383 backfiring?

[BoomerSoonerVA]

I have a 72 challenger just finished rebuilding it. Took it for a drive runs great except when I hit the acceleration hard it backfires. Any ideas?

 

[Chryco Psycho]

Usually this is a lean condition so not enough pump squirt as the throttle is opened or play in the accelerator pump linkage or low float level in the carb or the jetting is too small . What type of Carb are you using ?

 

[js29]

Is it backfire exhaust or intake? just to eliminate A cross wire check the firing order. 18436572, counter clock wise. what is your timing set at?as chryco stated it could be A lean condition. it will pop through the carb . you could also check your spark plugs and see if they are white.

 

[BoomerSoonerVA]

Edlebrock 1407. timing at idle is at 25 advance if I try to put the timing at 10-15 the car won't start. Also its coming from the carb.

 

[moparleo]

Is that 25 Degrees initial timing or running ? It should be closer to the 10-15 degrees initial. Other wise way too advanced . Original harmonic balancer ? Is tdc true ? Type of ignition ? Tell us what we are working with beside a "rebuilt 383".

 

[BoomerSoonerVA]

New balancer, electronic ignition. I know it should be closer to 10-15 but if I put the initial timing to there and shut the car off it wont crank over until I move the timing.. Yes it is 25 advance of TDC.

 

[DetMatt1]

I'd start over with the timing. Take the number one plug out and and crank it over at the starter relay with the key off in order to be sure you find TDC on compression stroke. The compression will move your finger if you're holding it just over the hole but don't try to seal the hole with your finger. After you have done you're best to get the #1 to the top have a look at the timing mark on the balancer. If you're within a few degrees either side of TDC the timing mark on the balancer should line up with the tab on the chain cover.

 

[js29]

what mat said, then I would look to see were the rotor is pointing. you could be off on your timing chain,seen that happen before.

 

[DetMatt1]

Exactly, that's next.

 

[Chryco Psycho]

Form the first post you say it starts& runs well just backfires when you jump on the throttle so my guess is your timing marks may be out of line but 25* is working well , so I still believe you have a lean condition more then a timing issue , I always time engines by ear so bring the rpm up to 3000 rpm & advance the timing until until the RPM quits rising , turn it back slightly until you hear the RPM just start to drop again & your timing will be set right no matter what the marks say .

 

[BoomerSoonerVA]

So when I rebuilt the engine I replaced the spark plugs with the exact same that was already in the car. So I decided to order new ones today and I received them and the thread part on the plug is a lot shorter then the ones that are installed. So we will see if that's the problem.

 

[Daves69]

So when I rebuilt the engine I replaced the spark plugs with the exact same that was already in the car. So I decided to order new ones today and I received them and the thread part on the plug is a lot shorter then the ones that are installed. So we will see if that's the problem.

What are the numbers on both plugs?

 

[BoomerSoonerVA]

The new ones are AC delco 7. I will have to look at the other ones when I get home.

 

[BoomerSoonerVA]

The ones currently in the car are autolite 5224

 

[Daves69]

Standard Champion plugs for the 383 around '70 were J-11Y which were the short reach. No longer made. Their replacements were J12YC, and now RJ12YC, if available.

Specs from the Champion site.........................
COPPER PLUS - PART #14

Plug Type RJ12YC; Gap .036; OE Type | Per Vehicle Qty: 8



SPECIFICATIONS▲

Plug Type: RJ12YC
Seat Type: Gasket
Hex Size (Inch): 0.813
Hex Size (MM): 20.65
Thread Size (Inch): 0.551
Thread Size (MM): 14
Thread Reach (Inch): 0.375
Taper Diameter (Inch): 0.31
Installed Height: 2.24
Insulator Projection (Inch): 0.09
Gap (Inch): 0.033-0.038
Gap (MM): 0.84-0.97
Center Electrode Diameter (Inch): 0.1
Center Electrode Material: Nickel Copper Alloy
Ground Electrode Material: Nickel Copper Alloy
Classification: Champion Copper Plus
Resistor Plug: Y

The long reach plugs were for the small blocks.

 

[BoomerSoonerVA]

Thanks going to install the new plugs and see what happens.

 

[Adam]

You can verify the balancer timing marks with one of these tools. bring up the #1 piston until it stops, and then do it again from the other direction. TDC should be exactly between the stopping points. only $16.

http://www.autozone.com/test-scan-a...tyle-heads-on-top-dead-center-stop/85871_0_0/

 

[BoomerSoonerVA]

I changed out the spark plugs to the right ones. Now it only does it once in a while when i pouch it..

 

[BoomerSoonerVA]

So i went got the TDC tool. Found out TDC is saying 17 on the balancer thanks for the help.

You can verify the balancer timing marks with one of these tools. bring up the #1 piston until it stops, and then do it again from the other direction. TDC should be exactly between the stopping points. only $16.

http://www.autozone.com/test-scan-a...tyle-heads-on-top-dead-center-stop/85871_0_0/

 

[moparleo]

Yep. Always start at the beginning when diagnosing. Always double check the obvious.