The Hobbit Tillotson

The traditional view in the vintage moped scene is that pumper carbs are an intriguing novelty, but whatever advantages they offer are outshined by the performance of conventional float / venturi carbs. AND YET - they are the standard carburetor of basically every snowmobile, jetski, gokart & ultralight aircraft from the 70's & 80's. With that kind of pedigree its understandable why many have tried, even if only a few shall succeed. (RIP myself)

For the Puddle Cutters Anniversary raffle bike, I wanted a larger carburetor. The dimensions of the Hobbit subframe meant either using a manifold that placed the carb outside the subframe, or embrace the hellish tedium of removing the subframe & carb for every carburetor adjustment. I prefer the look of the enclosed carb & was concerned that an external carb might interfere with pedal operation, especially if its next owner was a noob. These factors encouraged me to take another chance on a pumper carb. A bet that cost me exactly $19. The device was a clone of a Tillotson Hl-324A carb. Approximately 20mm bore, if it worked this could've been the cheap and possibly easy solution to many a Hobbit owners woe. ALAS! My 11th hour panic to locate an airleak had me eject the tillotson, So I'll never really know what might've been.

If you have no idea what Im on about, a Tillotson / pumper carb does not use physical jets to regulate stoichiometry: "jetting". In a traditional PHBG, SHA or Bing type carb, you swap out jets and needles and slides until you arrive at the preferred combination of parts. And off you go. A pumper uses the collapsing atmospheric pressure from the engine cranking over to pull a spring-loaded diaphragm that squirts / pumps fuel into the venturi. A pumper has no jets, but two screws to control the amount of fuel getting pumped. One screw for low speed, one for high. The procedure for balancing the two is pretty straight forward. If the crankcase pressure is too little or great, the spring can be swapped or modified. Easy Peasey, right?

Sorta not really. On the plus side, the HL-324 drops RIGHT IN. The studs on the stock manifold are within a millimeter of the mount. You'd need to drill holes to access the adjustment screws, but the location of these holes doesn't interfere with the case, wiring, variator, etc.. The overhead clearance between carb & frame isn't enough to prevent damage, so the carb must be flipped diaphragm side down. This is where it gets kinda tricky, because it requires a custom bracket to reorient the angle the throttle & choke cable. The choke is also complicated by needing to be held open through a spring. (wack)

Meanwhile... The Manifold and reed also need some prep. First, the stock manifold was bored out to fit the larger venturi diameter. I used a stepper bit, followed by a forstner bit to open the manifold. The stock reeds were upgraded to a boyensen set. I was unclear as to how the reed valve might interfere with the pulse line the carb needs so I located a usable spot on the reed valve and drilled out the reeds with an acrylic bit. I then drilled a hole in the manifold isolated from the carb venturi. The hole then aligns with a channel on the spacer plate between the manifold & carb. Finally the channel aligns with another hole facing the pulse hole.

To reiterate, I pulled this carb off the bike literally hours before a raffle in my desperation to resolve an airleak. That airleak was ultimately due to a bad seal, so it's a reasonable assumption this set up would've worked as expected. Unfortunately, I'll likely never know. I posted all this in the event that another hobbit builder wants to give it a shot. If that's you, Godspeed & happy hunting!