Friday, March 13, 2009

Tillotson Carburetor!

A while ago, Michael Mike mentioned that he was working on putting a carb on a bike that didn't need jets. No jets you say!? So I've been stoked on this idea and have wanted to try it out myself. I have been collecting the necessary parts for the last couple of months, and now that I've done it myself, I'm sure that a determined person could do the whole conversion in a day once they have the carb.

I'll try to explain my process and what I know about the carbs, but bear in mind that I'm a mediocre source of knowledge at best. Tillostson carbs were originally used for things like chain saws, lawnmowers, etc. The main feature of them is that since they are diaphragm carbs, they can be mounted in any position. No float means no dependence on gravity. Eventually, go karts started using them for the same reason.

So I got myself a carb, bought a new gasket kit at the lawnmower store, and rebuilt the carb while riding on the bus!

This guy on an arborists message board did a really good job of showing how to rebuild this carb.

The first problem that had to be solved was the intake mount. Unlike a Dellorto or a Bing, these carbs don't have either spigot or clamp mounts, so we had to make something that would work as a spigot. Luckily, an exhaust flange from a tomos, or puch mounts up pretty much perfectly. So after some cutting, I have two spigots ready to be bolted onto either side of the carb.

I made a couple of gaskets from a beer box and bolted it all together:

Since it's a diaphragm carb I am going to need a pulse line. Since I'm using a 4 petal reed cage, it's going to be super easy. As long as the nipple is tapped below the reeds on the intake I get the same benefit as if I had tapped right into the crankcase.
I bought a 90 degree nipple and drilled and tapped my intake, put a dab of grey goo on the nipple, threaded it in, and then dremeled the excess threads from the inside.

Once that was done I bolted it all together.

Uh oh! Since the carb is mounted forward, the throttle lever wants to be pulled to the rear of the bike. In the future, I am going to mount the carb facing the rear of the bike and this won't be a problem. However, for today, I really wanted this thing running. So I drilled some holes into a piece of sheet metal, and using a throttle cable elbow I was able to get my throttle cable to pull the correct direction. (See my video to see this all in action)

Per Michael Mikes advice, I primed the carb for the first time by loosening all the screws on top of the diaphragm so that the carb would flood with gas a little.

Of course, to my horror, at this point I messed up my throttle cable. The ends became all frayed after I routed it and I was dead in the water. What would moped tuning be unless something miniscule upset the whole process?

I ran to the store and bought a cheap bicycle cable set and using some knarps I was back in business.

Once everything was on, it was pretty much just the simple matter of getting the bike to fire, adjusting the low circuit until it idled, and then riding the bike and twisting on the high circuit until it felt right. Of course it will take some time to get the carb tuned perfect but for now it is certainly ridable. There you have it.

Tillotson Carb mountup from Joel Kvernmo on Vimeo.

Note: I really could not have done this without help. Thanks for the instruction MM.


  1. Rad Joel, very raddy radical. I like seeing simple innovation like this. Great videos too. And yeah, get it so that it mounts backwards, it'll look a whole lot better that way I would think. Later dude.

  2. Your videos are 100% better than any other moped blog video's I've seen. Nicely done!

  3. awesome. tillotsons: wave of the future, or at least pretty handy.

  4. this is wild, i definately want to try this, do you have any specs on the carb? what model, what it might be off of specifically, the size? i love it!

  5. I don't really recall the exact model number of this carb, I believe it's an hl-265. It's about a 20mm but with the butterfly in there it's more like an 18mm. I think the hl-360 is the one that karting folks use a lot. If you dig a little bit you can find karting sites with hi-flow banjos, varying pop-off springs, manifolds, cable mounts, all sorts of goodies.

  6. i have a brand new walbro carb that i wanna try now just for fun, i know its not as good but its worth a shot for jetless rippage!

  7. A lawn mower carburetor is smaller and less complex than a regular automobile carburetor, and even a motorcycle carburetor. It works with the same principles that a car one does, only it is supplying fuel for the engine of the lawn mower instead of an automobile. Air and gasoline go in, the vapors mix, and it powers the engine with combustion.