Back firing out of carb

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perko

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Hello,
New to the forum so I hope i'm putting this in the right place, I could use a little advice.

I've been getting backfiring out of my carb on a 318 duster under load, but not when its idling. I've been told that its most likely a worn cam shaft not lifting what it supposed to be lifting. Is there any way to verify this before going all the way to the cam? It would be really disappointing to do all that work for nothing. If i'm going to all that work I might just try and rebuild the top end while i'm at it.
 
pull covers and just look at the rockers when you turn over the motor by hand. Even a flat lifters will show a worn out lobe, especially one thats not lifting an exhaust enough to vent the cylinder. Also make sure your firing 18436572 CW
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Also check your timing and make sure your advance plate is actually moving: pull cap and suck on vacuum line, your plate should move and snap back smoothly.
 
Is it a random pop through the carburetor or is it rhythmic and repetitious? If it is repeated and has a rhythm and changes with engine speed, it is probably an exhaust valve that is not opening. Whether that is a worn cam lobe and lifter, bent or broken pushrod or broken rocker arm, you will know on teardown.

I agree though to double check the firing order first. Always check the easy stuff.
 
is it getting enough gas maybe a lean condition or a pick up coil .
 
Sorry, I should have added more information. The timings fine, plugs are where they should be, vacuum is good, just replaced the pickup coil and finally got it started which was when I noticed the backfiring. It is a consistent rhythmic sort of popping under throttle. I don't hear a lot of lifter noise or anything like that but its a little hard to tell it doesn't sound so healthy.

I guess If i'm going to open it up I should just replace everything within reason huh?
 
Well, a compression test, and a leakdown test, could help.But might not either.
Say, was this condition pre-existing to the pick-up coil change?
I guess this is a steady state thing, not involving any throttle tip-in?
It could be a bad pug too
Hard to imagine the vacuum would be good with a valve going down. http://www.earlycuda.org/tech/vacuum2.htm
 
I'll check the vacuum again, but I think it was good when I did the timing after putting in the new distributor. You're right, though I would expect that to change something. I think it was an existing problem, but I bought it recently and haven't been running it to hard because the tires are so scary. I was on my way to get new ones when the pickup went out. It appeasers to be a steady state thing, not tip-in.
 
if its coming from carb its the distributor. turn the engine to drop dead center and do a napkin test to find #1. then go from there, your number one is probably wrong. I had this problem and it was my dist the whole time. My #1 was completely wrong. Then keep moving the dist little by little to get it to the right spot and wala your back on the road.
 
I am going to say at this point it is a mechanical failure between an exhaust rocker, pushrod, lifter and cam lobe.
 
Unfortunately I think rustyratrod is on the right track. I'm pretty sure I got the distributor and timing right.

I'm thinking about tearing into it and re-doing the top end while i'm there. I'm sure it needs it anyway. If one valve isn't operating as it should the others are probably following like lemmings. I was hoping to enjoy the thing a little longer before I had to get crazy but that's life i guess. This would be the biggest project yet for me. Does anyone have any suggestions as far as resources? I've seen a few books out there on "how to rebuild the chrysler small block", but not a lot of detailed write ups on websites. I would really like to have everything I need in a nice pile instead of taking over the garage for a year

PS thanks for everyone's time, i can use as much advice as possible.
 
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