Backfire...revisiting

[4eyedblind]

Most modern gasket sets come with a nice silicone gasket for the China wall. Just a dab of silicone at each end and good.

[Broncojunkie]

Just an update on this:
I finally got back in town and just about got everything back together, when I was called back out for a week. Got back in from that trip and finished buttoning it up this afternoon. As I mentioned earlier, I did go ahead and pull the heads off. I upgraded the valve springs (they actually looked identical to the springs I pulled off, but who knows, without checking the spring rate). Heads, valves, chambers, pistons..everything looked perfect, other than some carbon buildup. Excessive carbon was due to me tuning it "rich" in an attempt to fix the backfire issue.

I set the timing at 14 degrees initial and it seems to be running fine. Before, it would barely run at 14. I'm pretty sure a leaky intake was my issue. I can't really take it for a test drive. We still have snow melting off and I have a couple vehicles and parts in the way. First halfway decent day, and I'll take it for a spin. There seems to be a little crackling and popping, but no big backfires like it was doing. That's probably due to an improperly tuned carb and the dumped exhaust (no cats, just some Thrush welded mufflers and offroad h-pipe.

[Broncojunkie]

I finally got some time off work and trying to get this thing lined out again. Backfire issue is still there. I'm fairly certain there is no intake leak (there probably wasn't before). I'm getting ready to pull plugs again and see what they look like.... I'll also double check my header bolts. Timing is fine, from what I can see. I may stick the wideband back in and see if that tells me anything. It basically only does it on deceleration.... most of the time, when I'm slowing down using gears. It will backfire occasionally (although not quite as bad) when letting off the gas after getting in the throttle.

[Tigger]

The one time one of my cars had a backfiring issue was due to the timing/ignition module. The dwell was all over the map which caused the backfire. I chased my tail for two weeks. It was dumb luck I noticed it as I had my tach/dwell meter set on dwell by accident instead of RPM and noticed the huge swing when I blipped the the throttle. I replaced the ignition module and no more backfire.

[Broncojunkie]

I've exhausted every possible ignition-related issue I can think of. It has had a few different spark boxes in it, distributors, plugs, wires, coils, and has been rewired more than once.

I may have spoken too soon, though. In my previous attempts to fix the lean issue, I progressively tuned the carb to be rich. After hooking up the wideband again, I found 11:1 at idle and 10:1 at part throttle. Plugs showed the same. I re-jetted and adjusted idle mix closer to 12.5:1. After a little run time, she began to clear up and power feels better. There was still a little crackle here and there, but no gunshots to scare the neighbors lol! I'm going to try and drive it some more tomorrow and see what happens.

[darkd0r]

I thought a little crackle was a feature, not a bug?

[Broncojunkie]

I thought a little crackle was a feature, not a bug?

Maybe just a little more than a crackle. Hard to explain, but very easy to hear that it's not running well.

Another update. I was busy with yard work today, but had a little time to work on the car. I drove it but it seemed to be a little worse. As I drove, it got progressively worse. On the way home, there was an obvious misfire and excessive backfiring. In the driveway, I noticed the fuel pump stopped running. I began troubleshooting the relay and found low voltage. Battery was definitely showing low and, once I got it started up again, I confirmed the alternator wasn't working. My 150a fuse was blown. Instead of replacing the fuse, I opted for wiring in a breaker, which I had on the shelf for this reason (just hadn't got around to it). After wiring the breaker and checking all connections, alternator still wasn't putting out. I removed it and ran down to the local parts store for bench testing. It tested good, so I spent time going through all wiring. I ended up making me some new cables and redoing some grounds and it's now putting out a little over 14v. Unfortunately, I had run out of daylight. I'll try to get out for another test drive tomorrow.

I'm not sure what it is about this car, but it seems like every job I do has to be redone at least 3 times.

[Broncojunkie]

Drove the car today and still backfiring. I notice an occasional miss. Timing is good at 14 degrees, firing order is correct, cam/timing gear looks great, plugs look fine. I did notice that the timing light would occasionally flash inconsistently. That may have just been a result of whatever is causing the misfire or possibly just because the idle is set pretty low. I'm going to try, once again, swapping in the msd distributor just in case there is an issue with the parts-store stock-style distributor. I can't see any issues with it, but you never know. At this point, I'm grasping at straws.

[Broncojunkie]

Distributor didn't help, but misfire seems more noticeable. I should probably try and diagnose the misfire at this point, since the two are evidently related. Unburned fuel igniting after the exhaust stroke is in process.

[854vragtop]

Distributor isn't off a tooth? Reluctor paddle is perfectly aligned with the distributor pick-up at 14 deg BTDC and the rotor is directly in alignment with #1 on the cap?

[Broncojunkie]

Distributor isn't off a tooth? Reluctor paddle is perfectly aligned with the distributor pick-up at 14 deg BTDC and the rotor is directly in alignment with #1 on the cap?

Yes. Timing is right on the money. I actually have it set to 13 degrees btdc right now, but either way, I've tried it with several different setting.

Today, I found the #8 plug wire was creating a misfire. Wires are Accel trim-to-fit set and one of the ends weren't crimped onto the core wire very well. I fixed it and I can't even tell a difference. Car just isn't running well. After some more testing/checking of ignition parts, I noticed the left bank doesn't seem to be firing evenly. If I move the timing light connector around to each plug wire, the right bank seems, for the most part, to have an even pulse. The left side skips some. I started probing around with the volt meter and found that there is very low voltage on the ignition coil primary connector. I just came in for a late lunch, but I'll be headed back out in a bit to start probing my wire looms to see what's going on there.

[Broncojunkie]

Definitely getting a weak spark. I'm beginning to suspect the spark box (Pertronix Digital HP). This particular box works about like an MSD 6AL. I have the main power (+ and -) coming straight from the battery, as per their instructions. The trigger wire is going straight to a switched source. Both of these are reading almost 13v with key on, and just over 14v running. Grounds were checked as well, and all are good. If I remove the + wire from the coil (+ and - leads straight from the pertronix box), I'm getting a reading of just under 2v. I've read that you can't get a good reading on it due to the way the box operates, so I'm really not sure if this is my issue or not. I tried another coil I had and I'm still getting a weak spark. I put a tester inline on the plug wires and I'm getting spark, but it's orange instead of blue/white.

I've been trying to find if it's ok to run a 12v lead straight to the coil, but I don't know if I can do it safely. If I can and the spark is better, I can rule out the coil and it will mean my spark box is bad.

[Broncojunkie]

After some more testing, I'm pretty sure it's either the box or the coil...or both. I have 2 coils that are compatible with the box and it runs the same with either one, so I'm inclined to suspect it's the box itself. I went ahead and ordered a new box and Blaster 3 coil. I would have ordered a different box, but I like the features of this one and it's small enough to fit where I need it. If a new one fixes my issue, I'll probably just see about sending the other one back for repair.

I hooked up my old 6AL box temporarily just to see if spark would improve and it did. I can't really drive it as-is (box is just lying on top of the engine with wires strung everywhere). I may try to secure it enough to take the car down the road tomorrow.

[erratic50]

Fresh ideas coming in..... I just noticed and started reading this thread now

Major valve events of the 4 cycles of each cylinder happen every 90 degrees. Certain things like both valve closed are easy to spot and give strong clues to if the cam is ground right and installed correctly, etc.

Have you fully inspected the wires? Are there any nicks or malformations — I’ve had bad wires cause an issue where spark jumped from one wire to the other and caused issues. Mine was popping out the carb because the intake valve was open when the plug was firing but other scenarios are equally plausible

A fairly generic set of premade V8 wires from a reputable manufacturer is an easy swap to make while trying stuff

is the front cover correct for the balancer marks?

have you checked the mechanical function of the distributor?

What about the advance functions specifically?

where is the advance hooked up?

How does it run if you turn the timing down then go advance it until it just starts to speed up then back just a freckle?

What about the cap? Is there something up with it? Maybe a crack or something allowing spark cross-talk

What about the rotor? Is it too small diameter vs the cap and allowing strange arcs across to different plugs

I’ve ran into all sorts of nearly impossible to diagnose problems that ended up being the rotor. Usually there is something odd with the wear pattern on the rotor and theres other issues like damage to the contact in the cap

I would tend to go to entirely factory ignition parts and see if you can get it to run right then go from there.

Unless there’s a specific reason an aftermarket ignition setup is needed, I give ZERO merit to them. They frequently cause trouble because they are not built to OEM standards for quality and reliability

[KevinVarnes]

Nice troubleshooting work. It sounds like you might be finally onto something solid which has to feel pretty good for as long as you've struggled with this.

[Broncojunkie]

Fresh ideas coming in..... I just noticed and started reading this thread now

Major valve events of the 4 cycles of each cylinder happen every 90 degrees. Certain things like both valve closed are easy to spot and give strong clues to if the cam is ground right and installed correctly, etc.

Have you fully inspected the wires? Are there any nicks or malformations — I’ve had bad wires cause an issue where spark jumped from one wire to the other and caused issues. Mine was popping out the carb because the intake valve was open when the plug was firing but other scenarios are equally plausible

A fairly generic set of premade V8 wires from a reputable manufacturer is an easy swap to make while trying stuff

is the front cover correct for the balancer marks?

have you checked the mechanical function of the distributor?

What about the advance functions specifically?

where is the advance hooked up?

How does it run if you turn the timing down then go advance it until it just starts to speed up then back just a freckle?

What about the cap? Is there something up with it? Maybe a crack or something allowing spark cross-talk

What about the rotor? Is it too small diameter vs the cap and allowing strange arcs across to different plugs

I’ve ran into all sorts of nearly impossible to diagnose problems that ended up being the rotor. Usually there is something odd with the wear pattern on the rotor and theres other issues like damage to the contact in the cap

I would tend to go to entirely factory ignition parts and see if you can get it to run right then go from there.

Unless there’s a specific reason an aftermarket ignition setup is needed, I give ZERO merit to them. They frequently cause trouble because they are not built to OEM standards for quality and reliability

I've literally covered every point here multiple times. Wear pattern on the rotor wasn't considered until a couple days ago, but it was fine with the factory style distributor. I went ahead and swapped the MSD distributor back in simply because there was a little play in the cap/adapter section. This seems to be common among the factory style distributors, as I had 2 aftermarket and a couple Motorcraft distributors lying around. With the distributor set and locked down, I could slightly alter the timing just by grabbing the cap and slightly twisting one way or the other. The MSD doesn't seem to have any play. I was certain this was not the problem, but since my problem doesn't appear to be distributor-related, I went ahead and put the MSD back in because of that.

I tried using the factory Duraspark 2 ignition box last year, but I had a problem with it. I tried a couple different boxes with the same issue. I can't recall exactly what the problem was, but I couldn't get it to work at all. I even had a friend come over who is good with this stuff and double check my work. I used wiring diagrams I found online...several with the same layout, just to confirm I was doing it correctly and I believe I even started a thread on here. I went through it enough to have the entire schematic memorized. It's simple, but I couldn't get it to work with factory or MSD distributor. I actually have one sitting in front of me now, so I may try it again. Since I tried it the first time, I've addressed a couple other issues, including a bad engine ground.

As for the timing, I'm 100% confident it's good. I even pulled the valve covers and watched the rocker arms as I turned the engine over by hand. Firing order is correct (351w/302HO) and timing marks are right on the money. The balancer is very hard to read. There are 3 sets of numbers for different timing pointer setups, I guess. Two sets of numbers at the front edge of the balancer and one set of numbers at the rear, which are almost impossible to read unless you're under the car and the engine is rotated away from TDC. I physically found TDC by pulling #1 plug and rotating the engine by hand until I had compression in the cylinder. I stuck a piece of tubing in the plug opening and continued rotating till it reached the highest point, then I crawled around the car till I could get a decent shot at the pointer, and sure enough, the rear set of numbers lined up exactly. I then rotated the engine around and marked TDC toward the front of the balancer. I found TDC again and made sure the pointer lined up with my line, which it did. I have a timing light with timing advance adjustment knob on it (and another one without the knob adjustment). l've set the timing using the vacuum method and by ear. I've even tried setting it slightly off one way or the other and problem persisted.

I have addressed a few issues that were hidden or intermittent, but none that have fixed the problem. I have done things that have affected my problem...a little...either for better or worse. One issue I had was that the volt meter (or does it measure amps?) on the dash reads good even if the battery is dead/dying. I had a blown 150a fuse and the battery was only at around 10v the other day and the gauge still read a little over half way. I fixed the alternator issue by rewiring it and making a new ground cable and cable from the alternator to the solenoid, along with a 150A breaker instead of the fuse setup. I have a 3G alternator using a PA Performance adapter https://lmr.com/item/PA-462802C1/Mus...G-Plug-Upgrade. Other than the gauge not reading correctly, the system is now producing good power and otherwise working perfectly.

[Broncojunkie]

Nice troubleshooting work. It sounds like you might be finally onto something solid which has to feel pretty good for as long as you've struggled with this.

Well, I've been onto it several times now but I think this car has a mind of its own lol! I'm pretty sure that my problem was actually 2 or 3 issues together. Now that I've found a couple issues, I'm revisiting areas I had previously checked. So we'll see if I can finally figure it out.

[Broncojunkie]

A quick update on this. The new ignition box didn't help. Same problem. I started thinking about how I programmed it when I installed it. You have to set the mode for the type of distributor you're running. In my case, it's an msd. You set the type, as in points/electronic distributor vs magnetic pickup distributor. I had it set to magnetic pickup (I referred to the wiring diagrams, which seemed to suggest that was the correct setting). It's a little more complicated than that, however. I had the option to choose 2 different power settings, multiple spark or single spark, and rising edge vs falling edge. After setting to rising edge/electronic-points style distributor, she woke up and spark was better. Set to multiple spark on the highest power setting, she runs much better. Still not 100%, but I may need to go back and recheck the timing. I also think the coil may be a little weak, so I'm going to try swapping it out as well. I took it for a short drive yesterday. A little backfire, but nothing like before. No obvious miss. Power is up quite a bit. I'll see what I can do with it later today.

[Broncojunkie]

Another update. Car still backfiring...occasional misfire that's not always noticeable (not a dead miss). It will run with the ignition box set to "falling edge" which doesn't seem right, but it works. It will barely start when set to "rising edge". Once again, I can run the car and it sputters and backfires. I can advance the timing and it clears up a little, but it's closer to 30 degrees btdc initial timing. I'm about at my wits end with this car. It's not a complicated setup. No computers, simple wiring, basic everything. I've never had this issue with any other vehicle and I've been around the block a few times.

I may start pulling plugs one at a time again...take ohm readings on each plug wire (since I did find a bad one the day). Other than that, I'm at a loss here. I'm evidently missing something somewhere. It may be about time to pull the cam and have a look.

[emerygt350]

You did double check your distributer is dropped in correctly right?

[KevinVarnes]

Any chance you can go back to a completely stock ignition setup to eliminate all of the aftermarket stuff at least temporarily?

[Broncojunkie]

You did double check your distributer is dropped in correctly right?

I've had three different distributors in and out at least a dozen times. I think I can literally do it with my eyes closed now. I've had a couple buddies, at different times, come and do it too, thinking I may have been overlooking something. They did everything the same way I did.

[mrriggs]

Another update. Car still backfiring...occasional misfire that's not always noticeable (not a dead miss). It will run with the ignition box set to "falling edge" which doesn't seem right, but it works. It will barely start when set to "rising edge". Once again, I can run the car and it sputters and backfires. I can advance the timing and it clears up a little, but it's closer to 30 degrees btdc initial timing. I'm about at my wits end with this car. It's not a complicated setup. No computers, simple wiring, basic everything. I've never had this issue with any other vehicle and I've been around the block a few times.

I may start pulling plugs one at a time again...take ohm readings on each plug wire (since I did find a bad one the day). Other than that, I'm at a loss here. I'm evidently missing something somewhere. It may be about time to pull the cam and have a look.

Did you reset the timing after changing to/from rising/falling edge?

If you have the box set to falling-edge [assuming the distributor is putting out a rising-edge signal] then the timing will retard as speed increases which is probably why you need to set the initial so high.

Switching to rising-edge will advance the timing even further which is why it is hard to crank over.

Reset your initial after switching to rising-edge and see if that helps.

Please, ignore if you've already done that. There is a lot going on in this thread so I may have missed it if mentioned.

[Broncojunkie]

Any chance you can go back to a completely stock ignition setup to eliminate all of the aftermarket stuff at least temporarily?

I tried...have ran it with the stock distributor and 2 different (working) msd 6AL boxes. I had trouble wiring in the stock ignition box for some reason. I believe it's because I'm not using the stock engine harness. Well, not completely anyway. I removed the engine harness and tied directly into the main harness for each connection I needed. Ignition box is tied into a 12v switched source from main harness. Oil pressure sensor, coolant temp tied into the gauge wiring, etc. When running the stock ignition box, you have to wire into 2 12v sources....one 12v in "start" only and another in "run". You also need to consider the factory ballast resistor. When I attempted this setup, I couldn't determine if the ballast resistor was correct/working/present. So I sourced an aftermarket resistor at the correct rating and wired it separately, where I could verify everything was correct. It didn't work. Resistor just heated up and I still couldn't get a spark. I believe I address this whole ordeal in another thread last year...and was an absolute ordeal. I ended up going back to the 6AL. It simplified everything so that I could continue ruling out specific parts. I used 6AL boxes that work fine on other cars. Distributors work fine on other cars. Coils work fine, etc.

I think my next course of action is to pull the engine and verify cam events. The grind could be off or the wrong cam altogether.

[Broncojunkie]

I should explain. I feel degreeing the cam would either solve or rule out a problem with the cam itself. Pulling the engine is probably easier than trying to do it in the car. I can pull it in an hour. And if I have to change the cam, that will be easier as well.