I've got a Matchless V twin that is driving me insane with behavior I can't rationalize. I'm posting this here because I suspect the answer is more common sense and ingenuity than Matchless.
The engine in question is a 1934 Matchless MX4, 50 degree water cooled OHV V twin. Single AMAL
89/011 carb, not new but not hideous either, and the jetting is at least close to where it should be. 6V, positive ground, points ignition, single breaker, two cam lobes, twin coil, wasted spark. Wasted spark requires that one cylinder fires 360 + 50 degrees after the other, rather than 50 degrees after the other.
The engine is freshly built. New high compression (7.5:1) pistons, new guides & valves (plenty of clearance, so not sticking).
The problem I've got is that I can't get the two cylinders to run the same. I can make one run, other the other run, but not both at the same time. I can start it and it'll run pretty well, but it'll spit out one tailpipe. It only starts with full advance (7/16 to 3/8 inch, depending on who you ask, which I think is around 36 degrees), which makes no sense to me. It backfires if I try to start it at full retard. When it runs, the "front" (driver's side) pipe gets good and hot, but the right barely gets warm. If I fully retard the ignition then the engine starts spitting and I have to keep the speed up to keep it running, but then the other ("rear") pipe gets good and hot and the "front" cools off.
I've set the ignition timing at least 20 times. I've checked the cam timing twice, swapped the cam anyway, taken the heads off twice, pulled the timing cover off a handful of times, swapped the coils side-to-side, swapped the plugs, run it with either spark plug out (it runs the same), changed the petrol (87 octane), and so on.
The points cam has one long lobe and one short lobe, I'm certain that I've got the correct cam for each cylinder but I tried switching it anyway and it didn't run much worse. I verify the timing in both spark plug holes, it looks spot on. When I had the degree wheel set up I stamped TDC and 7/16 BTDC for each cylinder into the flywheel, and a timing light says both cylinders are firing at the same times - I see the fully advanced marks for each cylinder superimposed on top of each other. The crank handle only turns the engine one way, so it's unlikely that I got confused and mixed up ATDC and BTDC.
Another weird thing is I'm getting water out of the "front" tailpipe. It's more than I expect from condensation, but not a whole lot. When I first put the engine together I had a leak at the head, verified with a fluorescent dye in the coolant, which showed up in the dribbles at the tailpipe. I surfaced the heads and cylinders, now there's no more dye in the water, but still some dribbles. I drilled drain holes in the mufflers, so I don't think there's standing water in there.
This engine is different from the X/X2/X4, it's an X modified for OHV, same cases but machined different with different heads, cylinders, front mainshaft, cam, etc. It's got a three lobe cam and some tight clearances in the timing side. Unfortunately the visibility into the cam gear is minimal at best, so maybe the rockers are tangling, but only when running?
I've got fuel, spark (kind of wimpy), air, yet both cylinders are somehow different. This engine should start and run just fine when firing right around TDC, but it doesn't.
So I've already checked everything, and it's all correct, but I'm obviously wrong about that since it doesn't run. Any and all ideas are welcome, even stupid stuff. Maybe especially stupid stuff, since I'm often too smart to think of the stupid stuff.