Backfiring question?

[Red90GTDude]

Ok I am not a super techie so I will do my best. I have a 1990 5.0 Gt 5 speed, Pretty much stock. I am the Second owner with rather babied 145,000. I've had her since 40,000 and do L.O.F ever 5k-7k.
After doing some reading on the net, I took the airbox silencer off around 85,000. I Recently put on a mac 2-1/2" offroad H-pipe, 2-1/2" intermediates, some v-force mufflers, and 2-1/2" tails. Then I put on a 70mm Performance Products T.B. and opened up my stock intake manifold opening to about 69mm(was pretty slow getting it there with a dremmel and $4 cutter tips LOL) I also have my timing advanced to 14 degrees. Since these changes I get a small backfire sound near under where the hpipe is. It is mostly a cold problem but sometimes hard acceleration too and doesn't happen that often. When I put on my new C&L 76mm mass air, it gets quite worse with a LOUD BANG backfire in the same general area... sound wise that is. My exhaust guy says a throttle bodied car shouldn't backfire unless there is a "leak" somewhere? I asked him where he thinks it cold be? and he just kinda shrugged and said who knows?
So that is why I am here? Does this exhuast leak sound right or am I getting to much fuel to air mixture? If that is the case how do I solve it?
Other info. I have rather new 10k bosch platnum2 plugs, new cap & rotor, new fuel filter, recently cleaned/oiled k&n air filter. Real Old (like 100k)8mm Twin Core SplitFire wires. Other than the bakfires I REALLY Like the Sound & Power gains! Thanks very much for the time.
Tom

[~The Jester~]

Wrong end of the log there buddy. INTAKE leak is the first place I would look. Changing throttle bodies makes it worse? Well, that's the first place I'd look. Bad gaskets, on TB, Uppper and lower intake, etc etc. Second thing I'd look at is timing. Maybe she don't like 14* of base timing. But I'd bet my butt you have an intake leak, and she's suckin' air.

Check for intake leaks first, then go from there.

[Red90GTDude]

Hey thanks! Thats helps alot. My T.B. didn't seem to aggrevate the problem but the 76mm C&L mass air really did. I didn't even think of intake or T.B. leak but I really like the sounds of that being the problem What do you think if the air tube from the smog pump isn't tightly welded over the new H-pipe air tube inlet? It was originally just clamped and I kept blowing it off. Then I had it welded on, but lookin at it just today it looks like just spot welds on couple sides and not all the around? Could that cause my bummer backfires? And again thank you for your time!

[~The Jester~]

I can't say for SURE that's the problem, but it certainly isn't helping the situation.

[GhettoPop]

although i dont think its related to your problem, i would take out the borch plugs and put in some autolites. platinum = long life, copper = PERFORMANCE. i would also change those olg plug wires with some ford 9mm or MSD.

[Jeff Chambers]

Exhaust backfire = exhaust leak. There will always be some amount of unburnt fuel in the exhaust stream. The backfire is caused when you introduce an amount of air sufficient to create secondary combustion. Look for exhaust leaks, particularly downstream of the collectors. This is the point where the dynamic pressure is lower than ambient pressure, causing the exhaust to 'suck in' fresh air. This is the same reason that an exhaust leak will cause an A/F meter in the tailpipe to read lean.