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Backfire out exhaust

AkMudr

AkMudr

1/2 ton status
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Dec 30, 2004
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'85 K20, 350/400/208 124k miles.

Ok, I just rebuilt my carb cuz it was pretty bad (VERY rich). Now I'm gettin about 14mpg. It runs awesome,, idle mixture is set perfect, idles awesome has ALOT more power. Problem is when I get on it pretty heavy then let off it backfires out the pipes and sometimes theres a little flame, which tells me there are unburnt vapors (obviously) and possibly a lean mixture could explain it running "hot" enough to ignite.

As usual I used a vacuum gauge to do the idle mixture and set it to the highest I could get with it still idling good, 17 inches of vacuum on both sides.

I have no cat, dual flowmaster 40's out the side. Initial Timing is set at 6*. I'm runnin Delco R44T's and new wires,cap and rotor. New float, everything in the rebuild kit was used where applicable. Also, I epoxied the main wells before I put it back together, they were pouring fuel down the intake pretty badly. lol.

I did have to swap out the base becuase the secondary shaft was a little wore out but reset the idle mixture. Everything is functional on the carb.

It wasnt doing this before I rebuilt the carb so something isnt set right, what are some things I should look at?
 
my truck will do the same thing you speak of with a slight header leak.
Like when you develop a crack in the gasket and just start to barely hear it leaking, i'll get on the gas pretty hard and when i let off BOOOM! it'll suck some cold air in there and ignite the fumes is what i'm guessing?
just a thought.
 
I dont have any leaks but I was told the big backfire could be related to the cold air the AIR pump was pushing into the manifold. But that kinda doenst explain the flame? lol :D
 
I would of said check your spark plugs but you said it was'nt doin it before u removed the carb, well i just checked ac delco website...they call for R43TS spark plugs but that's probably not the problem here.

The only other thing that comes thru my mind now is maybe the EGR valve,is that workin well? i know it will cause pre detonation if not operating but im not sure bout backfiring:dunno:
 
Just wierd, it wasnt doing it until I pulled the carb and rebuilt it. I didnt change the timing, no vacuum lines, plugs are good and didnt cause any backfire issue before I rebuilt it. And it is doing it out both pipes.

Whats the best way to check the EGR valve? Only thing EGR i can think of is the 2 tubes on the pass side of the intake right beside the carb, both are hooked up.
 
try richening the mixture.

Also, check to see if your Diverter valve is working correctly. This is part of the AIR system.
 
the egr valve is bolted on your intake on pass. side, its round about 3 inches diameter, now; u should be able to stick your finger(s) in it to see if it moves freely,if not its gone(replace it). when your eng running, u should have vaccum gettin to it and the little dish inside should be at top. Exhaust Gas recirculation valves get the Nox out of the exhaust and cool down combustion chambers:)
 
Come to think of it I dont remember seeing it, maybe it just hasnt caught my eye. I'll check on that later
 
No egr valve. One of my buds told me my intake doesnt look stock which wouldnt make any sense because all/most of the emissions BS is still on the truck.

No cat, no evap can and no EGR valve. Only egr thing I have on the intake is the 2 hard lines that go into the carb on the pass side.

I remember a while back the I'M referee station mentioned that '85 was the first year of a HD 350 3/4 ton and that my truck didnt come with alot of thigns hte other years did. Anyone else hear that?
 
Update Backfire/Flame thrower

I pulled the belt off the AIR pump and its running great. No backfire, no flames or anythin. But, I cant just leave it off, I hafta figure out why or what in the AIR pump system is causing it to do this. Is it possible the pump is going bad? Possible leak int eh system causing a back flow of pressure when I let off at high rpm? Is there any kind of pump filter that could be clogged?

I'll take any idea no matter how goofy it sounds.

You guys have any thoughts? Suggestions?
 
I hope its not, that litle f n thing is expensive! I'll pull the hoses off and see if anythin is gettin by. That reply never caught my eye, Thanks

If infact it is the diverter valve, is it pure coincidence it took a crap the day after I rebuilt the carb or does it stem from something else I might have done?
 
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how did you fix it?


The carb rebuild and the AIR breaking isn't related. Things like that happen a lot.
 
Pulled some hoses, pump is puttin out air, so I pulled the lines off the diverter 1 by 1 til I found out what wasnt workin. Just to verify I had a good port vacuum and it was actually the diverter valve, I plugged it into another potr onthe cabr, still ****in did it. Its not the pump, thats pushin air so theonly thing left is the valve. Theres no leaks in the air system, but air is being bypasses out the valve thru the little metal screens.

For now the belt is off the pump til I get a new valve, its not very improtant to me but if its a cheap part..fine. I'll be yankin the motor and all the EGR end of summer anyways.
 
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