Question: I need help with the ignition timing for my 1966 Triumph 200cc T20M Mountain Cub. It has a weird idle but it goes like hell. I set up a degree wheel and found it has a Harman & Collins race cam with 104 degrees overlap and a 10:1 piston. I set the ignition timing at 32 degrees full advance and used 101-octane unleaded and it ran great until the exhaust nipple loosened in the head and damaged the thread in the port.
When I removed the head to repair it, I found rust damage in the cylinder and had to bore it to 0.100 over. I then used a 9.5:1 piston and reset the ignition timing to 36 degrees full advance and tried 91-octane gas. It promptly seized and started melting a spot on the piston dome. I’m thinking of sleeving it back to a standard bore, using another 10:1 piston and switching back to 101-octane gas. I’m using the plug recommended by Triumph for Cub high-compression pistons, but the standard length comes about 4 threads short of being flush with the combustion chamber surface so I’m using a longer reach plug with the same heat range with 3 copper washers, which then comes flush. Would this contribute to hotter running?
I’m running a 190 main jet, #3 slide, 0.106 needle and #20 pilot jet in the Amal 15/16 monobloc carburetor as also recommended by Triumph for high-performance Cubs. I’m thinking of going back to the 32-degree ignition timing. But why does Triumph recommend 40 degrees for racing and 36 degrees for high performance street? Please help before I melt another hard-to-find piston.
Andrew Granovsky
North Hollywood, CA
Answer: The Triumph Cub is a strange historical anomaly. Yes, some people got decent power from it, but that was because what came before it were glorified scooters like the Mustang that won its class on Catalina so long ago. The Cub has a long reputation for a weak lower end. Compare its cooling fin area to that of later designs and you will see that, at a given power level, it has to run hotter. This hotter running puts it into the “severe” class of engines whose operating temperature pushes them close to detonating (combustion knock). That in turn requires fuel with a high anti-knock rating.
Another important point: The compression ratio printed on the box that the new piston comes in is probably not accurate. That makes it necessary to actually measure the combustion chamber volume with a syringe with the piston at TDC, and compute the actual ratio. Air-cooling makes pistons run very hot because all there is to cool them is contact with the cylinder wall—which is none too cool itself. During World War 2, Wright Aero had endless trouble with R-3350 (B-29 engine) pistons scoring and seizing—until the company adopted piston-cooling oil jets in postwar engines for aircraft like the A-1 Skyraider. The Cub is under-finned, so it has a special problem with piston temperature.
Your original build, running at 32 degrees ignition timing, was probably somebody’s proven combination, intended to run on good gasoline. Pump gas today is poor stuff, which is why racers run on race gas.
Here's what I suggest: Come back to no more than 9:1 compression, run race gas (not so-called "Turbo gas", which doesn't evaporate very well), and begin with the 32 degrees of ignition lead and a cold spark plug (very important!) recommended for that engine. If you know how to read spark plugs, you can then advance the timing bit by bit, looking at the spark plug’s center wire to see if its original sharp edges are softening (like the end of a snapped-off glass rod, held in a Bunsen burner flame). If the edges soften, go back half a degree. Be sure to break the engine in before applying anything like full power. It is a chronic problem of older engines that they will not break in on today's oils containing strong anti-wear agents. If the rings do not establish a seal in a full break-in, hot combustion gas leakage will push piston temperature up very fast.
I don't know what you plan to use this bike for, but older British bikes don't take to today's highway speeds. Souped-up Cubs like yours were run on dirt-tracks, where their lack of cooling was to some degree compensated by the fact that the riders were on and off the throttle—no long periods of full throttle, but periods of cooling between periods of throttle.
The Cub is a classic, but its care and feeding were always a special art.
Send your “Ask Kevin” questions to cwservice@cycleworld.com. We cannot guarantee a reply to every inquiry.