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Discussion Starter #1
OK, I'm pretty much at my wits' end with this, and I'm sick of spending money replacing parts I don't need to replace. My engine is popping and banging out the exhaust starting at 4000 RPM. Nothing I do seems to change the RPM or the loudness of the popping. I'm talking about just free revving in neutral, and it does it whether it's a quick rev or slowly increasing the RPMs. (It does it under load too, but I haven't studied that as extensively.)

Here's my setup:
454BBC, open chamber iron heads, vintage Edelbrock TR2X tunnel ram, Holley 660 center squirters rebuilt by Sean Murphy Induction, Aeromotive S/S pump, -8 line from pump to Holley 12-803 regulator, -6 lines from reg to carbs. Fuel pressure read right after reg and set to 6.2 PSI or so. Crane solid flat tappet cam with recommended springs set at just a little less than the spec height, should be approximately 10.2:1-10.5:1 compression, 2 1/8" headers, dual 3" exhaust with no crossover, MSD Digital 6 Plus ignition box, Accel 71000E "Billetproof" distributor with mechanical advance only, Accel 8140HV coil, MSD Super Conductor 8.5mm plug wires (I ohmed each one and they're all good), total timing 38° all in at 3000 RPM.

Here's what we've observed and checked:
It idles OK and has great throttle response with small and medium throttle blips. If you go from off-idle to WOT, it stumbles, so it needs squirter work, but that'll have to wait.
If you look into the carbs as you're slowly revving the motor, you can see fuel start to come out of the boosters, and it's a dribble: not at all atomized. We've tried it with the fuel pressure as low as 5 PSI, and it still does it. Fuel level is set such that it just seeps out of the sight plug holes with the fuel pump on and the engine not running. Jets were 76 all around, and then we dropped to 70s in the primaries and 73s in the secondaries, which didn't help at all. It gets as rich as 10:1, which is not good, but I'm not convinced that's the problem. Joe Sherman says the jetting in these carbs rarely needs to be altered more than a couple numbers, there are no power valves, and the plenum of this tunnel ram is the size of an Olympic swimming pool, so I'm doubtful that I should go down any further than #70 jets.
Sprayed ether onto every conceivable vacuum leak. The secondary throttle shaft of the front carb seems to leak a little (the engine picked up about 100 RPM when we ether'd that spot).
Checked the primary and secondary resistance of the coil. Primary should be 0.6 Ω and it's about 0.8 Ω. Secondary resistance should be 9.5 kΩ and it's about 10.6 kΩ. When I was mocking up the coil and bracket for mounting, I remember I had the coil sitting on the heater core cover, and I knocked it off by mistake, so I might have ruined it. Tried an MSD Blaster HVC coil, and that slightly smoothed things out, but it still popped at 4000.
I had Champion RV15YC4 plugs in it, and my Ford friends hate Champs, so we tried a set of Autolite 24s, and that didn't help either.
We've also played with the timing, setting it perhaps as low as 34° total, and that makes no difference either.
Double- and triple-checked the plug wires for correct firing order and the ignition for correct wiring and settings on the little dials. Ignition box is grounded to passenger side cylinder head, battery is grounded to block. Power goes from the battery to a terminal on the starter, and the MSD positive feed goes to an eyelet on the same starter terminal. That doesn't strictly comply with the instructions (connect power wires directly to battery), but I called MSD and the tech I spoke to said the way I have it should be fine. Just to cover my bases, I switched the polarity of the magnetic pickup wires, and it didn't even try to fire up, so I believe I have that right. Tested the ignition according to MSD's instructions, and there's a big, fat spark in the ~1/4" gap between the coil wire terminal and the valve cover. The light on the box flashes when it sparks, and then stays OFF. Not sure if it should stay off, but it comes back on if you cut power to the box and then turn it back on. One strange thing is that if you hold the coil wire and you're not grounded to anything, you still get shocked when you test for spark. I even stood on a spare tire, not touching any part of the car, and I still got shocked!


Anybody have any ideas?
 

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Discussion Starter #3
It'll rev over 4k, but it pops and bangs quite loudly.
 

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Discussion Starter #5 (Edited)
Yes. I don't have the pink or blue wires connected to anything, but the 2-step was set to 4000, so I thought that might be it. I reset it to 7200 or something like that, and there was no change. The retard is set to zero, and again, it's not connected to anything.

As far as springs, I had the motor together for 2-3 years before I got done with all the changes I was making to my car, so some of the outer springs were compressed for that length of time. But it revved OK to 4000 on just the outers, so it seems like we should have gotten a few more rpm from re-installing the inners. We also installed 0.015" shims when we put the inners back in.
Edit: maybe I should see if I can borrow an on-car spring tester from someone? Are you thinking the valves are bouncing on their seats?
 

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Discussion Starter #7
But it runs so well under 4k. I have friends who wiped a cam, and then wiped the replacement cam, and they say it ran crummy at all RPMs. After we eliminate the possibility of ignition problems, we'll put a dial indicator on the pushrods and see if we still have lobes on the cam.

Here's why I personally would be surprised if the cam was wiped. For break-in, we slightly overfilled the crankcase with Valvoline VR1 straight 30 and Rislone zinc additive so that we had at least 1675ppm zinc, we had generous but not globby amounts of Crane assembly lube on the lobes and lifter faces (but NOT the sides of the lifters because that can prevent them from spinning properly), and we only used the outer valve springs. The lifters are Howard's Direct Lube with the 0.021" oil hole. We prelubed the motor, turning it over a little at a time until we had oil coming out of all the pushrods. We fired it up and went to 2500 RPM immediately, and varied it from 2300-2700 or so for 35-40 minutes. If all that didn't keep the lobes on the cam... then perhaps I should live in a padded room and wear a straight jacket.
 

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Discussion Starter #8
I should add that I deburred the oil holes, verified that each lifter had a crown to it, and thoroughly cleaned them with carb cleaner before lubing and installing them.
 

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Discussion Starter #10

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Butch
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How much should I jet down? Go to a 66 or so?
May not be in the jetting

At 4000rpm you could only be tipped into the throttle .030" or so on each butterfly, and if this is the case the butterfly is barely open, you can drill an air hole in EACH far corner, or each inner corner butterfly so when it is tipped a little but still somewhat flat, your letting more air in the engine to lean it out

I would start with like an .031" hole in 2, such as the two outer ones, or the 2 front inner ones, then if I needed more, proceed to the opposite 2, and keep going from there
 

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Discussion Starter #12
The butterflies are open enough so that I can see fuel dribbling out the boosters. Does that change your recommendation?
 

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Butch
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The butterflies are open enough so that I can see fuel dribbling out the boosters. Does that change your recommendation?
Nope

If you can see the top of the butterfly, its closed far enough to where the bypass hole will lean it out
 

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Butch
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As to the vacuum leak, cover up the carburetors with your hand and see if it keeps running or stalls
 

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Discussion Starter #15
OK, can you describe a little more where to drill the holes? And are you sure I should drill such a big hole right off the bat? I didn't think my cam was rowdy enough to require holes in the butterflies.
 

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Butch
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A .031 is a LITTLE hole
X2, and why I suggested doing say 2 front barrels then if it needed more, do the 2 rear barrels

With a 750, 1.6875" butterfly, 1.401" venturii, I run 1 .125" hole in each butterfly. Yes the camshaft is large (278/284 @.050" 106° lobe center) but we also ran that same carburetor on the Camaro. One of the fastest carburetors on every engine it has been on
 

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Butch
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OK, can you describe a little more where to drill the holes? And are you sure I should drill such a big hole right off the bat? I didn't think my cam was rowdy enough to require holes in the butterflies.
Just so we are on the same page, as far as description is concerned, lets talk about a standard 4150 carburetor sitting straight on the manifold

As you open the throttle, the primary blades towards the front fuel bowl raise to pull them wide open.

As you open the throttle, the secondary blades towards the rear fuel bowl raise to pull them wide open.

So, as you can see, an air hole on the side of the butterfly facing the metering block will have the biggest impact on adding air to lean it out

Now, lets say the throttle is closed and the butterfly is sitting at 9 o'clock and 3 o'clock, at that low RPM of 4000rpm, I would be willing to bet the side of the butterfly toward the metering block is at about 10 o'clock and 4 o'clock, you can see how the air bypass on the metering block side would have a greater effect

With poor signal your way too rich, it would take extensive mods to make it work, but I would rather have a high volume of fuel and have to lean it out at a given throttle %

An .031" hole, is a little bigger than the emulsion holes in the metering block.
 

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Discussion Starter #20
OK, you're right, 0.031" is a pretty darn small hole, especially compared to the 1/8" holes some people have in their butterflies. I've read about people starting with holes as small as 0.019," which is why 0.031" seemed big.

Plug gap was 0.042 or so with the Champions, and we opened it up to 0.045" with the Autolites. MSD says you can run as much as 0.050-0.060" gap with up to 10.5:1 compression.

I took the carbs off last night. The primaries were exposing about a square of the transfer slot, which I think is darn near perfect, and quite possibly one of the reasons the throttle response is so fantastic. If I drill holes in the butterflies, it's going to throw off the nice idle I have going on. If the popping is because of the rich condition, then I really think the problem is the fuel coming out of the boosters. It's supposed to be an emulsion, but it sure doesn't look at all emulsified to me. I'll see if I can get some video tonight...
 
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