I frequent this website but spend almost all my time in the Super Duty section. I’m having issues with my 1974 Ranchero and since there isn’t a forum for that body style I’m posting my questions here figuring little to no difference in the ignition systems, so here it goes.

It’s not that hard to do but I’m stumped and looking for any ideas as to what is going on with my ignition timing. I’ll describe what’s happening and how I got to this point in agonizing detail in order to not miss a detail pertinent to any solutions. Here goes.

1974 400M with 120,000+/- miles
Hooker Super Comp long tube headers
Edelbrock Performer aluminum intake
Holley 4 barrel carb
Comp Cams RV cam
Double roller timing chain and gear
Duraspark ignition system

Engine is getting tired with a fair amount of blowby and a small tick from cylinder 2 valve train when warm but engine oil stays surprisingly clean and no strange or worrisome noises from the lower end. It ran fine but I wanted to see if I could dial in some more pep with an ignition timing recurve. So I bought a Mr. Gasket Duraspark ignition advance kit #925D (consisting of 2 identical soft springs) and prepared to dive in and curve my first Duraspark. (My only other experiences in ignition timing have been with GM HEI systems and only a handful of those.)

All timing described is with vacuum advance disconnected and plugged.

I checked original timing and found 15* initial, 27* at 2300 RPM and 32* all-in at 3,0000. This doesn’t make complete sense now, since I later found the advance-stop slot to be 13L, providing 26* centrifugal advance. 15+ 26 = 41, not 32. So all-in must have been at a higher RPM, or…?

The original advance springs were one stiff (high tension) and one medium spring. The Mr. Gasket kit provided 2 identical soft (low tension) springs. I started with both soft springs and found the advance WAY too fast, like 42* all-in at less than 1000 RPM. I moved up to one soft and one medium spring and that’s where I started getting some backfires in the tailpipe. I couldn’t tune it out by advancing (twisting distributor) or retarding the spark so changed the springs again, going to a soft and the hardest spring, but it still backfired at 2,000 +/- RPM. I changed springs again, this time back to the original ones (1 hard and 1 medium) where it had been running fine before the preceeding changes, but now I’m still getting the backfire at around 1,800 - 2,000 RPM where advance measures 22*. As an add-on, the vacuum advance (ported) worked as designed before. Now the engine bogs and wants to die when vacuum advance is applied (throttle is cracked open).

I’ve checked resistance across the orange and purple wires on the magnetic pickup and measured 595 ohms, within spec from what I’ve read.

I feel like I’m flailing now and didn’t go any further until I could get a better grasp of what’s going on and how to address it. If you made it all the way through this post and have any thoughts, ideas or insights as to what’sgoing on I’m all ears…. TIA!

 

When you put in the timing set I assume it was put in to gain back the factory's 4* retarded emissions timing. Next you went about recurving the distributor to accommodate the new timing position. All in at 3000 rpm and 34* to 36* total mechanical is perfect for an other wise stock 400 motor. Verify your 13l slot. (13L slot = 26 degrees centrifugal advance = .488”)
I would start with one original spring and one medium.
Until this is established we can't move forward so with the spring combinations and no vacuum advance connection we have to check and recheck until we've established the above perimeters verifying the mechanics of the distributor are getting us to where we want to be.
Check with a vacuum pump to verify the vacuum advance is functioning as it should. Verify you have an adjustable type vacuum advance. Then road test it in the following way to tune the vacuum advance. Follow the instructions in this link under vacuum advance

 

First, thank you for taking the time to read and respond to my post.
I’ve previously read the article you inserted the hyperlink for. I just reread it to see if there is anything I missed. I don’t think so but plan on starting again from scratch to adjust my timing and curve.
I’m still stumped as to 1) why there’s an exhaust backfire that begins at around 1,800 RPM, and 2) why there’s a huge bog/stumble off-idle with vacuum advance connected (ported).
BTW, my vacuum advance is fixed, not adjustable. I have verified it’s working by sucking on the vacuum line but have not measured the vacuum with a pump/gauge (which I do have).

 

Hey, I know this can be frustrating and I've been there. I'm not there but what you describe leads me to believe the vacuum advance is not performing properly. It can have a developed leak weaked spring ect' The adjustable type is preferred for more advanced tuning. They can be had for $25 from Rockauto. The "Standard Motors Products" versions are the adjustable type.
It's also important to verify the slot distance (13L) as mentioned above. It will be that distance that at the end of the day that can be dialed in to give you the precise mechanical timing that best suits your circumstance..

 

I just reread this. The timing set along with all the other performance upgrades were installed about 13 years / 5,000 miles ago. The timing set was replaced while I was installing the cam as preventative maintenance and was installed at original factory spec.
Correction: I still have most of the paperwork from back in the day (2007 or so) when I made all the mods including the timing set. I’m sure it was set straight up and not factory.

 

In that case the recurving of the distributor was not required nor recommended and in fact I would change back to an original Dura spark II distributor for this motor or change the timing gear and proceed with the recurving process. The only reason to recurve the distributor is when you change the timing set position.

 

It IS the original Duraspark distributor, I’ve only changed springs and then the magnetic pick up when I destroyed the original one. The original springs and initial advance are reinstalled but the two issues didn’t disappear and they weren’t there before I started messing around. Probably a rookie error when I get to the bottom of things. I’ll post back after I get back under the hood this weekend.

 

The 13L slot is original?

 

Yup. I haven’t measured it to verify the stamp but it doesn’t appear to have ever been touched.

 

What vehicle did the motor come from? and on the drivers side valve cover is the emissions sticker still present? and if so what does it say the timing should be set at? That is how the original dura Spark II distributor was set if it is original to the motor. I still think regardless of all that the vacuum advance is not functioning as it should. It's whole purpose is to handle timing between idle and WOT so if it's back firing there where you say It is the first place to look,

 

The motor is original to the vehicle. I replaced the valve covers years ago so no decal. The information wouldn’t be relevant anyway since the cam has been upgraded. Also to clarify, the backfires happen with the vac advance disconnected while I ‘m throttling up and checking the timing curve. I haven’t had it on the road.

 

This is the problem with slipping a Ranchero in with the F-series forum. There was no 400 motor in our trucks prior to 19771/2 . That's why I questioned it's originality. Also 1974 is the first year of the electronic distributor and it is likely the Dura spark I and not II. I would say it is likely the spark box then given all that has been revealed in your last post. In fact are you sure you have the correct part for your distributor when you replaced the "magnetic pick up". Verify the ignition system you have. Perform the following tests to verify it's functioning properly..
You have to be willing to double check your work if you're going to solve this. Good luck

 

Just throwing this out, but is there a chance the advance weights are sticking and staying swung out? Then slowly returning. On a side note I bought a 77 Tbird from salvage yard once with bad engine, drove it home. Someone replaced the DS pickup and forgot to replace the roll pin on the reluctor, you could spin it like a top.

 

I don't have any idea where my '77 351M is set initially. OEM Duraspark ignition. I used a recurve kit that lets mechanical advance come in faster, and it also limited the amount to less than OEM. I set my timing after marking my harmonic balancer up to 45 or 50 degrees BTC.
I unplug the ported vacuum, then with light trigger hooked on #1 plug wire, I shine the light at the pointer and set the distributor so the light flashes at 35 degrees BTC after all mechanical advance is totally used up. Then I lock it down. Vacuum advance is then reconnected to port. Remove light, done.

I played with springs, etc back in the '80s. All I can tell you is mechanical is all in before 2,000 rpm I'd say. Works for me.

106K miles 351M, Performer intake, Edelbrock 1406 carb, Holley FPR set at 5.5 psi, Crane "Fireball" hydraulic cam with cam timing set straight up, otherwise is stock.

 

Duraspark I, not II you say? That wasn’t even on my radar until you brought it up. Since then I’ve spent a lot of time reading and learning about the different versions and associated modules, grommet colors, number of wires, etc. I also have the Ford service manuals for my vehicle and the Engine manual has a section on the breakerless (Duraspark) system including wiring diagrams. The problem is, my vehicle’s wiring, which I’m positive is factory original, does not match anything I can find. Not the Ford manual nor any of the numerous diagrams I’ve uncovered online. I think I stated before that electrical is not my forte. It’s taking me a lot of time to figure this out but last night I had an epiphany and the electrical path through the ignition circuit started to make some sense. I still don’t understand some things, like why does my system have a condenser attached by a bracket to the module, wired into the incoming side of the module, from the factory? Also, why does the existing ignition module (NAPA TP35) have only 2 wires going out into a 3 wire factory harness plug? Still researching and learning…


The weights move freely with no sign of sticking. Timing light verifies a return to initial after revs.