Misfire at cruise, popping/afterfire at 3000RPM

[Righty Tighty]

Hey FABO -

I've been trying to solve this issue on my own for a couple months now, but I think I need help. I have a 440-6BBL that idles nicely, but presents with a random but frequent misfire at a light to moderate cruise. At 3/4 to WOT, the misfire gets much worse and the engine begins to stumble, pop, and backfires through the exhaust (afterfire). Here are some details to get started:

440-6BBL
UNKNOWN CAM
BLUE ECU FROM HALIFAX HOPS (THANKS!!)
NEW 4 PRONG RESISTOR
MAG PICKUP DISTRIBUTOR W/ VAC ADVANCE CONNECTED
STOCK TYPE COIL
AUTOLITE 85 PLUGS GAPPED TO .035
NEW TAYLOR 8MM PLUG WIRES
NEW BATTERY
CARTER HIGH PERF MECHANICAL PUMP
FUEL PRESSURE REGULATOR SET AT 5 PSI
92 OCTANE
OPEN HEADERS (SAME PROBLEM WITH FULL EXHAUST)

12 MMHG AT IDLE, NO COMPRESSION TEST OR LEAKDOWN PERFORMED YET. 20 DEGREES INITIAL, NEED TO CONFIRM TOTAL. FLOATS ADJUSTED IN THE CARBS.

IDLES NICELY, MISFIRE AT CRUISE. BAD MISFIRE, POPPING, AFTERFIRE AT 3/4 TO WOT. WILL NOT REV OVER 3000. DOES THIS BOTH UNDER LOAD AND REVVING IN NEUTRAL IN THE DRIVEWAY.

I've checked for vacuum leaks, but that doesn't mean there's not one I'm missing. I've adjusted the timing as far down as 10 advanced, but the engine doesn't like that at all. 14 advanced and it will idle, but roughly. I ended up at 20, and anything over 20 advanced doesn't seem to make a difference. So far, I've pulled the #1 and #6 plugs, and while they look a little rich (to me), the ground strap is pale and almost straw color. I'm going to pull the rest a little later, but pulling plugs on a 440 with headers is easier for me if I'm under the car.

Today I plan to check the reluctor gap, but I need to pick up some brass feelers.

The problem feels to me like an ignition problem, because I had this issue before rebuilding the carbs and it remains. The issue also remained while driving with only the center 2BBL connected and the outboards blocked. One possibility I can think of is maybe the vacuum secondaries aren't opening under heavy throttle, causing a lean condition. That's another test I'll perform. All throttle blades move freely, nothing hung up. I've disconnected the fuel line from the pump going to the carb to ensure ample fuel flow. What are some other troubleshooting jobs I can do?

Apologies for this being so long, I wanted to cover as much detail as possible right off the bat.

 

[MOPAROFFICIAL]

1 st Put your hand over the tailpipe and see if it sucks in pulse on your hand. Exhaust valve/s may be hung open... otherwise it night be ignition related...but sounds like exhaust valves are open , why is the thing to find out.
2nd, check carb for fuel dumping through mains.
3rd Leakdown test it, or try pulling valve covers and tapping the exhaust rockers with a rubber mallet.
Report back.

 

[eeka440]

Does it do it with the vacuum advance disconnected?

 

[Righty Tighty]

1 st Put your hand over the tailpipe and see if it sucks in pulse on your hand. Exhaust valve/s may be hung open... otherwise it night be ignition related...but sounds like exhaust valves are open , why is the thing to find out.
2nd, check carb for fuel dumping through mains.
3rd Leakdown test it, or try pulling valve covers and tapping the exhaust rockers with a rubber mallet.
Report back.

I tried feeling for a suck at the exhaust, but it was hard to tell. All I have right now is open headers, and the exhaust was too hot to get my hand close enough to feel anything other than the outward pulse. Center carb dumps plenty of gas when I hit the throttle, nothing excessive at idle. Starting a compression test now, followed by a leakdown test. Will come back with some numbers.

Does it do it with the vacuum advance disconnected?

Yes it does, no change with the the vacuum advance disconnected.

 

[67Dart273]

The vacuum secondaries have nothing to do with this assuming the carbs are otherwise OK. This sounds like ignition, and it sounds like not enough voltage OR A CROSSFIRE condition or maybe rotor/ cap phasing

Get/ make yourself a spark test gap. LISLE makes one that is adjustable I think they are under 10 bucks. "Rig" it to the coil tower with a SOLID core wire and adjust it for about 1/2" Crank the engine USING THE KEY. You should get nice blue snappy sparks "in rhythm"

Trying to determine if spark is hot enough "running" is more difficult. You might try to drum up a friend with an ignition scope.

Measure the coil + voltage "running" Should be somwhere between 8 and 10V "Rev" it a few times to see if it sags. Measure battery voltage running at fast idle with battery/ engine warmed up. Should run at 13.8--14.2 nominal, not below 13.2 and not above 14.7 or so

I don't like throwing money, but consider trying a different coil. Check the ECU is GROUNDED. Remove it, clean the mounting ears, and firewall around bolt holes and remount using star lock washers. Same with voltage regulator

Google "rotor phasing." "It happens." The spark MUST happen when the rotor is at least "corner to corner" in alignment with cap tower contact. The contact point changes with vacuum advance. Two causes of this are reversal of pickup coil wires (even though connectorized/ polarized) and incorrect installation of reluctor. Look at the reluctor. There are two pin slots visible from the top. A B/RB must be installed for CCW rotation

Also I would get a multimeter and check all plug wires for continuity including the coil wire. They should show similar readings "adjusted per foot" if they are a resistive/ radio suppression type wire

 

[67Dart273]

Reluctor key: See the arrow at top of photo? There is a second pin slot up there which is for small block/ CW operation. On your B/RB the pin should be in the BOTTOM slot

Better photo: This pin is in the correct spot (bottom of photo) for a B/RB engine

 

[Righty Tighty]

Okay, I’ll steer my focus away from the secondaries then. I was thinking that the engine might be starving for fuel if they weren’t opening.

Regarding crossfire, I’m guilty of not dressing the wires neatly for sure. There are two places the wires cross, shown in this pic. How bad is that?

 

[Early a body]

How old is the distributor, my guess is bad reluctor/ reluctor gap . Is the distributor the mallory type or is it older? Is it a mopar performance distributor or the new crap you can but at oreily/ autozone now- they are easier to work on but not well built imo. What advance springs are your running. What is your timing at 3,000 rpm and at 3500 rpm? if you set the initial at 20 you could be well past 35 mechanical advance at 3,000 rpm- that could cause a problem- Are you sure the vacuum advance is working does not have a damaged diaphragm. Put a mity vac on the vacuum pod and see if the distributor plate moves when you apply vacuum. What coil do you have, I like to run the msd blaster (2)? coil - cant remember i always remove the deals and paint them black

 

[Righty Tighty]

Pulled the plugs to begin the compression test, and right away I see two problems. #6 and #7 plugs are sooty (previously I misspoke, it was #8 I pulled, not 6). And 6 has cracked porcelain.

 

[Righty Tighty]

How old is the distributor, my guess is bad reluctor/ reluctor gap . Is the distributor the mallory type or is it older? Is it a mopar performance distributor or the new crap you can but at oreily/ autozone now- they are easier to work on but not well built imo. What advance springs are your running. What is your timing at 3,000 rpm and at 3500 rpm? if you set the initial at 20 you could be well past 35 mechanical advance at 3,000 rpm- that could cause a problem- Are you sure the vacuum advance is working does not have a damaged diaphragm. Put a mity vac on the vacuum pod and see if the distributor plate moves when you apply vacuum. What coil do you have, I like to run the msd blaster (2)? coil - cant remember i always remove the deals and paint them black

I didn’t mean to pass over your post, give me just a sec

 

[Early a body]

I didn’t mean to pass over your post, give me just a sec

no need to apologize, did not even cross my mind.

 

[Mattax]

And 6 has cracked porcelain.

Well that could explain some of the misfiring!

There are two places the wires cross, shown in this pic. How bad is that?

The cross wire isn't too bad IMO. Long runs of the wires parallel are also not good.

12 MMHG AT IDLE

I assume you meant inches of mercury?

MAG PICKUP DISTRIBUTOR W/ VAC ADVANCE CONNECTED

20 DEGREES INITIAL, NEED TO CONFIRM TOTAL. FLOATS ADJUSTED IN THE CARBS.

For the moment, based on all you've posteed, lets assume fuel level is OK.
Based on what you've written, theres's a good chance the the initial timing is probably too high for that distributor.
A. Just because the engine runs well at idle in neutral doesn't mean the rest of the timing will be even close to correct.
Setting initial timing works with factory distributor because the amount it advances with increasing rpm has been matched to be correct when the initial is set to spec.
B. Setting the initial by advancing the timing until no further increase in rpm is wrong. I know its often recommended, but the reason it is wrong is without any load on the engine, the engine can be run very lean. At idle the goal is not lean but to make the most power under very low rpms. This way the engine doesn't bog or die when placed in gear.

I will follow up with another post regarding timing.

The issue also remained while driving with only the center 2BBL connected and the outboards blocked.

One possibility I can think of is maybe the vacuum secondaries aren't opening under heavy throttle, causing a lean condition.

Okay, I’ll steer my focus away from the secondaries then. I was thinking that the engine might be starving for fuel if they weren’t opening

OK. You now know that the AFR is not related to whether secondaries open or not. When the secondaries open they should provide the same air fuel ratio as the center carb.

Center carb dumps plenty of gas when I hit the throttle

You are seeing the "accelerator pump shot" which is needed when quickly opening from closed or low throttle position.
When driving at 60 or 65 mph and faster, the boosters respond to increasing throttle without the need for much if any pump shot

 

[retroron]

To my untrained eye it looks like you've got at least 2 bad plugs. Maybe that would be a good place to start.

 

[Mattax]

A. Just because the engine runs well at idle in neutral doesn't mean the rest of the timing will be even close to correct.
Setting initial timing works with factory distributor because the amount it advances with increasing rpm has been matched to be correct when the initial is set to spec.

Here's why its critical to plot the timing from as slow as the engine can be run to rpm it appears to stop advancing.

Let's take a look a the initial timing of 1967 Plymouth 440 Hi-Po and how it works with mechanical advance.
The Dodge FSM says initial should be 12.5* BTC at 650 rpm. Look at this with specs for the mechanical advance.

If we had that distributor and measured the timing with increasing rpm we'ld get a plot like this.

But if we accidentally had been given the distributor intended for the '67 440 with Clean Air Package, we'ld get a plot like this.

That's because the advance in that CAP distributor was set up for an engine with inital of 5* BTC at 650 rpm.

Since your distributor is an unknown, you'll have to measuring the timing from lowest to higherst rpm you are comfortable doing.

Another approach to setting timing is to set it at a moderately high rpm, and then see where the inital ends up. Even with this method, while it will give better driving performance, it still will be really helpful to know the advance curve.

 

[AJ/FormS]

Put a dial-back type timing lite on it and watch the timing mark. Do not concern yourself with the actual numbers.
Make sure the Idle-timing is relatively stable.
Next, slowly rev the engine up while watching the TDC mark. Quit when you get to 2500 or if you see the mark jumping all over the place from advance to retard and or missing or double strobes.
If this happens to you, the Pick-up polarity is reversed. Simply cut the wires and flip them.
After that, your base idle-timing will have to be reset.
Then repeat the test. You want to see the TDC mark move smoothly in the advance direction only, with no skipped sparks.

The hi-tension wires should NOT be bundled together. They should be kept at least 1/2 inch apart to prevent induction firing, AND the numbers 5 and 7 wires should NEVER run side by side in any case.

By the look of your plugs, your engine has got some serious fueling issues as well, but you have to clean the ignition up first.

Afterfires with open headers is sortof common. It has it's origins in a couple of places;
1) if it starts as a lean-misfire, then the unburned fuel charge moves down the primary, and gets lit up by the following exhaust cycle and it explodes in the pipe making the pop you hear.
2) if it starts out as a rich mixture that didn't find enough oxygen in the chamber,it exits into the pipe, and finding oxygen there, it finishes burning in the pipe and makes a weaker pop.
3) If the manifold to flange gasket is leaking, you are almost guaranteed to have afterfires as atmospheric pressure forces air into the pipes any time the pressure in the pipe is lower than atmosphere, which is most of the time.
4) If you have a long-overlap cam, idling very slow, the header puts a pull on the intake plenum, and tugs fresh fuel charge right across the top of the piston at the very beginning of the intake stroke. That air-fuel charge has the right A/F ratio to burn, so if something lights it off, you get a pop.
5) numbers 5 and 7 are consecutive firing (as are 4 and 3), so if #7 gets induction-fired at the same time that #5 is firing properly, but #7 is on the end of it's intake stroke just beginning it's compression stroke. , well you can imagine that the force that drives the piston back down the way it just came, is not good. Yeah yur gonna feel that too.
6) If one or more sparkplugs has a cracked center-electrode insulator, the spark will leak away, and not jump to the ground strap. So then it is possible for the A/F charge not to light and you get a misfire. But sometimes, the thing that caused the electrode to crack, is still in operation, and the A/F charge gets lit at the wrong time. If early, then it could drive the piston backwards. If late then the charge will not have enough time to complete burning in the chamber, so it enters the header still burning energetically. But more often than not, that charge goes straight into the exhaust unburned, and with a 4-cylinder you feel it as a major power loss.
7) if your cam-timing is off by more than say 8 degrees (all I have tried), all kinds of weirdchit can happen
8) kudus to the guys who mentioned valve problems, But I'd start with checking the pick-up polarity, and separating the plugwires.

IDK, that's all I can think of.
Happy HotRodding.

BTW
your engine, at idle, in Neutral/Park will like a lot of advance; I mean a lot. And it will tell you so by the fact that the rpm keeps going up, along with the vacuum.
If you had a timing computer, this would be fine, because as soon as you put it into gear, you could program the computer to get rid of it, cuz you can't drive it when it's been maxed out like that. It is impossible to build a STREET timing curve from that point, and you're engine is guaranteed to experience detonation.
And if you happen to have a hi-compression engine with a 4 speed, you cannot even drive it slowly with all that timing.

BTW-2
In all likelihood, your big-cam engine does not like to idle with less that 20 degrees because the AFR is all messed up. After the pick-up is proved working correctly, you will have to sync your transfer slot fueling to your mixture screw setting, and set the idle-speed with timing. Then to get it to a happy quality of idle, you will have to give the engine some by-pass air.
Do not feel that your engine is demanding timing. I have made a 367 (mine) idle just fine at 5* with a 292/292/108 cam, down to 550 rpm. It ran well enough and smooth enough down there, to pull itself on a hard, flat, smooth, surface ; but did not have enough power to climb much of an incline.
But what it does demand, is a reasonably close AFR, that cannot be measured with a gauge, under those conditions.

 

[Righty Tighty]

Well, FABO has not disappointed once again. Thank you everyone for the great info -- most of which I'll have to come back and read when I can take the time to comprehend it. I'm pretty new to this and it takes a couple or three times to understand the technical information.

In the meantime, here are the numbers from the compression test. I'll be moving on to the leakdown test now.

CYLINDER

1 150 150
2 155 155
3 150 150
4 160 165
5 155 160
6 165 165
7 145 145
8 180 175

Cylinders 7 and 8 are the odd balls.

 

[Righty Tighty]

I assume you meant inches of mercury?

Haha, yes...sorry. We deal with blood pressures at work, so I think that's what I had on my mind.

 

[AJ/FormS]

what heads are on it?

 

[67Dart273]

If you did not break that plug removing it, that is suspect. The two wires NOT to bundle for a certainty are 5 and 7

 

[clementine]

I wish I knew as much as you guys. truly impressive amount of information.

 

[Righty Tighty]

what heads are on it?

Just stock cast iron 440 heads. I didn't build the engine, and unfortunately the guy I bought the car from didn't have a build sheet. No cam card, no nothing.

If you did not break that plug removing it, that is suspect. The two wires NOT to bundle for a certainty are 5 and 7

No sir, I saw that it was broken before I removed it. HOW it broke, I have no idea. The wire set I bought came with ceramic boots, and the #6 boot is also fractured.

I'm afraid all I had time for today was the compression and leakdown tests, so here are the numbers from the leakdown:

1 50%
2 40
3 40
4 85%
5 35
6 35
7 35
8 45

Clearly there is an issue with #4. I couldn't tell if it was leaking through the intake or exhaust valve, but certainly wasn't coming from the oil dipstick tube or radiator. I connected the hose directly to 100% shop air, and I could hear a loud rush of air in the cylinder head. Nothing coming from the header or carb. Not that I could hear at least.

I'll study everyone's notes tonight and get back after it tomorrow.

 

[67Dart273]

Check and be certain the valves were not open on no4

 

[AJ/FormS]

1 50%
2 40
3 40
4 85%
5 35
6 35
7 35
8 45

CYLINDER

1 150 150
2 155 155
3 150 150
4 160 165
5 155 160
6 165 165
7 145 145
8 180 175

These numbers do not support each other; one of these tests is no good.
And I'll go out on a limb and say the leakdown is wrong.......... because the numbers are way too big....... I'll guess by a factor of 10.... at least.

A LD of 3.5 to 4.5 or even 5% is acceptable and corresponds to your pressure numbers. But that #4 result is suspect for sure. And the pressure in #8 if correct, is way to high for pump gas and iron heads. Even 165psi is pushing the detonation limit with pump gas, at WOT, with open-chamber, quenchless heads.

So, IMO, the first thing to fo is to redo the LD in #4, and verify it is correct.
I assume you know how to correctly perform an LD test. If you suspect your method is off, just ask and someone will walk you thru it.

And the second thing to do is to redo the pressure test in # 8. If you oiled the cylinders, you will have to pump that oil out first. Just spin her over for about 15 seconds, prior to the test.Then crank the engine, with the gauge screwed in until you get to successive readings within a few psi of eachother.

 

[Righty Tighty]

Check and be certain the valves were not open on no4

How do I do this? At first that’s what I suspected, so I backed up the crank and then moved it forward to see the best numbers I could get. I also tapped the rockers with a rubber mallet, in hopes either valve would seat.

These numbers do not support each other; one of these tests is no good.
And I'll go out on a limb and say the leakdown is wrong.......... because the numbers are way too big....... I'll guess by a factor of 10.... at least.

I thought the same thing. Add in the factor of someone who’s only done one other leakdown test in his life, and you end up with lots of room for error.

Maybe I’ll try another leakdown tomorrow.

 

[Phreakish]

Is this a new build, or did it just recently start misbehaving?

If it were me, I'd pull the valve covers and rockers shafts/pushrods. It wouldn't take long and would tell you real quick if something is fubar up top. I've seen issues like this before turn out to be a bad cam, bad rocker arms, failed lifters, etc. The issue being present when revving in neutral is what makes me suspect valvetrain.

These types of issues are often easy to spot with the valve covers off, and especially if you crank the motor over (with ignition disconnected!) and watch the rockers.
You could also set TDC on #1 and watch the springs as you remove the rocker shaft and see if any of the valves are being held open somehow. At any rate, lots to see and determine under those valve covers.

Not that anyone above is off the mark, but no amount of ignition fiddling will fix a mechanical problem, then again no amount of mechanical fiddling will fix an ignition problem either!