Scooterotica

ive got an old race vaga that runs a tillotson in my garage!but its only 20mm.
pump works of a pipe to the crank case.

I used Tillotsons on my first national 100cc group c kart when I was a kid. In the day they were pretty good but have been surpassed now. They have no float bowl so instead they run a pressurised fuel delivery chamber which is supplied by the pump. There is a "pop off" valve arrangement in this chamber to govern the fuel pressure. It is a clever solution but no without it's problems. Predominantly they were used on small capacity 2 stroke engines particularly those on hand held machines or those that pull multiple G and get through around upside down etc such as chain saws, slab cutters and military drone aircraft. Essentially the characteristic of these engines is "idle or full chat" on relitively short running times.
Strangely enough, in the kart application the carbs are run upside down on purpose. The reason for this is that they were originally used on the disc valve group C engines where the drive sprocket was directly under the carb. Inverting the carb ment that the fuel banjo and pump/delivery system was out of the way of the chain (as there would be no space for it, the other way round). The orientation just kind of stayed that way when the newer reed valves took over plus it help with the position of the mixture adjustment screw position too.

Either way, to get back to Lambretta engines with a Tillotson carb, a couple of things happened. The first was power, the max size of the venturi for the HL tillotson series carb was 24mm. This was too small for the GT186 kits which ideally required a 27mm venturi (we changed to the PHBL25 and power jumped by 2 BHP and Dellorto cost less and would easily convert to a cable choke). Secondly, there is a problem with heat and fuel vapourising in the pump & delivery chamber. This then gave a lot of problems with warm starting. To over come it meant a special Tillotson with a recirculation circuit...it all got a bit "chasing your tail" when there was a standard issue PHBL waiting to go.

If you ever have money burning a hole in you pocket and nothing to do then one thing I didn't try was a double tillotson back to back on a GT kit. The reed valve used on a GT kit is the same used by the Parilla, KTM, IAMI and Rotax 100 reed valve engines. Stangely enough, the stud spacings on the reed valve are the same as those on the Tillotson and there is perfect space to run two tillotsons back to back on a GT reed valve, straight in to the reed valve, no manifold. It would mean a little conversion plate and naturally, you have to do something about the frame tube but that would be interesting.

There are loads of different versions of the HL Tillotsons, some with pumps that can be driven externally from a pipe and others (the kart versions) that line up with a hole in the manifold that leads to the crank case. In many ways the Tillotson is a natural progression of where the Wal Phillips should have ended up. A couple of years ago I did some drawings to make a version of a Wal Hpillips with a Tilotson style fuel pump and delivery system. When I got back in touch with tillotson my helpful contact had left and they didn't want to help at all. A shame really as essentially a modern day Wal Phillips that was integeral to the GT manifold with an integrated fuel pump and recirculation circuit, there was a good chance of resolving a whole bunch of problems associated with the original fuel injectors.