Backfiring through carb..why?

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c4mo
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Backfiring through carb..why?

Post by c4mo » Thu Dec 29, 2011 6:59 pm

I recently put together a sbc, in which I am having a problem with the engine wanting to back fire through the carb right before it starts. The timing is 16 degrees initial, and I have verified the lash and other mechanical aspects. I do know that the carb is rich, and this is making me
Think it's possibly the problem. Any ideas? More info can be given as needed. Thanks
Morgan Allen

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Re: Backfiring through carb..why?

Post by JohnsnsFord2@aol.com » Thu Dec 29, 2011 7:15 pm

firing order correct?
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c4mo
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Re: Backfiring through carb..why?

Post by c4mo » Thu Dec 29, 2011 7:27 pm

Yes, once the engine starts it is fairly smooth, until a high no load rev, when it will pop, but not backfire.
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Re: Backfiring through carb..why?

Post by FastBuick » Thu Dec 29, 2011 7:37 pm

Yes a rich condition will definitely cause that. You are actually lighting the excess fuel that is laying in the intake. A lot of guys have that problem trying to start a race car in the cold weather by pumping the throttle too much when starting it or taking too long to start it which also floods it or causes a lot of excess fuel to be in the intake.
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Re: Backfiring through carb..why?

Post by Dodge Freak » Thu Dec 29, 2011 7:41 pm

I would double check the lash.

What do you mean the carb is rich, s the exhaust smoke black, does it just seem rich, if its a non heated intake and its fairly cold outside the exhaust will tend to stink more till it warms up.

Why not just drop the jets down 2 numbers or jet it at stock settings and go from there

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Re: Backfiring through carb..why?

Post by ProPower engines » Thu Dec 29, 2011 8:14 pm

c4mo wrote:Yes, once the engine starts it is fairly smooth, until a high no load rev, when it will pop, but not backfire.

Is the pop continuous or just a single pop?
Also how much time is on the engine? Is it possible an intake valve is not sealing completely causing the pop or backfire issue.
Lastly have you confirmed the valve lift is correct on all the exhaust valves? Heaven forbid you are loosing an exhaust lobe has that been on your check list? A bad valve seal or exhaust lobe issue could also cause the issues you decribe.
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Re: Backfiring through carb..why?

Post by raynorshine » Thu Dec 29, 2011 10:01 pm

popping/backfiring thru intake, carb.....

1) too lean..
2) valve(s) adjusted too tight..
3) removing lobe(s) from camshaft..usually exh lobe..
4) timing incorrect..
5) bad plug wire(s), sparkplug(s), cap, rotor, etc..
6) rinse, repeat..
7) and likely many more..
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Re: Backfiring through carb..why?

Post by c4mo » Thu Dec 29, 2011 10:28 pm

The engine is fresh, just got done with the cam Breakin, I used Valvoline racing 20w-50 with Lucas Breakin for 20 min at 2000 rpm, once started it down not pop, except at high rev it will pop out the exhaust. It is a single plane, not heated. Fastbuick what you said is basically what's going on. I'll recheck the lash, and then take some jet out of it. It smells rich, Thats why I thought it was, I have a wideband, I'll hook it up also, and go from there.
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Re: Backfiring through carb..why?

Post by Matt Gruber » Fri Dec 30, 2011 7:07 am

had what sounds like the same problem 12 yrs ago.
friend said it is quite common on his low buck rebuilds.
he said just drive it around.
i did, and within 50 miles it broke in (valve seat to valve) 1st 10 miles were awful :lol: i've since seen stubborn guys take off theheads and have them touched up. :roll:
the problem was, i bought bare heads, and the pre-installed seats had a slight distortion from being pressed in. being a low buck build, i wasn't about to take them to a shop.
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