Distributor Advance Curve/Initial Timing

[Jdgephar]

Hey Everyone,

Since I upgraded my old points distributor to Duraspark, I've been doing some reading on the timing curves. Now that it's starting to get warm enough out to take the Bronco on some test runs, I've found that the new distributor timing doesn't work so well.

Here is what I have found. I started at 6* timing at idle. Winding up the engine brought me up to about 26* total advance. I'm not sure at what RPM, but total is somewhere close to 3000 RPM. I didn't have a helper to check the RPM. I've opened up the distributor to confirm the 10L advance curve, so I know the total mechanical advance is correct.

On the road test drive shows performance drop significantly after 3000 RPM. I then bumped the timing up to 15* at idle, for a total of 35*. Much better, but still some hesitation at RPMs above 3000.

I feel that it's still not enough timing at higher RPMs, but I'm not sure how high I can safely go. I haven't heard any pinging yet. So, this evening I took apart the distributor and changed the advance to the 15L arm. Now I'll get 30* mechanical advance plus the initial. My plan is to start at 8* initial, for a total of 38*, and keep the initial timing lower for easier starts. I'll have to test it out tomorrow, if it doesn't snow again tonight.

A couple other notes. Vacuum advance line is pulled and plugged for this testing. 302 engine, has a mild cam (wish I knew the specs), stock heads for a 70s 302.

Has anyone else here done work on the recurve of the dizzy? Any advice on the settings to try out? I really wish I did more research on this before I removed the old dizzy and sold it. I'm thinking it just needs more advance to run well at the higher RPMs like it used to. I'm assuming that the old dizzy was also on the 10L arm. If it was, that might explain why I thought the initial timing was way too high (over 20*). But that was the sweet spot where it really liked to run with that dizzy, which I'm guessing was close to 40* total. The old dizzy was real picky about running well through the lower RPMs and not pinging under load.

If anything, I'll just keep tweaking and testing. But that's why we all own one of these, right?

 

[broncnaz]

Most likely your old dizzy was at a higher setting. There are several different dist timing slots thru the years or application.
38 degree could be to much I really wouldnt want much more than 34-36 total.
Usually you can run 10-12 degrees intial and the 20 -24 mechanical and you'll be good. The main part of recurve is the springs. I got a spring kit for mine and got rid of the heavy spring. I ran a stock light spring and one of th lighter springs out of the kit this got all the mechanical advance in by 3000 RPM
During your driving tests Id hook the vacuum advance back up.
Also your hesitation may be due to fuel issues timing is more or less a non factor after 3000 RPM.
While you do want all the timing you can get without pinging generally anything over 36 degrees is to much but 38 may work for you. Especailly if your still runing the stock springs as you may not even be getting full mechanical advance until 4000 RPM.

 

[jckkys]

Most of these engines will run well with a lot more initial timing than the factory specs. I run my 351 with 21 degrees and most small blocks will take 16. Until you get pre-ignition with a warm engine you can advance it more. My '87 351 has 8.7 to 1 compression while stock Bronco 302s have 8.3 or 8.0 to 1 after '72. The 10 degree centrifugal advance limiting slots are preferable when you have full initial advance. You need a tach to find the best curve for your engine in your truck. Cheap engine analyzers taped to the dash will tell you at what RPM pre-ignition occurs. Changing advance springs and the preload give you the best curve. I have a Sun distributor machine so the final set up is easier for me, but it's not necessary.

 

[73azbronco]

Rest of the story? Tranny, tire size, axle ratio?

If you have an aftermarket cam, it just might be a really torqy low RPM one.

Compression check? Never know. Fuel pressure at 3000 rpm under load?

 

[Naughty Bronc]

First get a digital timing light so you can see the RPM and see exactly where the total is at what RPM.Cast iron heads over 36-38 degrees total is asking for problems. Give your motor as much initial timing as it wants. If it takes 20 and with a hot motor it still cranks without hesitation(cranking hard) then give it 20. Then add the 14 degree bushing/arm for a total of 34 degrees. The spring rate will adjust when it actually opens at total. My mustang is at 2800rpm. This is where the digital timing light pays for itself. You can actually watch and adjust the dial to see where your total is and at what rpm it is maxed out at. The hesitation etc could be due to carb issues like lean mixture or not enough fuel. Hope this helps.

 

[pcf_mark]

You should definitely install the vac advance for test driving/driving. This will add another 12-14 degrees of timing. At part throttle this extra advance improves fuel economy and makes the throttle response more snappy. It may also bring on detonation when combined with you higher mechanical and static timing.


I have a C4 and I ran 16 initial, 20 centrifugal and another 12 in the vac can when I had a carb. That can be 48 at light throttle cruising speeds. Ran great no detonation.

 

[jckkys]

You have to remember crank degrees are twice distributor degrees. The 10 degree limiting notch on the centrifugal advance is 20 degrees at the crank. 20 degrees initial advance plus 10 degrees in the distributor is 40 degrees total. I've looked through piles of Ford Duraspark distributors for an upper shaft with an 8 degree limiting notch. They're rare but by adding a weld bead a 10 degree can be shortened. I found an instruction on the net that describes the amount of notch per degree but you can figure it out by measuring the difference in marked notches. While tuning the initial and centrifugal advances it's best to disconnect the vacuum advance. There are dial back to zero timing lights that help in doing an advance curve, but a tach is a separate device. The cheapest, are part of the engine analyzer I mentioned above. Search Fordmuscle timing is everything, for a good step by step instruction on setting up your distributor.