Eccentric
Mister Wizard
The Tillotson HL63's were the HL variants most often used on front-tank Macs (and are easy to find) from what I've seen. HL19's were most often used on top-tank Macs (saws built like your 640). HL103's were used a bit on top-tank Macs as well, as were a few other models. What you want to look for is an HL with right side throttle and choke arms, and a choke that is spring loaded to the open possiton. Some of these HL's will also have an additional left-side choke arm (one of my HL19G's did). If your saw has a governor and you want to keep it in use, then you'll need an HL19G (those were used on governed top-tank Macs). You can use an HL63 if you don't care about the governor. The HL63 (or any other carb taken from a front-tank Mac) will have an air filter stud that will need to be cut off or replaced with a short bolt. Top-tank saws such as your 640 don't have an AF stud on the carb, as there's a crossbar in the carb box (over the carb) that contains a captive nut for the AF cover knob screw.
I've swapped two HL63's (an HL63D and an HL63E IIRC) onto flatback equipped front tank Macs (a 650 and a 550). I have also swapped HL19G's onto a couple top-tank Macs. IF you can get the flatback squared away there's no reason to swap it out other than curent and future diaphragm availability. Sugar Creek does make new flatback diaphragms, and OEM diapragms are available still from a few sources. I swapped my flatbacks out because one just wouldn't cooperate, another had a worn out primer, and I have several other saws (both McCulloch and Homelite) that run HL's. Made sense to me as I always keep RK-88HL and DG-5HL kits on hand (and those kits are cheap and easy to get from several sources).
I've swapped two HL63's (an HL63D and an HL63E IIRC) onto flatback equipped front tank Macs (a 650 and a 550). I have also swapped HL19G's onto a couple top-tank Macs. IF you can get the flatback squared away there's no reason to swap it out other than curent and future diaphragm availability. Sugar Creek does make new flatback diaphragms, and OEM diapragms are available still from a few sources. I swapped my flatbacks out because one just wouldn't cooperate, another had a worn out primer, and I have several other saws (both McCulloch and Homelite) that run HL's. Made sense to me as I always keep RK-88HL and DG-5HL kits on hand (and those kits are cheap and easy to get from several sources).
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