New some advice

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mase

mase

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Recentley i swapped the heads, intake and carburetor on my 318. after which the engine has started backfiring out of the intake when its in gear and i give it "too much" throttle ( too much being about 2 mph faster than a granny)..since then i have replaced the following: spark plugs, plug wires, distributor cap, rotor button, ballast resistor, coil and had the carb taken apart and inspected. But it still backfires! everybody at the shop i work at is at a loss as to what it could be..Any suggestions?
 
Why replace all that? Seems if it didnt back fire before and it does now it has something to do with the items you replaced right? Timing checks out good?
 
If you didn't already. I agree with checking your timing before you do anything else.
Oh! Make sure all your vacuum lines are connected too.
 
timing was the first thing i had checked after i finished the swap, i replaced all of it just too eliminate any possible causes, and im restoring the car so i figured it couldnt hurt to put in some new parts
 
What intake and carb do you have now ?
and what did you have on your 318 ?
Sounds to be out of time,or a bad vacuum leak.
 
the carb is a 600 holley, intake is cast iron, not sure what from though..Before It had the factory 2 barrel carter and factory intake, im not sure about the specifics though, this is my first resto so this is all pretty new to me
 
Could be an intake valve hung open, a cam lobe going/ gone flat, bent/ stuck valve/ broken spring.

Remember you changed heads. If someone did a valve job and "sunk" some of the valves without adjusting stem height, you could be "hanging one" open

Run a leakdown/ compression check

Determine if you can WHAT cylinder it is by doing this:

"Rig" a way of grounding the spark at each plug. One way to do this is to pull the dist. cap boots up, and run small brads/ nails down beside the wires. Then use a clip lead/ probe to ground one plug at a time. Look at an idle for a "weak" cylinder ---compare RPM, and rev the engine with each one grounded, looking for the backfire. IF you find the one that "changes" when grounded, you can concentrate on that cylinder.

MAKE SURE you did not switch two plug wires, or that you have a bad wire, or cracked spark plug.
 
what kind of intake 600 holley is a square bore carb isn't it could be on a spreadbore intake. what does the intake look like?
 
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