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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.

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OK, stupid stuff.... You mentioned checking for atdc, and a degree wheel...

I have no idea about how the throws are aranged in this engine, so, have the throws somehow been twisted out of time? Closer or further apart, than what they should be?

The discussion about the advance and retard lever reaction suggests that maybe the throws are twisted, or I might ask are you CERTAIN you have the correct crankshaft, or mag, for the engine/magneto combo you have?

You said the heads have been off, twice, with new valves installed, but was a valve job performed, correctly?
New valves don't mean the valve seat is sealed....

The ...side? with the water problem, have you had that head and cylinder [ if it's iron ] magnafluxed? or Zyglowed [ if alloy ]? To check for a cracked unit?
Are you sure it's water, and not fuel?

Have you checked to be certain there are no air leaks in the intake tract?

Can you better explain your term "backfire"? Are you getting a bang out the carb, or exhaust pipe, or a flame, or is it trying to throw you over the handlebars?
Brett
P.S. Someone ought to be able to tell you how to contact the OLD bike list... Pre-war bikes.

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Well'ard Rocker
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Originally Posted by Nark
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.




I've got a Model X but that's no help because it's not together.

But I'm going to have to solve the same problems you have some day, and here's what I'd be thinking.

Can you treat it like two singles like we tune up parallel twins? Just disconnect one cylinder and pull the plug, then get the other cylinder to where it starts and idles well by itself. Then disconnect the other one and get the first cylinder to where it starts and idles by itself.

Then see what happens. Maybe its some interaction while it's running ....

Lannis


Be guided by facts that you can observe yourself, along with knowledge of how people have behaved during similar periods in history.
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You need two 3volt ignition coil in series , to make it work on a 6v system.
I use same setup on a j.a.p. 1000 sv. engine ,but converted to 12v, and use two 6v car type coils in series.Honda had a 6v double coil , some forty years ago on small engines with 360° cranks, but they seem hard to get. Early citroen 2cv with 6v may have something similar.


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Thanks for all the suggestions. I started to reply to some of them, but all I could do is tell people that they're wrong and I had already checked everything. I mean, things like an air leak? It's a single carb, so an air leak wouldn't affect only one cylinder, that's obvious. Except that it does, and that was the problem. There are compression fittings connecting the T intake manifold to the head, and one of them wasn't compressing.

The stupid thing is that I'm not a bit surprised that there was an air leak, I probably might have even laid odds that there was an air leak, but I just couldn't believe that it could affect one cylinder so much more than the other. Considering that I was running it with the choke on, I wouldn't have thought the leak could have been big enough to still make it that lean.

So, it's got a nice thump-thump now, like a good old twin should. We're going to get it down on all three wheels this week and give it it's first outing in at least 30 years.

Thanks again for all the suggestions!


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I ran across my old post when googling for Matchless info. I hate to respond to my own old post, but here it is.

We did eventually positively identify the root cause of our problems, and it wasn't an air leak.

I'm fond of saying that 25% - or 50%, or 90%, depending on my mood - of all carb problems are electrical, and the same percentage of electrical problems are carbs. Well, this one turned out to be an apparently carb problem that was 100% electrical.

When we rewired the trike we used replica cloth jacketed wire - PVC insulated wire that has a cloth layer woven over it for the authentic look. Very practical and very good looking. It also makes the wire a bit thicker - so 18 gauge looks like 16 gauge, 16 gauge looks like 14 gauge, etc.

The wire to the (6V) spark coil was simply one gauge too small and we hadn't noticed, since it was the diameter of the correct gauge wire. The 6V coil can be dodgy at times when everything isn't just right, but we had made sure that everything was just right, except for the size of the one wire feeding the coil, so the coil was current limited, causing a voltage drop, causing the flux capacitor to short out and cross the streams, etc.

Within minutes of noticing the undersized wire I put a jumper from the battery to the coil and the engine fired up on the first pull of the crank. I replaced that one wire, without changing anything else, and it started first crank almost every time and never spit back again.

Another lesson learned. Again.

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Hi,
I very much doubt the size of the wire was the problem, It is waaay more likely there was a poor connection in the line
You found that when you bypassed the wire the problem disappeared , This is a method I have often suggested to people who call me with
wiring problems (and use myself)
6v systems are very susceptible to poor connections and oxidised switch contacts

That said today I spent quite a few hours going through wiring connections on my newly bought MGB, oxidised bullet connectors everywhere
a new non working sidelight wasn't working, I found the bullet was crimped on the insulation but not on the bared wire end !!

John


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