Ka-pow!

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mopar head

mopar head

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I have a strange issue with my rebuilt 440, on initial cold start I get a back fire out the exhaust, just one pow then it runs good,
After it warms up, I can turn it off and restart umpteen times with no backfire, heck it`ll start just using the key, no pumping accelerator.
Anybody experience this before? if so, what`s the culprit?
Timing is @ 18 initial.
Thanks, Grant
 
Could be that once you shut it off and it sits for a good while you might be getting some gas leakage (carb) down into the intake the when you start the car the excess gas ignites in the exhaust manifolds/pipes. But I would double check the timing, especially if you back it off and the problem goes away!!??

treblig
 
Treblig said:
Could be that once you shut it off and it sits for a good while you might be getting some gas leakage (carb) down into the intake the when you start the car the excess gas ignites in the exhaust manifolds/pipes. But I would double check the timing, especially if you back it off and the problem goes away!!??

treblig
Click to expand...
The timing thing could be the ticket, when I first fired it up I set it at a more conservative 15deg. I don`t remember it backfiring, That was back in the spring, It was`nt fired since then. "continuing the restoration" I`ve been tracking down an overheating issue, and was thinking advanceing the timing would might make it run cooler.
I`ll have to buck it back down and see. New carb, hopefully that`s not the issue.
Thanks
 
with too much timing, it will kick back when it's hot. how much are you cranking to get it to start?
 
Do you know which bank? I thought back fires through the exhaust were ignition related. Have you verified the wires are on snug and not loose? Have you looked at the plugs?
 
dano said:
Do you know which bank? I thought back fires through the exhaust were ignition related. Have you verified the wires are on snug and not loose? Have you looked at the plugs?
Click to expand...
Sounds just the dr. side. I`ll check the plug wires and plugs tomorrow and inform the findings
It does smell real fuely in the garage more than normal, maybe that carb is leaking down somehow?
 
Funny, my riding lawnmower does the same thing.
 
"Back in the day" my Harley and my 72 duster let out a big ka-pow when they started- LOVED IT!
Like you said only when cold and no other problems. It was a stock 80k 318 I put a 340 size cam, headers, and a holley 600. It would run perfect on the highest octane, but if I put anything less than the highest octane it would run as if it was only hitting on 5 cylinders.
 
JoeSBP said:
what carb?
Click to expand...
Holley 80508S- 4160 model
I just finished checking wires and plugs
Wires all seemed to be snapped on good.
All plugs were sooty and some a little fuel wet. I don`t know if the wetness was from starting and shutting down quick to just see if it still backfired, which it did, so I commenced on pulling the carb to look for anything suspicious, nothing.
The only reason I did that is that back in the spring I was initially first time fire up to break in the cam and had the glowing header issue. A member insisted that it was too lean, so I jetted up the carb, was not the issue, was tight new motor and timing.
I swapped back in the original jets and all seemed well. Weeks later I discovered a shiney ball bearing on the floor, could`nt imagine where it came from but I saved it non the less.
Last night I got to thinking if that was a check ball that fell out of the carb somehow?
I looked on holley`s site and looked at the carbs blown view. The only ball I coud see was on in the acc. pump, but upon inspection there is no way that could of fell out.
While the carb is off and apart do you guys have any idea if a check ball is used anywhere That could fall out if the carb was turned upside down?
That` where I`m at, at the moment, all input is appreciated.
 
have you ever had it backfire out of the carb?
might be the power valve dribbling..
 
JoeSBP said:
have you ever had it backfire out of the carb?
Click to expand...
No. I just got off the phone with holley tech, and he said there is no check ball that could fall out, so I`m stumped at the moment and am going to put the carb back on, fire it back up, lower the timing back to 15, let it cool down and see if it back fires on cold startup afterwards.
 
when it's cold, does it fire up instantly, or does it crank a bit? I know it back fires right away, but if you don't release after the backfire, will it eventually start?
 
Daves69 said:
When you turn it off, watch down the carb bores for any drips a couple minutes.
You might check your fuel pump pressure also.
Click to expand...
I`m seeing no drips. It does`nt like 15deg. timing after hot restart either.
The more I listen to it, it sounds more like a loud crack sound which I would only assume is a backfire.
I don`t know if my custom headers would have anything to do with it, no collectors, transition from 4 tubes to 2.5" pipe. I doubt it cause it won`t backfire after warmup.
 
Seal up your header and collector flanges. I have seen this many times where a bit of fuel in the header pipes explodes and causes the pop. This can only happen if air gets in there. And when you shut off the engine, the cooling pipes will pull fresh air in thru the leaks. There it sits waiting for a bit of fuel and a flame to make it's dastardly KAPOW!. Then the fuel is gone,and the fresh air has been diluted to below a reactive level, and it doesn't happen again until fresh air gets in there, perhaps the next cold-start.
You can check for leakage by severely restricting the tailpipes, at idle, and going hunting .
 
If your cold engine starts,as you say, without at least one shot from the accelerator pump,at this time of year, and you live in the USA, then there is already fuel in the intake, and that should not be there.
This is very difficult to prove, except as noted above in post #20 by Daves69 . You have to sit and wait, watching for the telltale smoke/fumes emanating from the airhorn or the vent-tubes.If you have a fuel-pressure gauge, you will see the pressure begin to drop right after shut-down as the fuel level in the bowls drains. It could be draining thru a perforated PV.

Could the ball-bearing be; the PV anti-backfire protection checkvalve?
 
AJ/FormS said:
Seal up your header and collector flanges. I have seen this many times where a bit of fuel in the header pipes explodes and causes the pop. This can only happen if air gets in there. And when you shut off the engine, the cooling pipes will pull fresh air in thru the leaks. There it sits waiting for a bit of fuel and a flame to make it's dastardly KAPOW!. Then the fuel is gone,and the fresh air has been diluted to below a reactive level, and it doesn't happen again until fresh air gets in there, perhaps the next cold-start.
You can check for leakage by severely restricting the tailpipes, at idle, and going hunting .
Click to expand...
I`ve already checked all header flange bolts, tight.. I`ll have to check collector flange bolts.
 
AJ/FormS said:
If your cold engine starts,as you say, without at least one shot from the accelerator pump,at this time of year, and you live in the USA, then there is already fuel in the intake, and that should not be there.
This is very difficult to prove, except as noted above in post #20 by Daves69 . You have to sit and wait, watching for the telltale smoke/fumes emanating from the airhorn or the vent-tubes.If you have a fuel-pressure gauge, you will see the pressure begin to drop right after shut-down as the fuel level in the bowls drains. It could be draining thru a perforated PV.

Could the ball-bearing be; the PV anti-backfire protection checkvalve?
Click to expand...
The PV seemed intact, I did`nt remove the block just the bowl, to swap out jets, how would the ball come out? and where is it located?
Thanks
Also, I give about a 1/2 throttle pump to set the choke, on the cold start.
 
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