaller chokes
Ever try to epoxy close all the holes. Except the one right over the butterfly's. Then mill that hole back to create a retangular transaction slot. Just a thought.. Larry
