Distributor advance curve with SU versus Stromberg carbs


(Geo Hahn 1969 Series 2 OTS) #18

There is a 5 digit number stamped on the distributor body (possibly begins with a ‘4’) that tells what the unit was made for and, if it is as original, details of the curve and vacuum.


(Dennismo) #19

I found a Distributor number (tough with it in place)

A0801A

Not sure if it makes sense


(Geo Hahn 1969 Series 2 OTS) #20

Could be 40801A - though that seems unlikely as that is a 4 cylinder distributor. Still anything is possible as a distributor can be build up from assorted parts.

Here’s an example of a number (my S2):

Number below is date of manufacture,


(David Langley) #21

The “correct” distributor for the 4.2 with SUs is the 41060.

-David


(Philip Lochner) #22

If you could imagine your car having just 1/2" of throttle travel? 1/4" is 50% and 1/2" is full throttle. Can you see that it would be rather difficult to modulate the throttle for a controlled drive? More so when the road is a bit bumpy?

OF course the above is exaggerating the situation but in the case I’m referring to, very little throttle produced substantial and rapid changes in rpm / power delivery, so much so that I knew the owner (a gentleman who prefers to cruise about lazily) would not enjoy the situation.

In the graphs I posted, you might see a bit of an upwards kink in the rising slope of the centrifugal curve. That was intended to bring in the centrifugal advance gradually up to 2000rpm (for a slightly more docile character) but then from 2000 to 3000, bring in all the advance that it wanted for max performance.The “racy” version had, IIRC, 30 degs at 2000 already, ie the kink was the other way round.


(Geo Hahn 1969 Series 2 OTS) #23

Something of a personal choice perhaps, I don’t usually “cruise about lazily” (though I am older and possibly a gentlemen) so my throttle response is fairly abrupt. The curve looks something like this (stock vs what I did):


(Philip Lochner) #24

Yep, that steep curve before 2000rpm is what gives it that sharp character.


(Dennismo) #25

Does the advance curve you customized in the graph above include about 10 degrees of static advance?
Or is the approximate 38 degrees of advance due solely to the weights being centrifugally flung out against the stop that limits them (in your case the limit looks like it is 38 degrees crankshaft ( 19 degrees in the distributor.)
The reason I ask this is that i am buying weights and springs from the distributor doctor in England and he has advised me to add metal to the stop that limits the advance in the distributor (called the Pecker) and limit the distributor advance to 10 degrees (20 degrees at the crankshaft).
There is a big difference between your 38 degrees crank advance and his 20 degree crank advance
Thanks


(Geo Hahn 1969 Series 2 OTS) #26

The oriiginal distributor for my Series 2 had a centrifugal limit of 20° (distributor) and I left that unchanged:

The static timing for that car is 5° and again I did not deviate from that:

The curve shown in my graph does not include static advance (the red line was what we got on the Sun machine which does not know about the static advance).

I am no distributor expert (far from it) and operated fairly unscientifically. I simply fitted a lighter spring for the thin spring in the works to get more advance early on. I liked how the engine responded, then when I had a chance to use the Sun machine I was able to see what I did.

The engine does not ping/knock with premium fuel and with 25,000 miles since the tweak seems okay with it.

Hopefully some with expertise will weigh in – but 20° total centrifugal advance sounds low to me. Of course you may be using a 10° static advance and perhaps your distributor has a vacuum advance (mine does not) so those numbers have to be considered as well…


(David Langley) #27

Geo,

I assume that, based on his comments, Dennis would have 10 degrees static advance plus 20 degrees (10 at the distributor, 20 at the crankshaft) centrifugal advance giving him a total of 30 degrees. He also has vacuum advance (which you and I don’t), so under light load I expect he will be getting 40 degrees or a little more total advance. The 41207 distributors that you and I have have an unusually large centrifugal advance of 40 degrees (crankshaft). The 41060 used on the SU-equipped 4.2s had only 19 degrees, whilst the 3.8s which used the 40617 distributor had 24 degrees. Both had 10 degrees static advance, and also vacuum advance. I presume that the advice he is getting to limit the crankshaft advance to 20 degrees (plus static, probably of 10 degrees) is based on the 41060 numbers, and is probably reasonable, particularly as he also has vacuum advance. When I recently measured my timing (using the 41207) in an attempt to understand the “exhaust chuffing” at idle, backfires on overrun, and surging at low throttle openings I found that I had 15 degrees static and 52 degrees at 2900RPM! I’m surprised it was running so well! I guess that the distributor clamp had slipped. As my distributor springs appear to be weak, resulting in a very steep advance curve, I’ve settled for 0 degrees static with it topping out at 40 degrees by 2900rpm. Hopefully I will have got my new Bluetooth 123 installed and tuned in time for the GCOL, with the Lucas as an emergency backup.

-David


(Dennismo) #28

Are the two advance curves you posted such that the curves show the advance in addition to the distributor 10 BTDC setting?

Therefore with red line graph above, at 2000 rpm, the actual crankshaft position is minus 10 degrees BTDC plus 25 degrees distributor advance equals 15 degrees on the crankshaft???

Dennis


(Geo Hahn 1969 Series 2 OTS) #29

Yes, but in my case it is a static 5° BTDC (some years used 10°, not mine).

Actually would be about 25° centrifugal plus 5° static for a total advance of a bit less than 30° at 2500 RPM (about 62 mph).


(D Barnes) #30

Assuming I have a stock 41207 distributor but with Pertronix on my 68 with Stombergs does anyone know what springs would be best to play around with at the beginning ? With my limited experience I would like to play around and perhaps learn more about advance curve with springs before diving into bigger changes.

David
68 E-type FHC


(David Ahlers) #31

Guys - I just want to make sure we’re all on the same “advance page”. George’s 20 deg. (dist. adv) advance delivers 40 degrees at the crankshaft. So if you set the initial or static crank advance at spec of 5 deg. You get 5 + 40deg or a total “all in” of 45 degrees. Remember the distributor turns at half the speed of the crank, so mechanical adv. is times 2 at the crank,

All Lucas mechanical advances have a number stamped in. It indicates the total mechanical advance. Dennis’s supplier in England is saying is 40 deg + initial is too much. Adding metal to the stops limits how far the weights go out reducing advance.

The XK engine is a torquer. To be responsive it wants as much advance without detonating at lower rpms.
The 69 advance curve was a first attempt at emission control. Jag gave up performance for lower CO & NOx. Slow advance = slow acceleration. They changed it after a year.

I limit my advance to 36 degs all in. Getting the mechanical advance in as quick as I can with soft springs.
10 initial + 13 (dist. mech) or 26 crank totals 36 all in. You can’t hear detonation at 45-5000 rpm. So be real careful going over 40 degrees all in. We’re not running 100 octane leaded gas like we had in 1969.
dave


(Dennismo) #32

I went to a local shop here in Vancouver and have gotten their Sun 500 distributor tester repaired. It needs a 1.35Volt Mercury cell! for the rpm measuring meter!

When it is running I will put my distributor on it and check the existing mechanical advance and report.

I will also put a friend’s 4.2 litre E-type’s distributor on it and see what it shows for mechanical advance.

When I get the new springs and weights from the distributor Doctor, I will do the same and report!

Does anyone have a printout of what the actual mechanical advance should be for a 4.2 litre series 1?

Thanks

Dennis


(Geo Hahn 1969 Series 2 OTS) #33

I saved this from somewhere (yeah, how’s that for a document trail?) –

I also copied this from a manual - not sure which one but it probably had others, I just saved the one for my 41207:


(Philip Lochner) #34

That it keeps on advancing even more beyond 3000 as much as another 10 degrees is VERY interestng!! It kinda confirms my view that further advance at high rpms might be a good thing due to reduced cylinder filling (and thus lower cylinder pressures at bottom tdc). I just did not know what “high rpms” that would be.

Another thing to go try out on the 123 BT distributor I installed for a friend.


(Dennismo) #35

I took the distributor out of my 4.2 litre with triple SUs with the following outcome. The distributor is a 41060 A which is the correct one for the 4.2 engine with triple SUs

I looked inside it and the MAXIMUM advance (as stamped on the Pecker) is 9 degrees (distributor speed) or 18 degrees crankshaft. This happens to be what the distributor Doctor in the UK says it should be. My weights are 17 grams each

I put it on a distributor machine and measured the advance at various rpm. Below is the result (already multiplied by two to get crankshaft advance). Note that this does not include the static setting which would add say about - 10 degrees

So the maximum crankshaft advance I can get is 30 degrees including 10 degrees static.
Yet, my performance tapers off at about 3000 rpm compared to a friend’s E-Type. His pulls pretty evenly to the redline while mine seems to lose the pull it had below 3000 rpm

Should I consider tinkering with it to get better performance? And if so, what do you suggest?

1000 rpm 0 degrees
1500 rpm - 4 degrees
2000 rpm -6 degrees
2500 rpm -6.5 degrees
3000 rpm -14 degrees
3500 rpm -16 degrees
4000 rpm -18 degrees
4500 rpm - 20 degrees
5000 rpm - 20 degrees


(Paul Wigton) #36

20 degrees is too little advance at 3500-up.

I ran ~36 degrees from 3500 rpm up.

Note: I live at 1600 meters, so my numbers may be too high at lower altitudes.


(Dennismo) #37

Sorry to mislead
The crankshaft figures are for the distributor only.
You need to add 10 degrees static to that so the maximum advance is 30 degrees