Help with Ignition / Backfire issue (fixed)

[skidd]

I need some help with an ignition issue. I sort of know what the issue is, but I can't figure out the cause.

The car is road worthy, but not yet road legal. I've only had it around the block a whole bunch of different times, both in GoKart and now BodyOn phases.
Prior to yesterday, it ran really good. Good idle, easy starting, plenty of power, no hesistations.. etc. Everything worked fine.
I have a remanufactured distributor from a 1995 Mustang. The one with Hall Trigger sensor, and vacuum advance.
Wired with a 1981 GM HEI 4-pin ignition module. (yeah... I know)
Sitting in a 1999 Explorer 302 with Weiand Manifold and Summit Racing 600cfm vac-secondary carb, and e303 cam.
Like I said, she's run really good up until now.

Short Version:
With vacuum advance connected to timed port on the carb, major backfires with any throttle input.
Initial timing is set to about 10deg (I think).

Long Version:
Yesterday while fiddling, it started running really rough, and died. Would not start. Nada.. nothing. Could not get it to re-fire. It would seem to try, but not fire.
I diagnosed a spark problem. I was getting a really weak yellow spark (using one of these). from the coil, and nothing at all from the plug leads. Assuming it was the coil, I went to NAPA and got a new Echlin IC24 E-Core coil. Just in case, I grabbed another HEI module too. Swapping out the coil did not fix anything. So, I swapped out the HEI module. That seemed to fix it... in so much as.. now I was getting a good spark on the tester at the coil lead. But really poor spark at the plug leads still. ??? Still wouldn't start. Ran to O'Reilly and grabbed a replacement cap and rotor... opted to upgrade a little for the one with brass terminals over the cheaper looking one that came with the distributor. It was killing me to replace pretty much new parts, with other new parts, but so be it.

Varoom! At last.. it starts!! and seems to idle great. Until I give it just a little throttle... coughin and spittin.

?? WTF is going on!!

I re-checked my timing.. and it sure looks pretty dang close to 10deg BTDC. I say it looks, because I had to make my own timing marker, and It's possible it's off by a degree or two.

Here is where I finally got somewhere.... I pulled the vac advance hose to re-check the timing. Forgot to re-connect it, and when I stabbed the throttle, it didn't cough. I thought I had it licked!! Until I re-connected the vac line.

So.. my current situation. Starts fine, idles fine, backfires under load or just partial throttle.

My Diagnosis:
Failed / Faulting Distributor. But in what way? Is the vacuum advance mechanism capable of failing in such a way as to way way way over advance the timing? Which would sure explain the backfires.

Here is a quick youtube video I shot of it doing that backfire with the advance connected.
http://<a href="http://www.youtube.c...W05VusWwe0</a>
https://www.youtube.com/watch?v=3W05VusWwe0
- Initially with vac advance disconnected. running well.
- Then, vac-advance re-attached. Runns like crap.
- Then again with advance disconnected, and left hanging. It's connected to a timed-vac port on the carb.

Anybody have any ideas?

[Itchief]

IMO The backfire sounds like a firing order issue I would verify that the spark plug leads are in the correct order after you replaced the cap make sure that at tcd on compression #1 is correct and work from there

The weak spark may be a loose power connection or a poor ground connection

Rick

[skidd]

I had checked, and re-checked the firing order a few times. It's correct. I initially thought firing order too.
As for the weak spark... It's a pretty good spark now. But, I'll re-check the grounds again tomorrow. Especially at the HEI module, I suspect it needs to have a really good ground to cleanly fire that coil.

But, now I'm even more confused. Went out into the garage just now to re-check the idle mixture screws on the carb. Hooked up the vac-gague, and found them to be just a tad bit fat. After fiddling around with the needles, they both went from 1.5 turns , to about 1.25 turns. Managed to pull about 14 inHg at 900rpm. Not a substantial change. But... get this. During the adjustments, and stabbing the throttle a few times.. very little backfires. Nothing like the video showed. ??? how does that happen? It still backfired a couple of time at very very light throttle, but that's it. All that changed since taking that video was I went into the house to eat dinner, and clean up. The engine had cooled off pretty good.. but nothing else changed. I haven't driven it yet to see how it handles load.

[Itchief]

You may want to check the hall trigger sensor on the rebuild distributor

Just a thought

[CraigS]

How long has the gas been in the tank?

[Norm B]

The timed port on your carb might not be the one you want to use. The vacuum advance will advance the timing when you open the throttle when connected to this port. This is opposite to what you want to happen. Try connecting the vacuum advance to the non timed port and see what happens. You will have to adjust your idle speed down.

HTH

Norm

[scottiec]

I’m probably not too much help here. But it sounds like your motor is trying to advance timing way too much for some reason. With too much advance it will have trouble starting and coming off idle. Now for a solution, I would start from scratch. Pull the dizzy, confirm TDC in #1, and set your initial. Also, disconnect your vac secondary and get a timing light on it. What does go to as you increase rpms. You’ll probably have to put a mark on your time wheel at the last notch, which is probably 34* or so. This way you can see it when you rev or blip the throttle.

[skidd]

Fuel is only a couple of weeks old. It's not that.

More ??? after some more fiddling this morning.

I'm pretty sure it's related to the vac advance... but I don't see how.

If I unplug the vac advance, I can manually advance the dist all the way from an initial as low as 10 up to 35. RPMs smoothly increase and vacuum increases.
Which makes sense to me, if I say.. set an initial of about 10deg, and hook the dist to manifold vacuum, it should advance another 10deg, giving me an idle timing around 20deg.
If I manually set the dist to 20, it runs smooth and steady.

I popped the cap, hooked up a small hand vac pump to the dist, and watched the sensor move into advance as I added vac to it. It started to move by about 7-8inHg. Once I pulled 20inHg with the pump, the advance bottomed out all the way, and the pump gauge held a steady 20inHg. no canister leak. The movement of the advance looked smooth and clean. Doesn't look like an issue. the gap in the hall sensor and the wheel looks to be about 15 thou. Just eyeballed.

Here is where it gets strange.

Fired up the motor, idle at about 900rpm. initial set to 12deg. With the hand vac pump, added 5inHg to the distributor advance can. nothing changed. Added vac now up to about 7.. starting to change a little. Slightly higher rpm. Added vac up to 10inHg... caugh sputter die. ??? WTF.

Tried it again watching the timing with my timing light. 12deg at 0inHg on the pump. About.. 15deg at 6-7 inHg on the pump. At 10inHg on the pump.. the light jumped all over the place.. and looked like it advanced to almost 60deg!!

Is that really possible?

I'm about to see if someone in town has a dizzy for a 1985 mustang and swap them out to see what happens. I just hate throwing $$$ at something not knowing why I'm throwing $$ at it.

[skidd]

Fixed!!

Short Version:
The 2 hall sensor wires from the Dist to the HEI module were.... wait for it... reversed at the ignition module.

Long Version:
A failed original coil, that in turn killed the original HEI module was the start of my problems. I replaced the coil and HEI module, and it did fire up.. but.. never ran right (see post #1 above). After throwing more and more parts at it... cap, rotor.. and yes.. I even went and grabbed a re-manufactured dist from NAPA. It was lnly $52 after core. On the up side, the reman dist is a motocraft .. better than the Chineesium one I had I guess. Anyway... after all those parts.. it still didn't run right. Check.. and re-check.. and re-checked everything!! I was really getting frustrated. Standing there talking to my neighbor.. and telling him the whole story of how I got to this point.... that's when it occurred to me.... "did I get the wires backwards on the ignition module?". Sure enough.. I did have them reversed. Flipped them around... and it fired right up, and now functions perfectly with the vac-advance. I knew it had to be something simple.. and it was. I'm a little miffed at wasing a few $$$ on unnecessary parts.. but.. c'est la vie!

I guess that's what I get for putting a GM part in a Ford. lol

[scottiec]

Glad to hear!!!!! Awesome man

[CraigS]

Hey, you now have some spare parts, and you learned a lot about your ignition system, and you have a real distributor in there now. It was a pain in the a$$ but all is well that ends well. Good work!

[skidd]

^ all true. Thanks.