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The vacuum advance module of my original Lucas distributor failed after 15 years of service. As a result, my 3.9 ran poorly (and threw an idle air control fault constantly). The 1994 models also had the amplifier mounted on the side of the distributor, exposed to heat and as a result, prone to failure (even more so than normal for a Lucas bit!). Land Rover moved the amplifier next to the coil for 1995 and sells a (very expensive) relocation kit for the '94s. Given that my D90 never got the relocation, a quick cost-benefit analysis tipped the scale to favor replacing the distributor entirely.
I chose to use the Mallory Unilite distributor that is a popular upgrade for the Rover V8. You can get a Mallory kit from at least one Land Rover spares source for about $600. Or you can source all of the parts (and more) on your own for half of that. I'd also recommend getting a spare cap and rotor now - it's not something that is available at your local parts store and I broke a cap in the process and had to stall the installation until spares arrived. You'll need it eventually, anyway.
The Mallory Unilite vs. the original Lucas distributor
Start by marking the position of the original distributor on the block and rotor position on the distributor (just in case). Note where the rotor position is with regard to degrees... The Mallory instructions suggest finding TDC, which isn't a bad idea, but isn't necessary so long as you get the new rotor in the same position. Remove the Lucas distributor and punch out the roll pin securing the drive gear to the main shaft. Next, note the orientation of the slot on the bottom of the drive gear against the rotor arm, then match the orientation on the Mallory distributor (mine was exactly perpendicular to the rotor arm). Doing this will ensure that it fits back into the motor without any adjustments (and takes less time than walking to the toolbox and getting a long flathead screwdriver). Follow the Mallory instructions for the drive gear installation (this is where you'll need a feeler gauge and a good 3/16" drill bit). Be sure to clean the drive gear and shaft to get all of the metal shavings off. When you're done, drop the new distributor into the engine and align it to (as closely as possible) the same position as the old one came out. Reinstall the retainer, but don't crank it down yet - you will need to check/adjust the timing once you have the engine running.
Installed in the Land Rover 3.9L
With the new distributor installed, it's time to move onto the electrics. Follow the wiring diagram in the Mallory instructions for this step - there is a version for using the resistor. I also used a power filter, which was cheap and can't hurt (it protects the distributor amp from power fluctuations from crappy electrics... Lucas = crappy in my book, so power filter it is!).
I chose to keep the stock wiring intact and simply run new wires for the new pieces. This will make it easy to revert to the original distributor should I need to do so for any reason. I also chose to mount the resistor and filter next to the coil for a cleaner look. Since the new filter was intended to be mounted on the distributor (a feat that seems impossible on the Rover V8), I used a connector that is compatible with what came on the Mallory (3 prong AMP) and made a plug-and-play ~1 foot extension, then mounted the filter to a piece of aluminum mounted off of the coil bracket. Be sure to keep the +12V ignition switch feed accessible, but aside from that one piece, you won't need to use the factory distributor wiring. I also used various connectors on my wiring as not to cut anything off of the factory wiring harness in any way.
Resistor, OEM coil (Bosch), and Mallory power filter
Custom wire extension (wrapped) with proper 3-prong AMP connector.
After a test to make sure you're getting spark at the cap, tidy up the wiring with some ties and wrap, then attach the leads to the cap.
Leads connected and ready for a timing check.
If you did a good job of aligning the distributor, then the car should at least start. If not, solicit a helper and adjust the timing while the helper cranks. If there is something fatally wrong, move the engine position to TDC on cylinder #1 and then re-orient the rotor; odds are you have the drive gear a tooth off, which is easy to do since the distributor is fundamentally different now. Once you have the motor running, check/adjust the timing per the shop manual. At this point, you'll need to address the vacuum advance before you reconnect the vacuum line. Mallory apparently sets their vacuum units to maximum advance. Get a 3/32" allen wrench, stick it into the diaphram, and turn it counterclockwise all of the way. This will essentially disable the vacuum advance for the time being, which I have found yields satisfactory results. If you don't crank the vacuum advance down, your engine will have horrendous pinking under load. And finally, if you've messed with the base idle like I did, adjust that too and clear the Check Engine light :)
Now go for a test drive and particularly check the performance under load. You can gradually increase the vacuum advance if you wish. If performance is poor or if you're experiencing pinging (particularly under load or at higher RPMs), it's time for trial and error with the re-curve kit.