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Initial Timing

3K views 18 replies 12 participants last post by  rickgto 
#1 ·
Looking for advice as to what I should set initial timing to for a good clean burn.



'68 302 with mild cam. Shelby intake and an Edelbrock 1406. Single diaphragm distributor with points. Both vacuum advance and mechanical advance are in specs


I am thinking 10 BTDC...


Thoughts? Thanks.


Dave
 
#2 ·
My setup:
1968 J-code 302 stock
1403 Edelbrock (500 CFM)
Stock distributor with Pertronix ignitor1 + flamethrower coil
Idle at ~800RPM
12* BTDC initial
38* "total" (= initial + mechanical advance with vac advance disconnected) all in at ~3500RPM
15* max vacuum advance, hooked to manifold vacuum

Rough AFRs:
Idle: ~13.5 - 14
WOT: ~11.5 - 12.5
Moderate acceleration: ~13.5 - 14.5
Cruise: ~14.5 - 15
 
#3 ·
Initial, total and rate of advance are all important. Usually rate is to have it advance totally by 2800 RPM. Total could be anywhere from 34 to 45 degrees. Initial could be up to 15 degrees for your setup, more for some. Trial and error and testing will pay big dividends in power gains.
 
#8 ·
If you want an easy fits all, advance it for best WOT performance and leave it be. Initial will fall where it will. If you do the opposite, (high initial without a limit) you're looking to destroy your engine. If you want performance at all levels, it takes a bit more than simply setting initial timing. SB Fords depending on build (compression cam etc) will run anywhere from 32 to over 40 degees total timing. They also like anywhere from 0 to 20+ degrees initial depending on dynamic cr, cam, head efficiency, etc. So, here's the backyard guide to max performance. Concentrate on WOT first. Remove the advance springs. Move the distributor until you get (what you feel or can verify) is the best performance at wide open throttle short of detonation. And I mean 3000 rpm plus not lugging the engine. Record it. Lets say it's 38 BTDC. Pull the cap and hardwire the advance so it can't advance. By that I mean replace the advance springs with wire to hold the weights from moving. Advance/retard for highest vac (assuming idle a/f is spot on) and verify timing. Subtract 2 degrees. Let's say (example) it liked 16 -2 is 14. So, it wants 38 total, and 14 initial. Send your dist out to be curved for 24 mechanical advance to a reputable shop that KNOWS mechanical distributors and isn't guessing with a Mr Gasket recurve kit. The number one overlooked performance piece to DATE is tailoring a distributor to fit the engine. BTW, initial means squat for performance. Initial is for a clean idle. Rate of and total are for performance.
 
#9 · (Edited)
#11 ·
Also, since you have the 1406, I just went through this, and posted it: https://forums.vintage-mustang.com/...1602-tuning-edelbrock-1406-success-story.html You can get the rods and jets from amazon. The tuning manual will tell you which ones. Also get the 1405 tuning kit and use the orange spring from there. For a starting point. Save yourself some headache.

Th 1406 is a CARB friendly carburetor. Not at all a true performance carb. You can rejet until your blue and make very little difference. The internal passages are restricted negating jetting changes. Trust me on that. If you want a performance Edelbrock 600 with electric choke you buy the 1405 and the electric choke kit. 1406 will work on a mild engine with rod and spring change but once cam specs get around 230 @ .050 it is not the carb you want. Mild daily driver? Kick *** carb.
 
#10 ·
You set the total advance to a point where the engine doesn't ping which is usually around 36* or 38* and your initial advance will be that number less whatever mechanical advance your distributor has built in. As long as the engine doesn't try to kick back when starting it you will be good to go, even if your initial advance ends up at 16* or 18*.
 
#13 ·
“The 1406 is a CARB friendly carburetor. Not at all a true performance carb. You can rejet until your blue and make very little difference. The internal passages are restricted negating jetting changes.”

The 1405 and 1406 bodies are exactly the same. Inside and out.
Only differences are the rods and jet sizes.
A 1406 can be tuned to perform just as good as a 1405.
 
#14 ·
The 1405 and 1406 bodies are exactly the same. Inside and out.
Only differences are the rods and jet sizes.
A 1406 can be tuned to perform just as good as a 1405.
That is my understanding as well. Moreover, with the exception of the venturi boosters and the accel pump nozzle size the 1403 500CFM carb is the same as the 1406 600CFM carb. My understanding is the carb body for all these is the same. I had a 1406 on my car and swapped out the boosters from a 1403 "parts carb" I had. Tuned it with the appropriate jets/rods (per the manual). Works better on a stock 302 engine than the 1406 although a 1406 is fine as well... it just seems that the 1403 has a wider "sweet spot" (and a slightly better throttle response) when tuning on a stock 302 engine.
 
#15 ·
“The 1406 is a CARB friendly carburetor. Not at all a true performance carb. You can rejet until your blue and make very little difference. The internal passages are restricted negating jetting changes.”

The 1405 and 1406 bodies are exactly the same. Inside and out.
Only differences are the rods and jet sizes.
A 1406 can be tuned to perform just as good as a 1405.

Yep. Carb bodies and appearance are the same. Inside is a different story. By your thinking/statement a Quadrajet is a Quadrajet and jetting applies across the board. Good luck with that. Do me a favor and look up calibration kits for the 1405 and 1406. Different kits that cover each carbs capability. Maybe Edelbrock screwed up? You'd think the kit would be interchangeable being the carbs are identical inside and out.
 
#16 · (Edited)
Yep. Carb bodies and appearance are the same. Inside is a different story.
To your point. From the discussion link below it appears specifically that the venturi booster assemblies (both primaries and secondaries) for the 1405 and 1406 are different.

https://www.forabodiesonly.com/mopar/threads/another-1405-vs-1406-thread.345986/

Q: Just curious, do you know whether the primary booster assembly has an effect on the idle circuit? My impression from looking at the Edelbrock owners manual is that the idle circuit (3,4,5,6 in figure 2) is part of the "venturi booster assembly" but I'm not sure. Thanks.
 

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#17 ·
If stock dist curve start with a stock initial. Me? I don’t care; I like 36 @ 3,000 rpm and let initial fall where it will; I recurved my body. A tac to get idle @ 600 or so and a vac gauge to touch up the carb and you should be good. Oh, I run points so I also check dwell.
 
#19 ·
Think what you want but the electric choke carbs (economy tuning) are far different than the performance version internally. A letter bit set and some knowledge can make a 1406 respond like a 1405 but simple jet/rod changes will not. Absolutely an afr gauge is going to show changes no matter...…. but...……...the 1406 in stock form CANNOT be calibrated to match the 1405's calibration by simply using 1405 specs. The different kits are due to the limitations of each carb respectively. Idle pickup tubes, idle air bleeds, down channel restrictions, main air bleeds......are NOT the same between the 2. Biggest complaint with the 1406 is poor idle and off idle with a big cam. 1405 squeaks by. Why? 1405 has adequate internal passages to support (low idle vac velocity) with other than stock cams. 1406 doesn't. Contrary to backyard tuners a large overlap cam needs more air, not more fuel, to idle cleanly. Low vac means low intake velocity which means fuel droplets vs atomized by more air. The 1406 in stock form has larger idle air bleeds, smaller idle pickup tubes, smaller idle/transfer restrictions, and larger main air bleeds than the 1405. Lean by design to meet CA use and LIMITED in performance use. Notice NOWHERE on the chart to calibrate the 1406 does Edelbrock recommend the stock 1405 calibration? The only calibration for the 1406 that comes close to the stock .100/ .070-.047 primary main jet/rod combo the 1405 has uses a MUCH larger rod to offset the main air bleeds in the 1406 to a usable a/f range. The reason for the different kits is the carbs capability/intended usage in stock form. If all was same between the carbs internally, the kit would be interchangeable. Same rods, same jets to produce same calibration. Lot's of mis-info out there about what it takes to tune a carb correctly and the biggest fault is assuming they're all the same other than swapping out jets and rods.
 
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