After running well my 42 Mercury-engined Canadian military truck wont start. Laying the plugs out on the block get good fat sparks.Gas is definitely getting thru to new Stromberg from Mad Max. Fuel pump working fine.Timing hasnt been changed. Distributor was originally set up by George haney. Battery fully charged and all grounding checked. Sometimes get a couple of cylinders firing but no more - often not even that. Even tried a 12v boost but no luck. Any ideas? Stuck valves?