318 Running Rough / Backfiring

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74Dart318

74Dart318

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Hey guys,

I have been researching this and I am not sure what's going on. I wanted to see what all the FABO experts think :)

The car has been running great the last few months. Just today I get in and it starts running really rough on idle and acceleration. Has some backfiring when accelerating also.

Car: 1974 Dodge Dart Swinger
Engine: 318 Bored .030 with KB pistons
~9:1 compression
Indy LAX heads
TTI shortys, full exhaust
Eddy air gap, Eddy 650 with elec choke
Comp Cams XE268H (Duration @ 0.006": 268° / 280°; Max Lift w/ 1.5RR: .477" / .480"; Lobe Separation: 110°; Intake Centerline: 106°)
Trans: Rebuilt 727, shift kit, stock converter
Rear-end: 8.25, SG, 3.21 Gears
Rev-n-nator
MSD blaster 2 coil
Initial timing set around 15°
Also running a Summit distributor

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-I have checked vacuum lines, nothing is disconnected
-Timing is still good
 
Water in fuel?
Dirt in carb?
Pull all spark plugs, are they burning even?

Give us something to work with.
 
timing chain jump? backfire through exhaust or carb ?
 
Yup, from scratch, double check/re set the timing. Check under the cap for clean contacts. Good wires!
Run fresh plugs.

Recheck the carb is clean, clear down the barrels. Float properly adjusted for proper fuel level.

Heck full advance with vacuum line off first, then check connected. Make sure the springs/weights/advance plate moves freely and come to a full rest.

These things if dirty, clogged, not fully creek living will cause issues.
 
Fuel pressure jumps around between 5&7 psi

I'll have to pull the plugs

Backfire through exhaust
 
74Dart318 said:
Fuel pressure jumps around between 5&7 psi

I'll have to pull the plugs

Backfire through exhaust
Click to expand...
Popping, burnt exh valve...but backfire is loading up unburnt fuel with heat setting it off down the line.
What fuel pump are you using?
It is possible you run champion plugs and an insuator came lose, it is misfiring.
 
Backfiring through exhaust means it is leaning out somehow. Check the fuel filter and make sure it's not slam stopped up.
 
Erratic ignition can also cause exhaust backfiring, when unburnt fuel goes down the pipes.... plugs, wires, distributor, coil and all ignition parts are possible causes.
 
Electronic ignition or points?
 
Electronic ignition.

Haven't had a chance to work on it with Christmas celebrations. Also been sick all week.

Going to try and look at it again tomorrow. I'll start with plugs.

I don't know much about distributors. Would I be able to take the cap off and physically see if a weight or spring has moved or come off?

Anybody think there is a chance the torque converter is failing?
 
Pulled plugs. My goodness they are black haha. Only about 1200 miles on the rebuilt engine and these plugs. Guess it's running pretty rich.

Weird that they are uneven with the amount t of foiling though. The darkest are the plugs from the front of the engine (plugs from cylinders 1&2) and they get lighter towards the back (plugs from cylinders 7&8).

Thoughts on why the front plugs would be so much darker?

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Rich... yes, but the cause of being rich? it may not be the carburetor. I would bet it would be poor performing spark, thus the reason for inconsistent showing on the plugs. Had the same thing with my 318 a few months back, and found my ECU wasn't getting consistent/poor ground. Car smelled rich when idling. Now it's doing great.

Just a thought......
 
For the leaner plugs in the back, I suspect you either have:
- a leak in the intake system near the back of the carb
- the secondary throttle stop screw is too far open in the

Either of the above will add air into intake, and due to the position of where the excess air is coming in, the back cylinders will lean out more. The overall mixture richness has been increased to make the rear cylinders idle OK, but that makes the front cylinders over-rich. Seen this happen before with a too-far-open secondary stop screw. Some cylinder-to-cylinder mixture variation is common but your 1-to-7 mixture variation is huge.

It also looks like the carb may be rich at other phases of operation, so IMHO, some carb work is in order. With cooler weather now, the electric choke is closing up more and staying on longer after a cold start, and that may be the base issue with all the carbon fouling on your plugs; simply adjusting the choke a bit leaner may help reduce the richness. Silly question: Is this your first carbed engine? It's just that the start up process for carbs versus FI requires a lot more 'user involvement' and sometimes tweaking on the choke system as the seasons change.

Things like idle vacuum and PCV and power valves can interact to mess up mixtures. What PCV valve is in there? (A Mopar OEM PCV valve won't match up well with this cam.) And what has been the idle vacuum level? With your cam and such, it ought to easily tune up to around 13-14" idle vacuum at 800-900 RPM. If you have less, then it needs work on PCV valve selection and setting the secondary stop screw.
 
I had a car backfire through the exhaust when I had accidentally crossed up two spark plug wires.
 
Fix the carb, im guessing not a 4 corner idle, rear plugs.Too much idle screw and maybe in the main circuit at idle., add the wrong pv and it only gets worse. Start with the carb and a new set of plugs.
 
Check the carb mounting bolts,could be loose. Also damaged gasket.
I agree it could be a fuel issue.
 
I would not be surprised if the carb is not set up correctly. The PCV valve is a 4343581.

I have tried tuning the carb myself but it's my first time trying that.

I have the Edelbrock AVS 650cfm carb 1806, I also have the 1840 tuning kit that has additional sizes for metering rods, jets, and metering rod springs.

Currently running .092 primary and .098 secondary jets. Also .065x.047 metering rods and the blue springs I believe.

Might be able to double check tomorrow along with idle vacuum and all that.
 
OK, that PCV should be decent IF you have and idle vacuum in the 10-11" or better range. The issue over the PCV is if the increase from the PCV's idle to cruise air flow settings occurs right around the idle vacuum level, then the PCV airflow is going to fluctuate up and down and mess with you idle and dump extra air in the part of the plenum where your PCV port is. That is in the front for your carb, and you are lean in the back, so that does not seem to be the issue now. I'd put that on the bottom of the list of possible issues.

I don't know Edelbrocks but there is 'something' that controls how closed the bottom secondary plates are at idle. (This is not the top secondary air-flow plate but the standard throttle plates on the bottom.) The top plate does not really close off the secondaries all that well so this bottom throttle plate closure setting controls how much air 'leakage' is introduced through the secondaries at idle and light throttle conditions. If too far open, it can be a source of excess air into the back area of the plenum. Holley actually has a stop screw that you adjust by flipping the carb upside down (or using an offset screwdriver) to adjust this.

I would also be checking the secondary air plate settings at the top. Make sure they are at the factory settings to start. There are online videos for that.

All the above is to try to find any source of extra air into the back of the plenum or intake manifold that is making the rear cylinders lean. And of course it could be carb gasket leaks or even intake manifold gasket leaks in the back on both sides. Seeing that the #7 plug is the one that is really lean (moreso than #8), I would be checking that back corner for intake manifold leaks.
 
Played with it some today. Just to give you some more findings.

Initial timing: 15°
Vacuum shows: 12-13"
Idle: 850 in P and 650 in D

Changed out the meter rod springs from Yellow to Pink. Pink is supposed to be for 14" vacuum but the next size down is for 10" vacuum.

Ran it around the block and it ran well. The fuel pressure is much more stable just over 6psi now. Adjusted the idle screws.

I couldn't find a leak around the carb base or the intake. I tried with carb cleaner, butane, and with flammable brake cleaner.

AJ I would love to test float valves and set the T-port sync but unfortunately I don't know how to do those things haha.
 
T ports, on a holley. You have an eddy.
Float valves-fuel pressure gauge, holds pressure engine off,valves are good.
 
Your vacuum and idle speeds sound good. Sounds like the leak theory is kaput with all the test you did.
 
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