The spark is yellow *at the coil* the wires and plugs are fairly new and the wires ohm out like new, plugs look good. The old distributor gear was fine. The new one came with a new reluctor, pickup and ignition module. Timing is controlled by the ECM.
I'm still concerned about "Yellow as a Daffodil" about four sentences into the OP and again farther down in the thread. Even if it's not HEI, this IS electronic ignition. Shouldn't it be popping a hot blue or white spark? Just to me, in time or not, there should be a HOT spark.
See your 1990 has an EGR valve, vacuum thermostat opens it at around 160*F and this is when your engine is going crazy. Typical for GM a dual diaphragm, top port can check okay, no way to check the bottom one,, but apparently leaking causing your misfire.
My 82 454 does not have one, mystery for me, required to burn non-leaded fuel to reduce combustion chamber temperatures, but with over 50K miles on it, CR is still great, 150 psi on all eight cylinders.
Mine was really out of timing, previous guy did not know that on the standard 454, timing marks are visible at the top of the engine. But no way to view these on a motor home, so were moved to the bottom. And the inductive pickup on your timing light does not go to cylinder #1, hooks unto cylinder #8.
Could have purchased a newer one, but already had my full with this computerized crap with other vehicles. But this is not the only problem with this older stuff, ethanol gas was never heard of when these things were manufactured. Don't sure love our government? Came out with ethanol in an attempt to cut our fossil fuel usage by 10%, but still burning fossil fuels to make this crap.
I wonder if it's terminology. Refresh my memory: Wasn't the name "HEI" first applied to the GM systems with the Electronics IN the Distributor AND the Coil in the Top of the CAP? If so, some would say a system with a traditional center contact in the Cap with a Coil Wire leading to a separate Coil, was Not HEI. The 350/5.7 in our boat was like that. It was a Delco ignition with separate Coil that turned out to be an automotive coil. Nobody called it HEI, maybe partly because it wasn't in a vehicle so the term wasn't familiar. The coil looked about like this:
. All I did was drill the rivet out of the "marine" coil and attach to one of the four holes in the corners of the lamination. Distributor happened to be "EST" (Electronic Spark Timing) which the OP here says this 454 is not. But I wonder if the system is "HEI" just with the Coil NOT in the Cap...
High Energy Ignition, introduced in 1972 by Delco when Delco was a part of GM, only the V-8 had the coil in the cap, were external for the four and six cylinder engines.
Was described as the easier cars to steal, just plug in 12V to that 12V terminal in the cap. Delco had to supply the complete distributor to GM including the 8 Silicone spark plug cable for $6.00.
One serious mistake was gaping the plugs at 0.065 inches, would get a series of misfires, I gaped at 0.035. also the year the catalytic converter was made law, misfires would toast that thing.
NASA also developed thick film technology to replace epoxy printed circuit boards, we kind of borrowed it, essentially one NPN power transistor replacing the points.
First ones had both centrifugal and vacuum advance, these two funtions were replaced by computers in around the mid 80's.
Major problem is that top bronze bearing would wear thin, engine oil could not get up their, other was soldering pigtail leads to the inductive pickup coil, pushed the ladies to fast for this, would get a flux joint that would corrode and loose contact. I installed an engineering sample in my 1970 Buick Riviera, direct fit with no more points to adjust. Oh, can't forget this, GM was using a resistance wire for the old ignition coil, had to replace that with a solid wire. 12 Volt coils were rare, Ford and Chrysler were six volt coils with a so called ballast resistor in series.