Backfire thru exhaust at idle

[Fran Blacker]

The problem pump number is Carter M6903.

 

[Griff]

Yes, it is a Carter Fran Blacker
Fits: 400, 440

Fuel Pump, Mechanical, Muscle Car Series, Chrysler, Dodge, Plymouth, Each

Free Flow Rate: 29 gph

Maximum Pressure (psi): 5.5 to 6.5 psi

Inlet Attachment: Stock

Inlet Size: Stock

 

[Griff]

Hey Fran...this is M6935

 

[dadsbee]

That came up as well in my thoughts, but I believe the alternator is the stock 30 amp 1968 / and when tested at O'Riellys they tell me it "passed"

I don't care what it tested at, it's not charging the battery. Like I said leave the charger on next time you run it up so you know you have 13+ volts and see if it misfires.

 

[Griff]

I don't care what it tested at, it's not charging the battery. Like I said leave the charger on next time you run it up so you know you have 13+ volts and see if it misfires.

Will do.

 

[Fran Blacker]

Something to look at. As dadsbee said altenator not putting out do you have a spare.

 

[Griff]

That was a thought I had as well, so yes I put the spare one on, after first round of knocking, and after I cleaned all the plugs the first time. The first one was a 1963 date code and it too "passed" when checking it at store along with the 1968 date coded one. Both are 30 amp

 

[eldubb440]

knocking?

 

[Griff]

knocking?

Car is not knocking, fires right up and very strong. reving it sound super strong and tight, then after warm at idle it bangs in exhaust. Sometimes a couple of big bangs. But, since I went thru plugs second time, the bangs at idle were not to loud. But, in fairness, I shut it down once it started to pop. It seems to be out of passenger pipe to me. A couple of firecracker bangs, 3 run sessions ago.

The wet plugs, it has to be running way rich. But, like I said, this was after about 3 hours of 15 min. runs before the first bang session.

 

[eldubb440]

Car is not knocking, fires right up and very strong.

you mentioned knocking in the previous post....... just clarifying

 

[Griff]

FYI - I've been running car every Saturday for about 10-15 min, since break in. About 3-4 hours total. Since I only have time on weekends, i'm trying to deduce the problem one step at a time. Like mentioned by WileRobby and Dadsbee: tomorrows run order will be: Take out all the plugs again, (I'm sure there wet)...do a compression test, while there out. Then vacuum test, again if someone can tell me best to pull vacuum from and numbers for both compression and vacuum. Then at same time of restart, I'll keep charger on battery during run charging at 12amp setting on charger (only have 2, 12, 50 or 250 amps on batter charger/jumper)
- or I can hook up small 6amp charger Dadsbee?

 

[Griff]

you mentioned knocking in the previous post....... just clarifying

ya, no knocking, I meant exhaust banging

 

[dadsbee]

Then at same time of restart, I'll keep charger on battery during run charging at 12amp setting on charger (only have 2, 12, 50 or 250 amps on batter charger/jumper)
- or I can hook up small 6amp charger Dadsbee?

Run the 12 amp and I'd do it first before you bother tearing anything else apart!

 

[Griff]

Run the 12 amp and I'd do it first before you bother tearing anything else apart!

Wiil do...ya plug removal is a pain
Then, move down my initial suggested list from tonight's great thoughts.

 

[Griff]

Well, I want to thank everyone for there thoughtful comments and suggestions. I had a good time chatting on this terrific forum. Beer and popcorn are now gone, lol. So, I am going to turn in and I'll let you all know here how tomorrow's problem solving tasks go.

Cheers all!

 

[Geoff 2]

The most immediate problem is the charging system. The high 13.XX volts you are reading after charging the batt is the surface charge. With any serious load [ when you crank the engine ] that drops way. 12.53 v, engine running, means the the batt is not being charged. After running awhile, the voltage will drop further & if it drops enough you can get no/intermittent spark....& exh backfires can be a low voltage symptom of a non-firing cyl.

Carb. Engine idling, look in the throttle bores. If you do not see fuel dripping, assume that the carb is ok. It might need fine tuning; black plugs can be from choke on/sticking, or too many cold starts/short runs whereby it never comes off.

 

[threewood]

Yes, it is a Carter Fran Blacker
Fits: 400, 440

Fuel Pump, Mechanical, Muscle Car Series, Chrysler, Dodge, Plymouth, Each

Free Flow Rate: 29 gph

Maximum Pressure (psi): 5.5 to 6.5 psi

Inlet Attachment: Stock

Inlet Size: Stock

I'd be really curious to know what the actual fuel pressure is from the pump. Much more than 5.5psi can open the float needles and flood the motor out. The 13 minute window is curious.

Fran is on to something. Once the car is running and starts to backfire see if you can clamp the rubber fuel line shut to stop the flow and see if it stops backfiring.

For plug wire issues, run the car in the dark and look for arcing from the wires to ground.

 

[Geoff 2]

The simple test in post #56 will tell if it is flooding. Plus, unlikely to idle if it is flooding.


These carbs will withstand a LOT more than 5.5 psi fuel pressure. The original Carter chart went up to 10 psi. It is posted on here in another thread.

The 5.5 psi fuel pressure quoted by Edel along with the 7/16" float level provides the datum point for the carb calibration. Changing either of these parameters will lean or richen the mixture.

 

[R413]

New engines need to be driven to seat the rings. Idling/running for 15 minutes every weekend isn’t helping anything.

The 500 carb is too small for a 440 with a 488/490 cam. So likely not enough manifold vacuum to hold down the metering rods and it’s too rich at idle. Also the plugs are always wet is a sign.

 

[turbine68rt]

Did you confirm that you had the proper lifter preload (as per cam manufacturer) during assembly?