On glowplug engines the timing is fixed (essentially) but determined largely by the compression ratio, the head shape and the nature of the glowplug (and the plug in turn has four independent variables!)-and the fuel components itself. The optimum timing and running characteristics will be quite specific for a particular fuel (and for the purposes of 1/2A-and assuming adequate lube content-it is the % nitro that has the most significant effect). The various aspects of the plug can be summarised as 'plug heat' [which is actually a bit of a misnomer-but a carry over from the days of ignition engines and full size spark plugs) but simplifying things-the more nitro you have in your fuel, the 'cooler' the plug and the lower compression required. Too hot a plug or too high compression leads to preignition (because the collective effect is to advance the timing)-this is manifested in reality by the engine being difficult to start-and/or running with a 'crackle' and being hard (or even impossible) to get a clean peaked needle setting on. The other extreme is equally possible-too low and too cold-also difficult to start-and may not sustain running. If it dies or the revs drop when the plug lead is removed-definitely indicative of a 'too cold' situation. (it could also just mean a dodgy plug!)
On the serious side-AMA speed for example, people would set up their engines for a specific fuel and plug-this would entail a head machined for 'that' specific fuel-a new plug each run-and the head shimming carefully optimised-and even then they would have to alter the head clearance minutely depending on the weather on the day (or alter the nitro content of the fuel slightly up or down for the weather-which achieves the same result)-in other words that engine was set up with every aspect of its performance optimised for a specific set of conditions-and an alteration in any of those would result in sub optimal running.
Now obviously a Medallion 049 is pretty much the opposite-a sport motor, ported and timed for general purpose use and easy handling-but limited by production tolerances and the head geometry. With Coxes you have no choice in plug heat-you get the standard Cox head with the standard Cox element-in either low compression (#325) or high compression versions (#1702)-so the only variables you have to play with are the nitro % and the number of head shims. In your case running 35% nitro with a high compression head almost cetainly resulted in an 'overcompressed' engine-and the addition of a couple more head gaskets might well have improved things by lowering the compression. If it didn't have much effect then you might have to revert to the low compression head as well. The effect is well demonstrated in our model diesels-where for a given load, on a given fuel, the engine will run cleanly and smoothly only at the right compression setting-too little and the engine runs with a misfire (because the ignition is effectively retarded) and too much and the engine runs 'hard'-and sounds laboured -the difference between the two settings may well be only half a turn on the compression screw-in real terms a few thousanths of an inch-the equivalent of a couple of head shims....!
ChrisM
'ffkiwi'