454 backfiring problems need help

[68black_betty]

So I got a 454 with holley 750 dp, air gap intake mild cam. I been having a backfiring problem when I get upwards around 3500 rpm if I let off the gas I get a pop but if I just ease off it its fine. Runs smooth at idle and pretty much most the time.my front two cylinders on each side are running rich but the back two are fine. Timing is at 17 without vaccfuum and like 30 with vaccuum. I have an advance curve timing kit in that is why its high. The wires are good and just replaced header and collector gaskets thinking it was exhaust. Does anyone have any ideas or ran into this problem before?
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[Shyguy]

I am running 10-12 degrees at idle on my 454 with the vac advance disconnected and plugged.

[68black_betty]

If I go any higher or lower by more than 4 degreed or so it backfires like crazy
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[turp mcspray]

A bad cam lobe will cause similar symptoms. If your ignition has been sorted out, I would pull the valve covers and see if any rocker arms are barely moving.

[Irishpilot]

My vote is high timing. It is always a pain trying to figure that out.

[Lee H]

When did it start doing this, out of the blue, new motor, tune up, cam swap, distributor swap, carb swap? Did it ever run fine? Hard to diagnose a problem without more info.

[LONGHAIR]

A pop when letting of of the throttle could also be an exhaust leak. When you close the throttle quickly cold fresh air is sucked in through the leaky spot. Cackles and pops are the result.

[cam.man67]

Got the same problem with my 350, this is helpful.

[cdowns]

i see lots of people tryin to set timing with not starting out and physically setting the engine to TDC to make sure the timing marks are accurate

[68black_betty]

lee h yeah it used to run fine with the stock manifold and quadrajet 750 that was about due for a rebuild, it started doing the popping with the new intake and double pumper, as for exhaust leak i just replaced the header and collector gaskets, i didnt even think bout the rocker arms i will definitely check that out tomorrow as well as messing with the timing

[aintmisbehavinn]

Sounds like the distributor is off a notch or something. If it started after the manifold, I'd trace my steps from there. Has to fire with exhaust open, obviously, something is way out of time.

[Lee H]

Ok, that makes sense now. First off you need to verify TDC with the timing tab and the rotor is pointing at the number one plug. Also verify all the plug wires are routed right.

[68black_betty]

all of that has already been done, checking the wires and makin sure they were routed correctly was the first thing i tried

[68black_betty]

what rpm range is best for setting the timing, is it at whatever rpm idle you want is there a certain rpm range you wanna be in

[kd71jimmy]

did you fool with the weights in the distributor? I have a 468 in my '86 and the timing was junk because it would not advance due to the springs being too light for the weights that the guy had installed...I put a stock E.I. and it did better...After further investigation, I also had to add a cowl hood to get more air...

[stich626]

Originally Posted by cdowns

i see lots of people tryin to set timing with not starting out and physically setting the engine to TDC to make sure the timing marks are accurate

x2. timing tape is cheap..
the stock ring has been known to move

[68black_betty]

i did, right now i have lighter springs in it, the advance kit i put in had 3 types of springs, so your sayin i should put the thicker springs in. also i planned on throwin a carb scoop on their just to get more cooler air, you think that would help as well