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Old 04-01-2012, 11:00 PM   #1 (permalink)
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Default Backfiring through exhaust

I recently started the car after having it sit for five to six months. For the most part, it has run very well. Starts quite easily and idles well.

However, it will occasionally backfire in the exhaust. Usually I hear a slight, "pinging" sound but every now and then there will be quite a loud "pop" that startles me. It mostly occurs when I'm revving the engine but it will sometimes randomly happen when it is just idling.

Before I let is sit, the car was running well and did not exhibit any backfiring issues. I have not touched the distributor in that time but I have removed the spark plug wires. I'm 99% sure they are in the correct position but I will double check that next time I'm working on the car.

Let me share some information about the car. I am using electronic ignition with a MSD 6AL box and distributor with mechanical advance. The carburetor is a fairly new Holley 4160 with non-adjustable floats. It is a 67 with a late model 5.0 roller motor.

At this point I'm attempting to compile a list of could be causes. What comes to mind thus far:

- Too rich of a mixture
- Switched spark plug wires
- Bad gas? (Gas is a few months old although I swear I added stabilizer to it)
- Plug not firing
- Timing (Seems unlikely to me but I can pull out the timing light just to verify)
- Valve issue? (I haven't messed with them so this seems unlikely)
- Exhaust flow issue? (Seems unlikely)
- Other ignition issues related to the MSD box.

What do you guys think? Let me know if I left off any information.

Thanks,
Emile

Last edited by airmenair; 04-01-2012 at 11:03 PM.
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Old 04-02-2012, 02:45 AM   #2 (permalink)
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I'm going to go with the timing/plug wires. My was backfiring like a howitzer and it turned out that the transport guy had messed with the timing because it would start for him to get it on and off the truck. Turned out that he ran it out of gas. So my vote is for timing/plug wires.
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Old 04-02-2012, 07:33 AM   #3 (permalink)
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Hi,
In addition to a thorough tune-up, check the engine vac and adjust the curb idle mix using the gauge. Checking the engine vac will provide some insight as to the timing, valves, etc.
Good Luck!
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Old 04-02-2012, 08:47 AM   #4 (permalink)
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You did follow the late model firing order, didn't you?
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Old 04-02-2012, 09:05 AM   #5 (permalink)
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Scott, yes I did. I arranged the spark plug wires so that the labels matched the number of the cylinder. I.e. the left bank is 1,2,3,4 right bank is 5,6,7,8. And they are hooked up on the distributor to match the late model firing order, but I will double check just to make sure. I don't ever recall removing the wires from the distributor.

Is vacuum usually checked through each cylinder or at the manifold? If it's checked at each cylinder, do you start the engine minus 1 cylinder?
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Old 04-02-2012, 01:42 PM   #6 (permalink)
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You won't be able to check vac at each individual cylinder. Attach a vac gauge to where you can get as close to the base of the carb, but below the throttle plates as possible. Vacuum should be steady at idle, with a stock cam, somewhere between 15 to 20 inches. You mentioned it has been sitting for about 6 months, it is possible that you have a sticky exhaust valve
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Old 04-02-2012, 01:53 PM   #7 (permalink)
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Might just be a little gunked up from sitting? Did you use stabilizer before parking?
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Old 04-02-2012, 07:25 PM   #8 (permalink)
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I just noticed you said exhaust. Usually exhaust is from the ignition cutting out and raw fuel building up in the pipes and then exploding when ignition comes back. Could be as simple and cheap as a plug gone/going bad, plug wires shorting or to the extreme as the MSD going bad.
Crank the car in the dark and watch the plug wires for arcing. Swap plugs. Two quick and cheap tries.
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Old 04-02-2012, 07:48 PM   #9 (permalink)
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I did notice that the boot came off of the number 4 cylinder twice while I had it idling. I put it back on and started the car again and there was still some popping but perhaps it is related.
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Old 04-02-2012, 07:58 PM   #10 (permalink)
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I find it typically to be a bad plug or wire or it can also be a weak spark. Fuel builds up in the exhaust and ignites. I always use silicone dielectric grease under my boots and I slide a toothpick between the boot and the wire when installing to let the excess air bubble out so it seats properly.
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Old 04-08-2012, 11:01 PM   #11 (permalink)
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Today I got started on giving it a full tuneup, even purchased a vacuum gauge. I pulled all of the plugs to inspect them, they all showed signs of normal combustion, tan coloring on the electrode. While I was at it, I went ahead and re-gapped them to .055". Previously they were .045". My thought is the high powered MSD box and blaster coil should be able to handle this gap. If not, that sort of steers my in the direction of faulty MSD hardware.

I double checked the firing order and the location of the wires, both on the distributor and on the plugs. As another measure, I checked the resistance of all the plug wires all of which fell within 2-3 kohms.

Re-installed plugs and wires and checked the timing with my light. It showed 8* BTDC I upped it to 12*. I've read that total timing should be 34*-36*. The stop bushing on my distributor is set to 21*, so I have a total of 33*. My real question is though, at what RPM should total timing be reached? I changed out the springs today to get the total timing to come in by ~3000 RPM I believe.

Got the vacuum gauge out to see how much vacuum I was pulling at idle and noticed that on two of the ports on the carb, the little rubber vacuum caps had dry rotted and were leaking. I replaced those and it smoothed the car out some. After addressing that issue I got onto getting the vacuum gauge hooked up. Once attached, I was able to get about 16 in Hg at about 600-700 RPM. A couple of questions on that though...

1. What should the idle speed be set to? I have a manual transmission car.
2. How steady should this mechanical vacuum gauge read? It was fluctuating maybe +/- 1 in Hg at a relatively low frequency. Is that normal?

With all that said, I didn't notice much if any backfiring through the exhaust. It did it once or twice when I started it up after letting it idle for probably 10-15 mins but after that I did not notice anything.

I also added some new gas and treated what I had left in there with some sea foam.

One thing I think I'm hearing (most noticeably at the exhaust tips) is an occasional...miss perhaps? I'm not sure how to describe it, sort a pause in the normal rhythmic pulse of the exhaust. As I describe it now it sounds as though I might have a cylinder misfiring on a semi-infrequent basis. Maybe this goes back to the increased gap? Although, I think this was actually present all along; I'm now just looking at it closer.

I probably should have gone ahead and checked for spark at each plug while I had them out, oh well. I'll do that next time I work on it. At least it can tell me whether I can rule out the ignition system or not.

I also should have performed a compression test while I was out there. I'll chalk that up to next week as well.

Will also check for vacuum leaks at all the external engine connections, carb and around the intake.

Thanks guys.
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Old 04-08-2012, 11:06 PM   #12 (permalink)
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Vacuum Gauge Pictorial
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Old 04-08-2012, 11:23 PM   #13 (permalink)
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Hi,
OK, read through your post. Nice job on your being thorough.
First, know that the wider the gap, the more stress you put on your ignition system, especially the wires. Remember the spark energy will take the path of least resistance. Therefore, any slightest weak spot in your cap, rotor, wires can and will be compromised. Long story short, go back to .045.
The needle should be somewhat steady at that 16 Hgs., but, not necessarily still. No crazy swings See:Animated Engine Vacuum Diagnostics
Use your vac gauge to adjust your curb idle mix. Reconnect and slowly adjust each mix screw to obtain the highest vac reading. If your idle increases, re-adjust the idle speed screw to return to your previous setting.
Advance "all-in" by 3000K is fine, some guys go even lower, but, it's a case by case adjustment. Every engine's requirements are different. This is why people fine-tune on a Dyno. What's good for me, may not be good for you. I think the 12 deg. initial is a good place to start. BTW, a too tight pre-load (hydraulic lifters) can cause a lower vac reading.
Happy Motoring.
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Last edited by kenash; 04-08-2012 at 11:28 PM.
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Old 04-08-2012, 11:26 PM   #14 (permalink)
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Oh, that is nifty thanks bartl!

I'll probably have to take a second look at the gauge with this in hand (might upload a video of it too) but based on what I see and what I remember I either have...

1. Normally operating engine (depending on what "steady" actually is)
2. Leaky valve (although I don't think it dropped more than 1 in Hg)
3. Loose valve guides (amplitudes of fluctuation aren't as high as what is described here)
4. Late ignition timing (depending on what "steady" actually is)
5. Plug gaps too close (I can't imagine .055" being too close, although it does seem slow in nature, frequency of oscillation that is.)
6. Carburetor out of adjustment (These seems kind of likely, what needs adjustment curb idle mixture screw?)
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Old 04-08-2012, 11:43 PM   #15 (permalink)
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Just read your post too Kenash, thanks for the input. Another great resource you posted there.

Sounds smart to un-gap the plugs. I didn't mention this in the last post but I did notice some wear on the tip of the rotor. I thought it looked a little odd because it wasn't uniform over the whole rotor just one corner of it. Sort of discolored and somewhat miss shapened, again pictures will help.

Most of the observation I did was at idle, now that I'm armed with some more information I can get a better diagnosis of my engine with my new tool. I'll report back with any more findings.

Regarding idle, what should I shoot for 800-900?
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