The distributor in my 1966 3.8S has been rather finnicky lately. When I got the car, the mechanical advance was stuck and I suspect it's again acting up, as the car is having some severe driveability issues.
I have been looking at A Pertronix Electronic Distributor which should eliminate a lot of issues- points being another one. However, there appear to be multiple units that may work.
According to XKS.com:
"Two versions are available: Standard units (part numbers 17-D177600 & 17-D179600) incorporate the original Ignitor ignition and vacuum advance. High performance distributors (part numbers 17-D171600 and 17-D171618) are negative ground only and available with our without vacuum advance. They use the Pertronix Ignitor II micro-processor ignition. Both types are ready for installation with only two wires to connect. They fit all pre-fuel injection six-cylinder Jaguars. The standard units are available in either positive or negative ground; the high performance version is negative ground only."
However, the Summit Racing links for the above-listed part numbers provide some conflicting information:
D179600: positive ground, Hal Effect, Mechanical and Vacuum
D177600: no ground specified, Magnetic, Mechanical only
D171600: negative ground, Hall Effect, Mechanical and Vacuum ("High Performance")
D171618: no ground spec'd, Hall Effect, Mechanical and Vacuum ("High Performance")
Anyone know which one I ought to use? Any idea what's "High Performance" about the last two?
Thanks for the input.
In reply to volvoclearinghouse :
Is your Jag negative ground? I'm not too familiar with British cars, but I seem to recall some were. Once you figure that out, does the stock dizzy use both mechanical & vacuum advance, or just mechanical? I don't think you necessarily need to keep it the same, but it's something to consider.
In reply to volvoclearinghouse :
The firing of your ignition depends on the mechanical advance built into your distributor. It doesn't matter if it's points or electronic. If it's bad with points it will be bad electronically.
Sounds like it's time to pull your distributor. Don't worry it's easy enough. Scribe a mark between the distributor and the block. Now pop your distributor cap off and with a magic marker make a mark on the distributor where the rotor is pointing. As you remove the distributor the spiral cut gears will rotate the distributor and where it winds up is the starting point so make a double mark there.
Now take the distributor apart, it's really simple and clean and polish everything. Oil it carefully before putting it back in place. Double check by working the advance against the springs.
Ive done it so many times I've forgotten how scary it is the first time but once inside it's really simple.
Worse case box it up. Put all the screws and springs in a little baggie. I'll fix it, clean it grease everything, anything bad I'm sure I've got parts. When all done I'll run it up on my distributor machine and check it out. In all probability the rubber diaphragm in the vacuum advance unit has cracked or torn so you'll need a new one of those. I use XKs unlimited but there are plenty of sources for parts.
But it will cost you, I prefer my payments in the form of wine. So the next time we meet you'll need to buy me a glass of wine.
In reply to frenchyd :
I did all this already. Fitted fresh points, cleaned up the mechanical advance, etc. I've freshened up distributors on my Volvo 122s before. Pretty straightfoward.
The links I sent above were entire, drop-in distributors. My 3.8S has negative ground, and points. It's a 1966. I'm not sure if it was originally negative ground or not, but that's the way it's wired now. Anyway, the replacement distributors are all new, mechanical &/or vacuum advance, electronic module, cap, rotor, etc.
The current distributor has both mechanical and vacuum advance.
Appreciate your offer, (and appreciate wine myself!) but the other issue with distributors I've found is that over time, they do wear out- shafts get loose, and it's then impossible to set good time to them. I appreciate the whole "keep it original" concept and have tried to do that with this car as much as possible, but like the fuel pump issue I was working on (where the original pumps were not even there- gone, replaced with something aftermarket already) this may be something whose solution involves a new, better replacement.
In reply to volvoclearinghouse : Welch and XKs unlimited both offer replacement distributors ( among others) but I doubt yours has reached that point. If they have grab one from a later 4.2 it will slide right in and cost less than dimes on the dollar plus it's electronic.
In fact if your engine is tired grab a good usable one from the 4.2 in a XJ6 and bolt that in place a little more power decent improvement in torque and better fuel mileage
In reply to frenchyd :
Do the 4.2 electronic distributors require an external box or other wiring modifications?
My other concern is the coil. It's mounted right up on the engine and I am not at all certain it hasn't expired. My thoughts involve fitting a new coil, and mounting it on the sidewall of the engine compartment so it doesn't get as hot and shaken up. I'll try that before swapping distributors, in case that solves the problem.
My 3.8 just turned over 100k miles. It's neither tired nor worn.
In reply to volvoclearinghouse : the later years do. The earlier ones were straight forward drop ins. I think things changed a little before they went to fuel rejection
you're right with only 100,000 miles your engine should still be good. Except the distributor! Nobody reads an owners manual. Nobody oils their distributor!
I'm not a keep it original including all the flaws kind of guy. While I object to a Chevy in a Jaguar it's because it's more work to get a swap right than fix problems. Besides a Chevy is like a belly button, everybody's got one.
Now a Jaguar all polished up and running smooth there ya got me!!
In reply to volvoclearinghouse :
Since you presently have both mechanical & vacuum advance I'd take the easy route and choose the negative ground model that also has mechanical + vacuum advance. Other than possibly dialing in weights/springs, or adjusting the vacuum canister - presuming its adjustable - it would be the closest to your stock setup. Since it's not a racecar, it should still be a slight bump in performance without doing anything crazy.
In reply to volvoclearinghouse :
Oh by the way any distributor used in a Jaguar will need to be oiled as well.
In reply to frenchyd :
I'm familiar with distributor oiling- the Volvo distributors need oiled periodically as well. (Not to mention the generator in my Corvair) But, like you said...no telling how the owners for the previous 99,927 miles treated the car. 
The distributor listings on Summit are a bit confusing...one says it comes in positive ground or negative ground in the description, but in the item details it only says positive ground. The XKS website for the same distributors simply says they fit all older Jags.
As I ought to have done in the beginning, I went to the Pertronix website.
BMC 6 CYLINDER ENGINE
45D style vacuum advance distributor 12V Neg Top Exit 79 D177600 D171618
45D style vacuum advance distributor 12V Pos Top Exit 79 D179600
45D style non-vac. distributor 12V Neg Top Exit 79, 121 D177628 D171600
Looks like the D177600 or the D171618 is my dizzy, fo' shizzy. And they describe the differences between the "standard" distributors and the Hi Performance ones, too.
