Gil ... glad you took it to the limit as my attempted run this afternoon was a flop .... well mostly a flop. I can say that one fill of my tank got me well over three hours run time, but I did have problems that caused me to lose track of actual time as the engine quit on me a couple of times, and wouldn't restart right away. I was also not right there when it stopped, so don't know how much actual run time was lost, though certainly less than ten minutes each time. Can't figure out what the problem was/is as it stopped again somewhere near four hours and now won't start, but there is no air showing in the fuel line so I know it hasn't run out yet.
I got 14.6ml from a high quality glass medical syringe (B*D Yale) into the tank, but I also had the needle valve on the carburetor open 1/4 turn so some of the fuel I was putting into the tank may have been lost into the cylinder. It did take me half a dozen flips to get it to start and it obviously started up very rich. Another problem I had was that while adding in the MMO I was trying for 25 drops, but around 20 drops got a squirt that probably doubled the oil I put in the fuel, so perhaps my run problems were due to too much oil in the mix. That was the very last of my old Coleman fuel, so I couldn't rectify the MMO overdose in any way.
The first time the engine quit, I thought it might be a break in one of the wires, the way the thing bounces around could easily pinch a wire, but no ..... I pulled the spark plug wire and gave the engine a flip, and YOWWWIEEEEEE, nope not an electrical problem. So I pulled the plug to see if it was fouled, and broke it (Oops) as my wrench had no room to turn and my socket fit at a very slight angle that was enough to break the ceramic when torqued. The plug was not especially foul, with just a little black carbon showing, hardly anything at all really, but the plug was really wet. With a new plug installed it took right off, but stopped again within an hour.
Another problem with my endurance run today was that I did a little tweaking of the springs on the counterweights and even though that did seem to slow the engine down some, giving me about three hits every five seconds, it also caused it to double hit some and even triple hit once in a while, so clearly not an optimum situation. Still I'm certain that I did get something over three hours run time (likely well over) and the tank isn't empty yet.
I'll try again later and see what I can do with fresher Coleman and perhaps a bit less MMO, and perhaps messing around a little with the timing.
Congratulations on your very nearly five hour run!!!
ps: Yes I fully realize I can trim the springs back to a shorter configuration, but thought I'd play with the "arch adjustable" concept a bit first, before I go whacking on them. Shortening them even one coil does make them less elastic (stiffer) so is not ideal. May see if I can get a few spare springs from Jin at some point, so that I can get back to a nominal configuration and have more options.