10-31-2019, 12:04 PM
(10-04-2019, 11:33 AM)doug tenney Wrote: Ever take apart a BDC-16 (Mc101A/A) kart carb and wonder why Walbro went to the trouble of using FOUR pumper plates, even though several of the passages and chambers in those plates aren't even connected to anything?? Walbro patent 3,738,623 from 1971 explains why.
The inventor was Charles H. Tuckey, whose name seems to be on all of the Walbro patents from that era.
Anyway, the so-called "double-pumper" aspect of the BDC-16 probably isn't what most of us thought.
I haven't read it thoroughly yet, but here's what I got so far.
The patent claims a novel fuel pump spring (described in later snowmobile carb diagrams as the "leaf"). Surprisingly, the write-up goes on to explain pretty much everything else about the carb... maybe to bulk it up a bit? No matter, it's all helpful.
The patent clearly pertains to Walbro's WD / WDA / WDA-1 / WR series used on snowmobiles, but the BDC-16 is nearly identical. Compared to the BDC-16, the sled carbs add the "leaf," plus three tiny springs on the fuel pump check valves and (spoiler alert) the 2nd pump. The sled carbs didn't need the BDC-16's duckbill circuit, since its purpose was to compensate for the carb being grossly oversized for the 125cc Mc101A/A (and especially the 100cc Mc9X's).
No doubt all of this will be "old news" to some of you, but I thought it was interesting.
Now... if I could only get my BDC-16's to actually WORK...
By the way, for those sled carbs, this thread is super helpful: https://vintagesleds.com/bs/index.php/to...980.0.html
Sadly, the pics aren't there anymore... fortunately, while you guys were out running your karts and generally enjoying life, I was holed-up, making a PDF. It's too big to attach (13MB). PM me with your e-mail address if interested.
Regarding the McCulloch BDC16 double pumper alcohol carburetor and the WD series of snowmobile carburetors I wanted to make a correction;
The WD is on par with the tillotson HD and is not for use on any of the kart engines, however the WR series is, and is on par with 34mm Mikuni/Burris or the HR Tillotson (same as BDC). That is the mid sized but will likely have a tiny primer tube and 5/16” flange mount holes, not ¼”. And yes as mentioned above by SteveO, there will be a ¼” fuel line nipple that runs to the crankcase for the pressure/pulse. Another thing that many of the WR/WD snowmobile carbs had was a fuel return line (to the tank) making for a total of four hose connections including the tiny primer line on the these carburetors. The sled carbs were phased out in the early 80’s as most manufacturers had or were already changing over to Mikuni VM Slide carbs.
Mark L./SWNY