As I was looking for an update on this thread, and knowing that you are going to give it another try on a new engine, I decided to re-read the whole post regarding the plug burn-up issue. It made me wonder at what starting bottle psi did you test your 100shot? (really 75whp jetting under Zex charts I think should be no prob on VHR). If it was really high 1200-1300psi or greater that could have caused an extreme lean condition or if the NMU plungers became stuck due to low voltage, high pressure, or long-term pressure where you didn't close the bottle off and purge out the lines after use. I came across this article that may be of interest the next go around...
Rebuilding a nitrous solenoid..
I've been getting questions on what would cause a nitrous solenoid to "stick" closed sometimes ??? This time of year (HOT Summer) this seems to become more of an issue. With the under hood temps getting unbelievably high there are a few things that can cause this. High bottle pressure, long durations of pressure against the solenoid, extremely heat soaked coils as well as low voltage.
1. As far as voltage goes you ALWAYS want to power your solenoids thru a relay that is supplied with a good battery power source. Most nitrous solenoids are pressure tested to operate at 12 volts. If your having issues with solenoid activation, testing voltage output on your relay can help solve issues.. Also good grounds are important too.
2. One of the things many don't understand as well is what pressure can do to the plungers in the nitrous solenoids. PRESSURE is what hurts a nitrous solenoid plunger !!! NOT a progressive controller !! ANY time your opening your nitrous bottles valve and or an inline 1/4 turn style valve you should at first SLOWLY crack the valve to allow pressure to steadily put pressure on the plunger, not SLAM into the plunger. Remember, pressure is what holds a plunger down/closed.
High pressure is a no no against the solenoid/plunger. The higher the bottle pressure is the harder it forces the plunger into the solenoids orifice or sealing seat. If you over pressurize the solenoid or leave pressure against the plunger for long periods of time it actually begins to force the center of the plunger out of the main body and nipples up into the center of the orifice. This nipple being forced into the center of the orifice now adds resistance to the lifting motion needed to open against the pressure as well.
3. There are two common styles of plungers (regardless of sealing material in them) used in many of the nitrous solenoids out there. We have been using what is called a "self compensating" plunger ( @ .474 diameter by 1.171 tall) for quite a while. Just recently (@ April of 2012) we made the change to what is refered to as a "pinned" plunger ( @ .474 diameter and overall 1.161 tall). You need to be aware of this when ordering rebuild kits for our's or any other brands solenoids from us. The stem that the plungers go into are also different depths and mixing these up can cause issues..