Wylie valve clatter is caused by LEAN mixture or advanced timing... period!
So you have replaced both systems.. lets talk about LEAN first. That carb is a bit too large and so it should be able to get TOO RICH quite easily. If it has an idle adjust screw for each barrel screw both of em in to STOP while counting the turns and 1/8 turns.. I would start with them about 2 turns out.. If this makes the idle advance to more rpm then I would turn the idle stop back to a good 700 or so idle..
Now to the OEM spark system. In good health this is a fine spark system and should work well. HOWEVER after changing the carb you MUST do the nutter bypass! Period! If not done the computer will "feel" of the exhaust and retard the timing.. It'll never run right till the nutter is done.
Maybe I would right away do a leak down test to see if one of the intake valves is leaking too much. A leaky valve causes randon missfire.
All of that said I am confused by the complete cutoff of the engine at times.. When it comes back on does it pop and crackle? If so it would be a spark issue.. When the spark does resume it ignites unburnt fuel in the system causing backfire.. If NOT then I would look to a fuel problem.. FILTER? Pull the filter and blow through it.. If it is not real easy THERE is the problem.. You may have a simple problem such as a cracked fuel suction hose.. Your pump is sucking AIR and not enough gasoline is coming with the air.. Also do the propane test of the new manifold gasket..But I ramble.. Check these out and get back to us.
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />