big backfire problem

[Irish_Alley]

some could of passed by the filter and clogged up the carb. but i would fix the filter problem and try it out

[bake74]

ok gotcha. and to clarify it is not like one or two cylinders missing, at the high rpm or wide open throttle every cylinder starts popping and backfiring badly. which I think rules out bad wires, especially since I just got new ones
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      Just a question here, how did you determine it was the cylinders popping and not the carburetor ?

k it was worse than I thought but I don't think they're was enough stuff in it to block the gas too bad.
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      Paper fuel filters are a funny creature.  What happens is they collect containment's and before they plug up enough to become blocked they will slow the fuel flow down.  So when you first fire up your truck it will run great, a couple of minutes later it will be running rough because the containment's in the filter settled when you took off the pressure, now the pressure is on and they flow back into the paper filter elements and slow the fuel flow.  Eventually they will plug all together.
      This doesn't sound like your issue but when they show signs of containment's you should always change the filter, I keep one in my truck along with a 1/4" nut driver just in case.

[75_chevy]

actually it could be the carb I suppose I just thought it was cylinders because I was hearing it through the exhaust.

the filter I was using is not a paper filter it was a metal round one I'm sure the same thing could happen though ?


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[75_chevy]

ok new filter on.. reinstalled charcoal canister.. adjusted timing a couple degrees.. adjust air fuel ratio screws.. better.. still idles low in gear , also pops around 3k now not so much 2100.. gonna keep timing an tuning screws


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[bake74]

     The idle in gear concerns me, your carb. has an adjustment for WIDE-OPEN THROTTLE, CALIBRATING THE PART THROTTLE (Cruise Mode, Power Mode), and CALIBRATING THE POWER MODE STAGING.  Make sure you read the manual you downloaded for these settings, it is where I looked to get the info and adjustments the last time I adjusted mine.

[rich weyand]

I dialed mine in with an A/FR meter.  The tune is on here somewheres.  It would probably work for your setup; your close to mine.  Also, you can probably dial that into 16* BTDC as since you are running manifold vacuum and not ported vacuum.

Ah, here it is.  Read reply #12 in this thread:
http://forum.73-87chevytrucks.com/smforum/index.php?topic=26985.msg225757#msg225757

Also, have you pulled the step-up pistons out and cleaned them lately, or at least shot some carb cleaner down the bores?  They get gummed up.  If they don't move up and down freely, weird stuff can happen.

[CoggedBelt75]

Since you said it ran alright before you replaced the wires, double check the firing order. Be especially sure about #5 and #7 being correct- they're most often the two that will be wrong. Easy to get the wires crossed even by people who know what they're doing. I have caught myself a few times.