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Old 04-17-2011, 04:48 PM   #1
Foosified
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Join Date: Oct 2006
Location: Edmonton, AB, CAN.
Posts: 152
SBC 350 Engine Backfire Through Exhaust and Carb

1979 350 block stock crank, pistons, rods. New rings and seals.

I added;
- Vortec heads stock other than new valve springs to allow for larger valve lift on vortec heads from Alex's Parts http://www.alexsparts.com/products/V...YD-ROLLER.html
- Howard's cam and liter kit 110991
.470 lift on int.and exh.
284 deg of duration
108 lsa
- MSD HEI ingnition
- new plugs and wires
- flotech headers
- Edlebrock Performer RPM intake
- Edelbrock 1406 carb

Let me know if you need to know any other info on the engine.

timing is at 12 deg adv.
compression is 155 - 165 psi (checked cold)

Backfiring happens on start up fast idle. Wife says it sounds like a popcorn machine. Pretty sure it is happening on both sides of the engine.

When I kick it down the popping stops, but when I rev up to 1700 - 2000 it comes back and if I hold it and then burb the throttle it sounds like I let off a round from a shotgun.

I have quadruple checked the lifter/rockers for overtightness. 100% sure that is not the problem. I have checked for vacuum leaks everywhere and cannot find any.

Not sure where else to look other than the POS carb, which is a couple years old, but has only seen maybe 5000km's. I had it on this same engine before all the add ons and while it was never tuned up as good as it could have been the previous set up never had a backfire through exhaust issue only the ocassional through the carb. It definatley could use some fine tuning, but I'm not sure if that is what would cause the backfire through the exhaust.

The second issue is the backfire through the carb. This I understand occurs beacuse of a lean condition, correct.

I'm at a loss. i have checked everything I could think of and researched. The only thing I have left to do is take it to a mechanic and bend over while I pull my pants down.

I'm about ready to put an LS motor in and be done with it, but knowing my luck I would have to show a whole bunch of loving too before it would run without issues

Any help is appreciated.
BG
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Last edited by Foosified; 04-17-2011 at 04:50 PM.
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