The 1970 spec cars use the 41322 distributor with the vacuum retard capsule which uses a manifold rather than ported vacuum source. So, the “static timing” value of 10 degrees BTDC is never actually achieved at idle, because the retard capsule retards by 10 degrees for vacuum levels of 8 inches of mercury and above. So, at idle, the timing is actually close to zero degrees. The mechanical advance spec of the 41322 rises to a nominal 25 crankshaft degrees BTDC at 4,000rpm, which when added to the 10 degrees static gives 35 degrees. I would therefore consider the 10 degrees BTDC static timing of the 70 cars to be an aberration associated with the retard capsule, and ignore it for the purposes of installing a different distributor such as the 123.
I actually have 2SU carburetors with vacuum advance connected to the 123distributor.
You two are making my head hurt.
Respectfully disagree with “very little difference below 3000” statement, Terry.
For a stock emission 4.2 engine (not cammed or ported) that’s where all the gain is to be had, (imho). We drive the cars in the 2000 to 4000 because of the torque! At 2400 its a significant 5 degres (S1 versus early S2) difference. My personal car has 12 deg. more than S-2 at 2400.
I guess it depends how and what rpm one is running at. I really don’t care what happens at 4500. Very hard to hear detonation at high rpm. I rarely go there with a stock engine. I want as much advance as I can add in and around the torque peak of 2800-3000 w/o detonation. I can hear that ping. It made all the difference in mine.
Its easy to change a Mallory to add “all in” timing with the weight stops. How much would you add on a bone stock engine now running 36 total from 3000 on up? I all ready have the softest spring combo available.
Dave
Hi Bill these curves are quite different from the stock set up, including the pollution control one. Assuming timing set to 10 degrees BTDC the stock distributor is:
900 rpm 15
1300 20
1700 26
2600 30
3000 32
3000+ no further advance
Pollution dist
1200 15
1600 22
2900 31
4400 39
4400+ no further advance
This is all mechanical advance. Is there also vacuum advance on your 123?
The curves you show for the 123 are retarded mid range, perhaps beyond what is needed. Mid range (2000 -3000 or so) is the period of greatest danger to the XK engine - it’s in this range that it’s most prone to detonation. The stock settings work well in this range, but may not be available to you, so you are running retarded, but that’s better than being too advanced.
George recommends curve 5- that one worries me with 36 at 3000 - that’s too much IMO just coming out of the detonation zone.
If you look at curve C you gain 21 degrees with 1900 rpm gain so if the gain in advance is linear you are at 31 degrees at 3000 which is pretty close to stock and safe IMO. Forty two degrees advance at 3900 rpm should be ok. Curve 9 would, if linear, give you 33 degrees at 3000 which would also work, but drop total advance to 40 degrees which would be safer.
I agree with you David - the more advanced the more power, and if you can get away with that much advance at that rpm go for it. That hasn’t been my experience.
Yeah, you’re right. I didn’t much like any of them.
Terry, after 50 years, every engine is different hey? Skimmed heads, leaky guides, worn rings, tight rings, sunken valves etc. Every engine a different kettle of fish with respect to it’s detonation threshold.
Unwise to make a blanket timing recommendation. Every manufacture subtracts timing to protect warranty claims based on their QC level. That keeps us tinkerers interested!
In your dyno time, did you ever increase high rpm advance on a stock motor? I’m still interested in your opinion on the 3000 rpm timing setting for best performance? Is it in the 40’s?
Brave man. Honest, too…
David I think the problem is like a school teacher with a couple of lesser lights as students - the lesson plan has to take them into account. I agree that all engines are different, and more importantly gasoline in different areas is different. My worry about mid range advance comes down to part throttle acceleration in mid range situations when the vacuum advance is still active. I used to frequently visit a friend who lived in Beaverton Or. Access to his place was off Hwy 405 onto Highway 26 which involved a very long uphill climb at 50 - 60 mph. If I was careful and held the pedal absolutely steady in 4th gear it was ok but the slightest bit of acceleration would invoke the most horrendous knocking. This occurred at factory timing. I went so far as to buy a MSD box that permitted remote timing changes from the cockpit so I could retard the timing going up the hill. With high mechanical advance at lower rpm and vacuum advance operating at partial throttle your timing could be quite extreme.
If you are running aggressive timing I would urge to to disconnect the engine breater from the intake system. Crankcase fumes have a tendency to reduce the octane effect - lowering it, and you can have detonation in cylinder 2 and 3 - which are the closest to the breather outlet. Took out the big end bearing on #3 in a car on a hillclimb by forgetting to disconnect it.
When I did my conversion last year, I just reused one of the S2 choke cables, and that seems to work. I agree a solid wire is probably better, but the length of exposed wire is so short there’s not much flex to it. My point is, that part of the conversion can be put off.
When you do it a section of cable from a generic lawn mower throttle cable works perfectly. Cheap and readily available
I just bought the generic choke cable kit from an auto parts store for I believe 7 bucks and the solid core cable solved all my problems.
I think this is the same kit…
A solid core wire will also ensure that the choke is completely off when you close it.
I used one of the old stranded cables on my 2xSU set up, which uses one choke cable. I added a light return spring at the carburetors and it works well.
Yes, I have one Strom choke cable pulling on my SUs. And I have a spare in place just waiting.
Pull should be no problem. Push maybe a problem.
Ha! 
I have a 4.2 ‘68 FHC with twin strombergs. I have a 123 distri. I run it at 10 degrees BTDC and use curve 6. I have plenty of power. Just changed over to 3.07 diff as the 3.54 diff made the acceleration a blast but highway a nightmare following my buddies in 308, 365GTB, ghibli, pantera and corvette split window,
Thinking of switching to triple Weber’s
You’ll need to do more than an extra carb if want to keep up with those guys…