Weird timing issue...

Back fire is caused by; Late ignition timing Lean fuel mixture Leaking intake valve Worn exhaust lobe on the cam Crossed ignition wires (crossfire or plugged in wrong)

The fact that you're seeing the timing retard with your timing light makes -that- the prime suspect.

Sorry Jon, I'm not buying the "everything was fine before the fire" bit. It's been 5 months, so, a lot can be forgotten, and

-something- had to be the source of the flame that started the fire when you had a fuel leak. Take the cap and rotor off, look closely at the mechanical advance mechanism. In 36 years as a mechanic, and [too] many of them as a driveability tech in a Chevy dealership, I've seen this problem more than once, and often times it traced back to something wrong in the mechanical advance. Ozone inside the cap from the arc causes rust, rust causes binding, from here it's either it's doing it because it is bound up or it's doing it because someone was in there before.

IIRC, you've had a vacuum gauge on this thing since you got it running, if the vacuum was steady, I'd rule out valvetrain problems for the time being.

Well, since the timing is retarding rather than advancing, (ie: later timing) that could be a problem. I did attempt to lean it out. No word on the valves or cam, but the long block is 2 years old.

It's possible that I miswired the distributor. I wasn't replacing one at a time, the wires were burned up.

Agreed.

LOL... *I* was the cause of the fire. As I said, I was trying to get it to lean out. It ran terribly rich after the new engine went in. Basically, the dude that put it in said the Quadrajet was in need of a rebuild. I needed a carb immediately, so I drove it as it for like a week, then bought an Edelbrock Performer 600cfm carb. That's when it started running rich.

When I was leaning it out (changing to smaller jets) in December, I had just driven home from work. The engine was still hot. I reconnected everything except the fuel line (Doh!) and when I tried to start it, the fuel pump caused a bunch of gas to shoot onto the hot engine via the loose fuel line that was laying on top of the intake.

When I tried to start it, it (obviously) didn't start. I heard a whooshing, boom kinda sound, and saw flames shoot out all 4 sides of the loosely closed hood.

No prior problems caused the fire. It was just me making a stupid mistake.

The mechanical advance opens smoothly. I checked the movement of the weights before I reassembled it. If that were the case, it should simply not advance right? It should retard timing should it?

Yeah, manifold vacuum is pretty steady at idle. It dumps to zero when I rev it.

~jp

When you twist the plate that the rotor screws down to, which direction makes the advance weights open up? Clock wise or counter-clockwise. Stated another way, pulling the advance weights out (as centrifugal force would do) should make the rotor (or rotor plate) move clockwise. If the advance mechanism is put together wrong, it will still open, but the timing will retard instead of advance.

I'll check... But it did work fine before the fire... When I had the distributor out, I did NOT removed, adjust, or otherwise screw around with the mechanical advance--AT ALL.

When I said "I checked the movement of the weights before I reassembled it" I meant, reassembled the distributor, not the mechanical advance. By reassembling the distributor, I mean, installed a new condensor, module, rotor, cap, button and coil, and plug wires.

But I will check it again.

~jp

What happens if the two wires that go to the pickup coil are reversed? It's possible that I screwed that up. Is one side of the coil grounded? Seems like it would have to be in order for anything to happen.

~jp

At this stage of it, I would try it.

----------------- The SnoMan

What are your thoughts on pickup coil polarity? Should it matter?

I'm thinking it shouldn't, however if one side of the pickup is grounded (as it probably has to be for it to work) then it's possible that the wrong side of the module is being grounded.

~jp

I found this:

"The polarity of the signal going into the HEI module is VERY important."

then...

"On an engine, a symptom of reversed pickup wires is ignition timing which is about 20 to 50 degrees retarded, and the timing bounces around a lot. When it is connected correctly, the timing is very consistent, and is close to where you set the static timing."

~jp

and... (yes I realize I'm flooding the thread but still, this is interesting)

/\ / \ / \

----/ \x /------ \ / \ / \ / V

"The ignition should trigger at the zero crossing, point "x". that is the most stable point vs RPM. If the polarity of the trigger is reversed, the ignition will likely trigger on the rising edge, ***THE POSITION OF WHICH CHANGES WITH RPM***."

~jp

THe rising wave well not change from its trigger point regarless of RPM BUT, the fall wave relative to crank postion can because the decay rate of coil in pickup is constant over a fixed time interval so as RPM increase it will cross zero axis at a further and further point as RPM increases (relative to crank postion) and in effect, retarding the timing signal. Incorrect polarity and/or wirng could cause your problem. The key to your problem is in the pickup and its inteface with ECM.

----------------- The SnoMan

Well rock on... Here at work, I get my best thinking done out on the loading dock (smoking area). I was leaning against the wall and it hit me.

This makes a LOT of sense. That is the ONLY way I could imagine spark actually retarding with RPM increases.

~jp

I have put some thought into this and I agree it does add up and could explain your problem fully. There is always a cause for a problem and you need to understand it to find a solution. Your comments of the trigger signal made it cyrstalize for me as to what was going on here. It all adds up.

----------------- The SnoMan

Yep... basic AC voltage theory...(I'm by no means an electrician, but I know a *little*) The wave WAS running in reverse. I just walked back in the apartment not 2 minutes.

THE TIMING ISSUE HAS BEEN LAID TO REST!

Currently set to about 13 degrees BTDC @ idle with the vacuum advance disconnected, pulling about 14-15" Hg off the manifold. The mechanical advance is doing its job, advancing with engine speed... No more backfiring too. I'll take it!

Many thanks for your help, Sir...as well to anyone else who replied with suggestions.

~jp

Eh... I mean, the wave was 180 degrees out-of-phase... :-)

~jp

You are quite welcome. I am here to help not to get brownie points. Your problem was curious as I never had seen one like that so I wanted to find the cause. Do not dish yourself on electrical knowledge because you knew more than most and enough to understand what was happening when I connected the dots for you.

----------------- The SnoMan

I can usually figure out the problem given enough time and a few decent hints.

I can't count how many good ideas I come up with on the loading dock at work, lol... more than I do at my desk, that's for sure.

Once I got a few 'keywords' down, I was able to pull up quite a bit on the subject via Google. I find it interesting that I've never heard anything about this before.

And apparently the 5-pin ignition module is uncommon enough that I'm very unlikely to find much on it. I may actually post some of this info on my site for future linking/reference.

Surely someone else will need this info someday.

~jp

Glad you got it figured out Jon.

I'm a bit puzzled though...

Been a few years since I was inside an old HEI but two things come to mind;

if it mattered I would think it wouldn't spark if installed backwards.....