DIY - 123 Distributor Ignition Timing with Carburetors

I'm embarking into new territory regarding the potential of the 123 distributor that I think is being overlooked. I am no engine expert but from my research on the web (dangerous I know), that old school tuners used to set initial timing with a vacuum gauge and/or a "highest idle" procedure. From what I have determined, engines typically like more initial or static advance than the owner's manual suggests (for emissions and/or erring on conservative), all you have to do is twist your distributor counter clockwise and see for yourself just how much. This indicates a more efficient fuel burn at idle. Free power. The racer discussions are in favor of setting the idle advance as high as the engine likes, until adding more no longer results in a higher vacuum or idle rpm, then back off a little to compensate for fuel quality etc. Especially if you have a bigger cam like I do. The issue that always comes up is that if you do that, then total advance (when you add in the mechanical advance spec'd in the distributor) you'll have too much all-in and your engine will ping at best or grenade at worst. However this argument is moot with the 123 because you can control the "mechanical" portion with a computer or iphone.

I have said before that one of the great things about the 123 with Weber sidedrafts is that you can adjust the throttle plates to the perfect position in regards to the first progression hole (a critical adjustment for off-idle performance) by compensating either up or down with the static timing to achieve proper idle rpm. This was not possible back in the day so people compensated by opening up the throttle at idle or putting in less than ideal jetting. I have proven this to myself too many times. So this has led me to the next step down the rabbit hole in that now I am experimanting with an even higher static advance because recently, just for kicks I bumped my initial timing up to where the idle responded to the max which turned out to be about 25 degrees btdc. Every engine is different based on age, compression ratio, cam etc and it will tell you when you've gone too far when the idle or vacuum drops off. I then programmed the distributor curve for 11 more degrees from idle to all in 3000 rpm so total advance is still 36 btdc which is in line with spec. No vacuum advance. The only way to do this with a traditional distributor is to have it physically rebuilt, it can be done any number of times and configurations on the 123 in 5 minutes and of course is reversible. So why not said the curious cat.

The "snap" off idle and initial acceleration took me by surprise, it's amazing. One race builder site says this is how they build their engines - a lot of initial advance and only a little mechanical. I've read that Webers "like" a lot of advance. As does a bigger cam. However my Webers at 25 btdc required the throttles to be closed quite a bit in order to keep the idle at or below 1000rpm and so was hard to get the right smooth idle mixture. So my goal is to see what the point is where I can maximize static advace and still keep the throttle plates in an optimal posiiton for idle quality/AFR and will post my findings here for any others interested (if any :p ).

In the meantime I'd appreciate any opinions or other precautions by heading in this direction. The off-the-line acceleration is most defintiely better than my stock timing specs. Here is the curve I'll be tweaking, I suspect it will end up somewhere between 25 and the 15 initial advance I am using now:

Screen Shot 2019-12-20 at 1.09.11 PM.png
 
Last edited:

autokunst

Well-Known Member
Site Donor $$
Messages
3,037
Reaction score
1,932
Location
Milwaukee, WI
This sounds great Steve. I have nothing to contribute, but will follow along closely and hope to learn from your research! Thank you for creating this thread.
 

Stevehose

Well-Known Member
Site Donor $$
Messages
11,254
Reaction score
3,474
Location
New Orleans, LA
Another point made moot by the 123 is the argument that if you have a lot of initial advance then the car will be hard to start. Advance doesn't kick in until 600rpm so it has no affect on cranking resistance, unlike the typical mechanical distributor.
 

Stevehose

Well-Known Member
Site Donor $$
Messages
11,254
Reaction score
3,474
Location
New Orleans, LA
I don't see why not, and the dual 38's won't be as finicky with the throttle plate alignment. You'd still get the benefit of easy starting, cooler idle temp, and adjusted curve from your 123. I'm writing a step by step now and will include how to do it with stock dual downdraft intakes.

Can the same approach be safely applied to an engine with dual 38 downdrafts?
 
Last edited:

Stevehose

Well-Known Member
Site Donor $$
Messages
11,254
Reaction score
3,474
Location
New Orleans, LA
Another benefit from advanced timing at idle is cooler idle temp. Back in the day they used vacuum to retard the idle timing to increase the heat of the exhaust to pass the smog test.
 
Last edited:

TodB

Well-Known Member
Messages
1,225
Reaction score
309
Location
Saint Augustine, FL
I don't see why not, and the dual 38's won't be as finicky with the throttle plate alignment. You'd still get the benefit of easy starting, cooler idle temp, and adjusted curve from your 123. I'm writing a step by step now and will include how to do it with stock dual downdraft intakes.
Awesome, thanks Steve.
 

Stevehose

Well-Known Member
Site Donor $$
Messages
11,254
Reaction score
3,474
Location
New Orleans, LA
First experiment has been succesful so far. I have determined for now that 20 degrees initial advance is where it is working best for my setup. Here is the process I used which will likely evolve:

For triple Webers:
1. Confirm with a timing light that your idle advance matches what your 123 curve says. If not, manually twist the 123 until it does. I'm assuming your base timing is around 10 or so.

2. Lower your idle adjustment linkage screw until the throttles bottom out and the idle doesn't drop any more. If the engine starts to die then keep it to where it will still barely run and not stall.

3. Inspect your linkage to make sure when you backed it out that the carb with the main throttle rod attached to it is still connected to the other 2 and that there isn't any play anywhere. I had to turn the main idle screw back in because carbs 2 & 3 bottomed out before #1 carb. Check that they all move in unison, this didn't show itself before when I had the idle screw in for higher idle speed. If you have the crossbar style linkage, you'll have check for slop also, I have no experience with that setup. Inspect all areas of linkage for no binding anywhere.

4. Turn the idle screw in so that it touches the linkage and maybe another 1/8-1/4 turn to be sure all carbs are just off bottomed out.

5. Do a quick synch of the carbs for airflow check. Doesn't have to be perfect. Afterwards, lower the idle back down to just above stalling if needed.

6. Twist the 123 counter-clockwise slowly and observe the idle increase. Keep going until the idle doesn't increase or starts to decrease/run poorly. Back it off a little and note the timing with the gun.

7. Alternatively you can use the "Tune" function of the 123 software to bump it up however the bluetooth version only allows +10, the computer version allows more. Note the point where the increase ends or engine stumbes.

8. If it's more than 25 degrees btdc, I'd set it to 25 for now.

9. Synch the barrels via the air bypass screws if you have them. Keep the higher airflow barrel screw shut and open the lower airflow barrel screw until it matches the other. Only one screw is used per carb. Then synch the carbs to the linkage.

10. Adjust idle mixture screws for best idle. Check synchs again and adjust if necessesary.

11. With engine off, unscrew the progresson hole covers and shine a light to see the relation of the throttle plate and the first progression hole. Ideally it should cover the first hole with the back edge of the throttle plate uncovering it at the slightest movement of the throttle. A tiny amount of space is acceptable. Adjust if necessary. All 6 throttle plates should match their respective progression holes. If not then you have a bent throttle shaft or something but don't sweat it as long as the carbs are drawing close to the same amount of air. If you have capped holes with no screw covers (DCOM - I tapped these out and added covers) don't worry about it for now.

12. Ok so what is the car idling at? If very low then you'll need to increase the idle via the linkage screw but preferably not because this will alter step #11 but these are old tech so nothing is perfect, if too high then you'll have to back down some timing. Hopefully you won't have to do either very much. Anywhere between 900-1000rpm is the target, preferably set where the engine runs smoothly with minimal change to step #11. Might be closer to 1000 for triples. Check synch and idle mixture again. Idle mixture is always the last adjustment.

13. My carbs pull 4 kg/H on the Synchronmeter at 950 rpm. I'm curious as to what other's pull.

14. Check your timing with the light and use this as your base timing in the software. I have mine starting at 20 degrees at 600rpm untill 1100 rpm. This will prevent any timing changes and idle fluctuations under real world conditions like temperature, humidity, shitty gas etc. from changing the idle speed and timing. At 1200 I have a straight line to 36 degrees at 3000. You can experiment with what rpm you go all in on, I have a Schrick 284 cam so I've gone in at 3k, maybe your engine responds better at all in at 3.5k or something else. Err on the conservative side in all of this. My graph is shown below. Snap the linkage at idle to see how the carbs accelerate. Take it for a test drive and check out the off-idle acceleration. Listen for any pinging under hard acceleration. Back off timing accordingly but I suspect if you don't go more than 36 degrees all in you'll be ok unless you have high compression. Most importantly listen to YOUR engine, not mine or anyone else's. That will dictate your final adjustments.

15. If your carbs have adjustable accelerator pumps (DCOM, Solex, Delorto) then you may need to adjust the off-idle squirt sensitivity if you've lowered the idle speed. If you get a pop when shifting or accelerating from a steady speed this is likely the cause.

16. I have manifold and ported vacuum take-off ports so I may experiment with a cruise advance of another 8 degrees or so but that's later. Not sure this would have any affect on improving fuel economy with sidedrafts. No vacuum advance at present and should not be used while doing these experiments.

For twin Zeniths/Weber carbs:
All of the above applies except for the throttle plate/progression hole adjustments, because you have the stock manifolds this adjustment isn't as critical. Obviously the barrel/idle air adjustment is not applicable also but your carbs should be working properly regarding the secondary opening etc. Your idle could be more in the 900 range depending on if you use your a/c in which case maybe a little higher. Do not use any vacuum advance or retard for this while doing this experiment. Vacuum advance can be added after for more advance at cruise and low load scenarios but vacuum retard should be eliminated under all circumstances whether you do this experiment or not.

I definitely have better off-idle throttle response and it also show up when blipping the throttle for downshifts. YMMV. More to follow.

Curve to this point:

Screen Shot 2019-12-21 at 9.08.40 AM.png
 
Last edited:

Stevehose

Well-Known Member
Site Donor $$
Messages
11,254
Reaction score
3,474
Location
New Orleans, LA
I believe another advantage to higher idle advance could be less idle drop when the a/c compressor is turned on, due to more power at idle.
 

Ohmess

I wanna DRIVE!
Site Donor
Messages
3,833
Reaction score
1,590
Location
Aiken, SC
Can't wait to follow along with this procedure using my car, but unfortunately I have not yet repaired the small crack that has developed in my exhaust system.
 

Willem Tell

Well-Known Member
Site Donor $
Messages
334
Reaction score
300
Location
Zug, Switzerland
Hmmmm.... Interesting; great info..... I wonder if I can apply this to my fixed-program 123 with D-Jet system? My 123 program is pre-configured, so I could only twist the dizzy and observe the performance change.
I'll have to research,, but since the D-Jet is a non-feedback system, I think the timing change would not affect the D-Jet operation, only the fuel burning aspect.
I love the BlueTooth 123 on my Tii, but this fixed-program is what came with my M30 motor.
 
Last edited:

Stevehose

Well-Known Member
Site Donor $$
Messages
11,254
Reaction score
3,474
Location
New Orleans, LA
I don't believe it will work because you can't adjust your all in timing on the fixed model, so if you advance your idle time it will affect the all in time, possibly to a dangerous level.

Hmmmm.... Interesting; great info..... I wonder if I can apply this to my fixed-program 123 with D-Jet system? My 123 program is pre-configured, so I could only twist the dizzy and observe the performance change.
I'll have to research,, but since the D-Jet is a non-feedback system, I think the timing change would not affect the D-Jet operation, only the fuel burning aspect.
I love the BlueTooth 123 on my Tii, but this fixed-program is what came with my M30 motor.
 

Ohmess

I wanna DRIVE!
Site Donor
Messages
3,833
Reaction score
1,590
Location
Aiken, SC
The "preconfigured" distributors appear to have a number of alternative curves according to this:


A couple of these curves have different all in advance, so I wonder if Willem might be able to use one of those? For example, if Willem is using curve number 2, which has 36 degrees of all in advance, it would appear that he could change to curve number 6, which has only 30 degrees of all in advance, and then dial in 6 degrees by rotating the distributor. This 6 degrees would be available throughout the entire curve, even when first starting the car, the rest of the curve would not kick in until 1000 rpm and vacuum advance would operate differently, but total advance would remain at 36 degrees.
 

Stevehose

Well-Known Member
Site Donor $$
Messages
11,254
Reaction score
3,474
Location
New Orleans, LA
If that's the case then yes he could use that and add 6 to the idle advance, I'd like to see those in graph form.

The "preconfigured" distributors appear to have a number of alternative curves according to this:


A couple of these curves have different all in advance, so I wonder if Willem might be able to use one of those? For example, if Willem is using curve number 2, which has 36 degrees of all in advance, it would appear that he could change to curve number 6, which has only 30 degrees of all in advance, and then dial in 6 degrees by rotating the distributor. This 6 degrees would be available throughout the entire curve, even when first starting the car, the rest of the curve would not kick in until 1000 rpm and vacuum advance would operate differently, but total advance would remain at 36 degrees.
 
Top