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Old 08-20-2008, 05:09 PM
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Afterfire coming down off of WOT

I recently changed out my intake manifold to a dual plane edelbrock and rebuilt the carb which is a 625 cfm carter AFB on my 350c. Problem is once I reach WOT and start to back off, I get one loud afterfire.

I believe it was running too rich so I swapped out the springs with the lightest and still have the same issue. Because I got the carb used I have no clue what the metering rods/jet sizes are. Anyone know what the carter came with? (cast # 9625 S)

Also on the rebuild I messed with the floats, but I'm unsure if that has anything to do with it.

Thanks in advance.

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Old 08-21-2008, 10:01 AM
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Update

Swapped out the carter for a edelbrock 750, and still the same issue. Maybe just a bad plug or the dizzy cap, need to replace the cap anyways looks like one of the terminals is a little worn. Could this be the cause?
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Old 08-21-2008, 10:44 AM
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Quote:
Originally Posted by Ashler
I recently changed out my intake manifold to a dual plane edelbrock and rebuilt the carb which is a 625 cfm carter AFB on my 350c. Problem is once I reach WOT and start to back off, I get one loud afterfire.

I believe it was running too rich so I swapped out the springs with the lightest and still have the same issue. Because I got the carb used I have no clue what the metering rods/jet sizes are. Anyone know what the carter came with? (cast # 9625 S)

Also on the rebuild I messed with the floats, but I'm unsure if that has anything to do with it.

Thanks in advance.
This isn't all that uncommon, backing down from WOT with the vehicle coasting puts a lot of vacuum on the underside of the carb. This combined with fuel flow inertia in the primary system where there is a momentary continuing flow even though there is no longer a need can combine to drive a rich mixture the engine can no longer use so raw fuel with some air flows into the exhaust where it often explodes.

You can do the usual things like checking float levels to be sure you're not encouraging this with high fuel levels. The air bleeds can contribute, it they are too small the fuel remains in larger droplets that have more inertia so they will flow longer when not need by a suddenly dropped throttle. But I wouldn't start the attack there as this can also be caused by excessive fuel in the circuit as well for many reasons. A dashpot can be used to prevent the throttle from snapping closed. The slight easing of the throttle to closed will give fuel flows a chance to track closer to the diminishing air flow and it helps prevent the sudden increase in manifold vacuum.

The vacuum advance can be connected to a timed port, this will let the advance rate back off when the throttle is closed. If it currently is sourced from manifold vacuum, as soon as the throttle is closed the advance is moved to it's full limit when it isn't needed.

Engines equipped with AIR systems get around this problem because the AIR system is injecting air into the exhaust close to the valve where temperatures are high enough that any unburnt fuel being expelled will be combusted rather than flowing into the exhaust system to go off with a bang. Sometimes drilling a small hole in the bottom side of the header tubes will silence this problem, such a hole shouldn't be larger than 1/16 inch. Placing a dimple into the tube so that the hole can be aimed toward the downstream flow helps keep air going in and exhaust from coming out of the hole. Sometimes putting reversion cones in the exhaust headers also helps by reducing exhust pulse signals from getting thru the cylinder and into the intake. This often helps stabilize the signal the carb sees with the effect of bringing the mixture to chemically correct without making anyother chances to the carb.

Bogie
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Old 08-21-2008, 05:12 PM
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Could the exhaust system be a contributing factor?

He is running shorty headers to true duel 3 inch with what looks to be 2 1/4 inch (possibly turbo) mufflers. This system was installed by the previous owner.
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Old 08-21-2008, 06:11 PM
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Quote:
Originally Posted by 70bird
Could the exhaust system be a contributing factor?

He is running shorty headers to true duel 3 inch with what looks to be 2 1/4 inch (possibly turbo) mufflers. This system was installed by the previous owner.
Could be, I'm probably throwing too much FLAK at once, he shoud start simple and go to the complex.

- If the rotor and cap look like they aren't what they should be, I' replace them first off. If your going to do that I'd also suggest wires and plugs. I guess it would be worth while to fire it up in the dark and look for high voltage that ain't in the wires. That's always a sign that weird things can happen.

- He says he messed with the floats, I'd be sure to get them back on spec toward a lower fuel level. One of the problems with carbs and the Carter and it's clones have is the good ald rich hook in the top end fuel curve, this is almost always a battle between too much jet area and too little air correction. You can lean out the power step a bit which might help, I'd certainly start with replacing metering rods or jets before drilling out air correctors 'cause fixin' them if you get it wrong is a real pain.

I didn't see anything about 3 inch duals, but that's a lot of pipe. If the car isn't being raced to where the RPM levels are really generating enough exhaust to fill 3 inch pipes with a high velocity flow, certainly any fuel in a slow flow will fall out and puddle somewhere in the system. When you back off that flow goes down pretty fast plus large diameter pipes have a lot of surface are so they cool rapidly which causes the exhaust flow to slow even more. A slow flow lets air in from the outlet side and allows fuel to condense and puddle out what it gets trapped in mufflers and eventually goes bang in there. I've seen afterfire explosions big enough to blow clamped mufflers right off a few times and the blow the their contends out the tail pipe on welded systems.

- This can be sparkplug with too cold a heat range that washout on a dropped throttle and then pick up as the mixture gets drying out. The same could happen from a too cold engine, or even too cold an intake. Just about anything that encourges fuel to wet out.

- So there's plenty of opportunity here that needs to be checked out.

Bogie
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Old 08-22-2008, 09:20 PM
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New cap and wires, and adjusted the floats to spec. Still the same issue. I replaced the plugs a month ago and were fine before the adjustments, think one is fouled? I should be getting the metering rods and jets in tomorrow.

The exhaust really is horrible and will get replaced soon (thanks god) but it was fine before this.

thanks for all your help.
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Old 08-23-2008, 09:53 AM
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Honestly, your exhaust wasn't "fine before this". Your system was horribly mismatched.

355 with flat tops
A very mild unknown cam
76 cc heads
625 cfm carter
Holley Street Dominator intake (open plenum)
3" true duel exhaust with 2.25" mufflers
2.56 gears in the 8.2" rear on a mono-leaf

We have installed Edelbrock RPM heads, an old school LT1 intake (duel plane), and an 8.5" 10 bolt with 3.73's on new global west multi-leafs and shackels (del-alum bushed rear). The car moves now.

The 2 carbs we are playing with are a Carter AFB 625 and an Edelbrock Performer 750.

A Holley 1850 600 cfm will soon be added to that mix (primarily to see if it is the Carter and Edelbrock that are causing the afterfire). We'll probably be rebuilding the Holley today along with messing with the metering rods of the Carter.

As is, the motor doesn't see much over 5000 rpm before the throttle drops and we get a good bang.

Just thought I'd chime in.
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Old 09-02-2008, 09:30 PM
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Just following up.

We put the Holley on and guess what, no more afterfire.
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Old 09-03-2008, 07:29 PM
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If you can't tune it, swap it.
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