HL63 variants were used on the front tank large frame Macs. 200, 250, etc.
Main dif between the HL63 variants (HL63A, HL63B, HL63E, etc) and the HL19 variants (HL19B, HL19G, etc) is the AF cover stud on the HL63 and the hole drilled/tapped in the top of the carb body for it. The HL19 was used on top tank Macs that had a crossbar over the carb with a welded in nut for the AF cover bolt rather than a stud on the carb and a knob/nut in the cover. The carb bodies of HL19 variants are not drilled for that stud. Mac also used a few HL103 variants in place of the HL19's. As far as I know, there were no HL103 variants with provisions for the air vane governor.
Many HL19 variants have the arm and roller on the left side of the throttle shaft for the air vane governor that many top tank Macs had. A few HL63 variants also have the provision for the governor (like what's on my 1-60 carcass...........think it is an HL63G).
An HL63 variant can be used on a top tank saw if you cut the threaded portion of the AF stud off (flush with the top of the hex portion). It'll then clear the AF nut bracket that resides over the carb on the top tank saws. You can use non air vane application HL (or 'flatback' Mac carb) on a Mac that originally had the air vane governor if you remove the air vane assembly and make a new throttle link (or modify the existing link) with a Z bend in the end to positively engage the hole in the HL63's (or non governed HL19's/HL103's) throttle arm. The saw will then of course be ungoverned.
In stock configuration, the governed carbs have a 'hook' throttle arm on the right side that is pulled closed by the throttle link when the trigger is released. The link does not positively engage the arm when the trigger is pulled (moving the link forward, away from the arm). It only "allows" the arm to move forward under governor spring pressure when the trigger is pulled. Governor arm spring pressure rotates the throttle shaft towards the open position, "following" the throttle link until air action on the vane overcomes the governor spring pressure.
I have done away with the air vane governor on Mac 740 and 790 saws by removing the air vane assembly, replacing the "hook" right side throttle arm with one that has a hole in it, then putting a Z bend in the throttle link to engage that hole in the arm. I have seen several of these saws that have had similar modifications performed sometime in their past.
Edit:
Forgot to mention earlier;
HL63's were installed with a cast adaptor plate when used on front tank saws. That plate is NOT needed when the carb is used on a top tank Mac (and it should not be used in that application). Don't toss it though............save it for use on a front tank Mac (or send it to someone that'd make use of it)....
