Ok - I have an engine very much like Tilley - it is a 1/2 breed
15HP natural gas two cycle that has been factory converted from
a steam engine to a two cycle natural gas engine that I run on
propane. The cylinder and main casting of the engine say
"Manwaring & Havens" & "Sheffield". I assume made in Sheffield, PA.

I'm sure there are lots of ways to run it but I found
what works for me is to burn the hot tube flame so the 6" stainless
steel hot tube (purchased from John Burns) is entirely red. 
Connect an accumulator in front of the gas/air mixture input made
of 4" pipe about 4 feet long fed from a regulator set to about
2PSI. In between the accumulator and the engine is a 3/8" DIAMOND
valve. This is Much more controllable than a larger size!
The head itself has one threaded connection that sticks
straight out as if to hold a spark plug. In this hole, Is a short
nipple that sticks out, a tee with vertical hot tube and another
nipple with a ball valve that opens to the air.

Then, to start the engine, you open the valve to the atmosphere,
close the diamond valve and turn the engine over at least twice.
More times don't hurt anything. Leave the piston at BDC. Close
the air valve, open the gas valve to exactly 1/8. Put your foot
up on a spoke and grab the top of the one flywheel (there is a
clutch on the other side - only one steam engine flywheel) and
give it a bit of a turn back into compression. As soon as it fires,
and it will 90% of the time with this method, you have to immediately
open the gas valve ever so little until it fires again. Then keep
your hand on the gas valve and do tiny tiny adjustments until the
engine is running nice and comes up to temperature. 

This engine runs like a four stroke when unloaded - i.e., ever other
compression stroke is only a small explosion and every other is a
good hard hit. If you crack the gas valve open farther, it will
hit hit hit but run too fast. I haven't had it loaded it but 
suspect if it had a belt load on it, you could give it more gas
and it would hit hard every time.

I am writing this at 2am and reserve the right to change the 
the 1) length of the stainless steel pipe, 2) the gas pressure in
the accumulator, and 3) the setting on the diamond valve because
I haven't run the engine since last fall but I believe what I have
written above is true and correct. 

I LOVE this engine bought off eBay, sight unseen, for $850 on my 
way to my first Portland show ever in the summer of 2001. The 
engine was 4 hours west. The show was another 7 hours farther west.