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!

Puddle Cutters 17th Anniversary Raffle Hobbit

Stacks on Stack on Stacks -was the battlecry. Almost a year ago, The Puddle Cutters were planning the next rally and trying to develop an operational budget. With some, but not much in the bank we were scoping the rally to fit our budget. (weak) I said that I could double the gang's budget if they backed me up on building a raffle bike. And that they did, Gang gang. The plan was to raffle off a bike during our anniversary ride (mid-march) and use the funds we made to pump up said jamz. Barney very generously donated a non-running roller PA50II Hobbit to the cause. The bike was a ridden-hard & put away wet ex-Uphill Battle bike. It obviously was the source of much joy & gaiety for its previous owner. I do love bikes with personality & it truly wouldn't have taken much to bring it back to operational status. But y'know what sells raffle tickets? Sex Appeal. So I said f**k it & my maximalist tendencies took over. Full frame up rebuild, here we go.

So for starters, here's what we had... The hobbit was missing most of its plastics, had no spark, likely engine rebuild candidate, the gas tank had holes in it and it had been crashed enough to prevent the pedals from working. #mopeds On the plus side- the rims, tires, forks, frame & subframe were in good shape and it had a complete wiring harness and most major parts, including a circuit pipe.

So I took the bike apart as far as I needed to validate every system & redo the paint. I washed the hell out of everything, before stripping the paint down to the metal. I made a few choices: I really like bikes that look complete & making everything work correctly was a key goal. The engine would get rebuilt with every upgrade that still allowed for a mostly-stock appearance. I went for the Metrakit, a stuffy crank, JBot/Treats variator, stock ignition, boyensen reeds & a 20mm Tillotson carb (for extra razzle dazzle).

Every Hobbit in the world is yellow, or red if you're unlucky. I wanted a color that that would make the raffle hobbit unique, but felt appropriate to 1978. I settled on Oldsmobile Bronze engine enamel. -bc 1. Gas resistant & 2. Sparkles. After welding up the gas tank I primered and sprayed everything. At this point, Travis, Kyle & Kristin from the club had all donated body panels & a rare AF OEM pedal crank to the cause.

If you've worked with the metrakit, that exhaust port is no joke. Even though the studs were a match, it seemed silly to restrict the new engine to the dimensions of the stock header. So I cobbled together a new header from pieces I had laying around. Getting everything to fit correctly in the closer quarters of a circuit pipe was a real challenge. But hey, at least it looked nice when it was done.

At this point in the process, I had blown up our planned timeline and missed the Anniversary ride deadline. Kyle / everyone was super cool about it. We had another opportunity a few weeks later at The One Moto Show's kick-off night. At this point I was assembling the bike, including the persnickety wiring. I really wanted everything to work, including the stock ignition switch. I was able to rebuild the old switch using a new lock column off treats.

At this point I had everything to finish the build of the bike & the One Show deadline was approaching quickly. I assembled the bike enough for photos, but the engine had a persistent air leak that wouldn't allow the bike to run for more than a few seconds. We were already selling raffle tickets and I was indeed panicking. As it was the only unorthodox component, I swapped the 20mm Tillotson back for the stock carburetor (which had been bored to 13mm). The air leak persisted even then. I was sweating bullets as the raffle was within 36hrs. I pulled the motor to look for broken gaskets, etc. The flywheel side seal was brand new from the rebuild kit, but did not feel snug. I replaced it with another seal I had laying around. After reinstalling the engine, by the power of Greyskull, the hobbit started and ran. (deep breaths) The jetting was all over the place, but had enough power to get me to the One Show event. The raffle Hobbit was the Belle of the Ball that night and we sold out of tickets (limited to 250). The Puddle Cutters did the raffle live on Instagram and a lucky fella from Washington state won:)

I got lucky again in that the winner of the Hobbit was not physically present at the event. This gave me precious *precious* time to correct the jetting. After a few rounds I had the Hobbit ripping hard and lifting the front wheel. 'Launching' is a good word for it, I see why so many guys dig these built hobbits. I put a huge amount of elbow grease into the project, but it takes a village or whatever. So many people contributed to donate parts, or worked to coordinate the raffle itself, or promoted online. In the end, the raffle bike was a tremendous success, doubling our raffle budget and allowing our small club to go hard on the rally. My heartfelt thanks to everyone who contributed to the project & everyone who bought tickets! Before:

After:

No Update, just an Engine Casting Video

WHHIPSNAKE: 75cc Puch Cylinder Project

Hey Gang, Like Alex from 2StrokeStuffing, Kilian Münscher, Doug Binder, Jurrian Mulder & Graham, I am also working on a custom-cast cylinder project. This is not my first, however. I cast a cylinder back in 2009 for the Decepticons ‘No Rules Race’, which was limited to 50cc & built around my Case Reed-Inducted Puch ZA50 motor. -But that’s a story for another time.

Ever since I started goofing with mopeds twenty plus years ago, the Gilardoni 75cc cylinder was the peak of off-the-shelf Puch performance. My second ever build in 2002-2003 was a Gilardoni Puch ZA50. Watching the parts availability flourish over the years from the ‘tractors & volkswagens’ state of things when I started, I’ve been waiting for the Gila’s successor. But it never came. Yes, a Polini ported just so, or a EuroKit / 80 Metra with the right mix of parts will beat a Gila. And of course the watercooled conversions are plain bonkers. But the Gila is still the gatekeeper. So I decided to design and craft a direct replacement for the Gila that outperforms it in every metric. Sure, I do love learning stuff & gaining skills. I have a stubborn DIY, Burt Munro style ethic, but that’s not my goal. The Goal is to beat the Gila. To make a cylinder that slaps onto a bike already set-up for a Gila with power output much more like the AutoScoot, Kart & Dirtbike engines that share many of the same limitations but are significantly punchier than classic mopeds. I recognize that a go-for-broke approach is dumb, and needs to be moderated against the tunability and transmission liabilities of a 40+ year old moped -and yet- I still feel the Gila’s 11-13hp leaves opportunity on the table.

So, I did a buncha of math. Primarily using A. Graham Bell’s  Two-Stroke Performance Tuning (2nd Edition) formulas and parameters. & Honestly, I can’t recommend that book any higher. -But also reverse ‘engineering’ cylinders with performance I’m trying to duplicate: Honda NSR 80, Kawasaki KX85, Malossi MHR series for Dio / Zip / NRG, Aprilia RSA125, Cristofolini, etc..  I landed on some porting numbers I feel are a good starting point.

I then designed a 3D model of the cylinder. Compared to the Gila, I added additional cooling fins, a larger intake system, a larger transfer face and an exhaust flange 1mm wider. The internal port flow is comparable to other modern cylinders. There was a strong emphasis on maximizing port area as well as preferable radial & axial port angles.

Most of the guys previously mentioned are using the lost-PLA method, which (trust me here) is much simpler than lost-wax casting. That process is ^ everything above, but then 3D printing the cylinder with internal structures modelled, adding gates, sprues & runners and then investing the model assembly into plaster. I chose not to go that route because of three reasons: texture, drift & scalability. 3D prints all have a texture, even at a high quality level and I needed the internal face of the transfer ports to have a different texture. Every 3D print I’ve ever worked with, had at least a tiny bit of drift -as in not precisely the shape that was modelled. Since the port features are so small, I needed to really sweat it with my caliper micrometer. Which is not something you can do to a fully formed 3D print. Scalability was also a thought. From experience I knew that it was going to take a few tries to get it perfect and the vendor cost and wait times favored lost-wax casting. Having said all that, there are many penalties to lost-wax and it kinda comes down to individual preference. I trusted my gut.

So I 3D printed the internals of the cylinder as a positive shape (known as a buck in mold-making), I then did some bodywork & sanding to get them exactly right. After that, I created a Silicone RTV mold of these multiple internal parts. This mold is used to cast the internals in plaster. These plaster parts assemble in a modular manner that allows me to make changes without having to redesign / remodel / reprint / remold the entire assembly. This is the pre-fabbed core of the cylinder.

I 3D printed & made a mold of the external cylinder form. The external form has registrations that match the registrations on the pre-fabbed core, so it plugs into the empty mold while waiting for the wax. The external mold was a multi-part assembly to allow for the wax positive to be removed without damage. The mold material is Smooth-On silicone rtv (Mold Max 25) and sometimes TAP Plastic RTV 30. 

After that, I had my wax cylinder loaded with a pre-fabbed plaster core. I then had to attach the sprue, runners, gates, vents, etc… and figure out the best location for these elements as well as a plaster mix that is durable enough but also fluid enough during investing. Pure plaster of paris is way too brittle and will not survive the burnout process, too much sand will cause the investment to freeze quickly and leave voids. I’ll share the recipe if you’re curious. 

I tried a variety of gate, runner & sprue locations -all with mixed results. The textbook method is a sprue that runs alongside the mold down to a gate at the very bottom of the mold, feeding the aluminum up through the bottom. Looking at youtube, some manufacturers simply pour through the center of the cylinder body.  Thus began my campaign of trial and error. Every attempt saw some experimentation with wax type, gate & vent arrangement, and burnout procedure. Across 2019-2020 I attempted around a 12-15 molds & pours with only 3 quality results. Of those 3, one was perfect except it had a tiny imperfection in exactly the wrong spot *rage scream at the sky* and the other two were machinable but very ugly and likely had internal voids. Locating the gates, sprues & runners is tricky but likely not the source of the many failures.

The real culprit here is the burnout process. Once you’ve invested your mold assembly into plaster and the plaster sets, you’ll heat the mold to get the wax (or PLA) out to create the void for the aluminum. Proper burnout is ESSENTIAL. Plaster at the microscopic level is quite porous. While heating the Plaster up, the wax or PLA will start to liquify and soak the plaster. So even if you heat the mold up-side down, to get the wax or PLA to flow out, it’s still soaking into the plaster. If not all of the wax or plaster is eliminated from the plaster during burnout, the hot aluminum entering the mold will cause it to ignite, blowing gas into the void and screw up your casting.

Trust me, I know from experience.

Ideally, you’ll need an electric oven / kiln that can get up to and sustain 800f degrees for days. Professional sculptors I know will burnout 3-4 days prior to pouring aluminum. This would also probably be in a facility that won’t burn your house down. The other thing is that the mold cannot cool down after burnout while waiting for the aluminum. So if you only have the one kiln or furnace, etc.. you're screwed. Also, you can’t burnout the mold one weekend then do the pour the next. Thems the breaks. I tried many different strategies to cheat this and none really worked. 

So I decided to get rid of the wax.

Last winter, in the smoldering aftermath of my most-recent casting failure -and considering the pro & cons of a tricky sandcast system, or a highly expensive tool-steel die-mold process (which is what the factories use), I decided to use the idea of my internal parts plaster mold and expand it to make a multi-part matrix of plaster cores that together, form one fully detailed waxless mold assembly. Without any wax, the prep for the mold is as simple as preheating it to a temperature to drive any moisture out.  

But that meant I had to design a system of plaster chunks that could fit together without altering the original dimensions of the cylinder. So I designed that. It’s an assembly of nine parts (plus the internals) that use the same registrations as the original design. I also tweaked some of the dimensions of the fins & intake tract as I saw opportunities for improvement after the original series.

So I 3D printed all the mold parts, did bodywork & modifications to make them more accurate. I then began the process of making molds for all of the 3D prints. That & living life took about a year. At some point I started calling it the WHHIPSNAKE! because of a funny video about an over-featured piece of equipment. I just recently finished the molds and began pulling plaster cores out last night. They look pretty good & I think it’s gonna work. So that’s everything up to this point, I’ll drop updates as they come up :)