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Old 08-22-2010, 04:01 PM   #1
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360 backfire only when warmed up.

He's a good one that has me stumped.

My 360 magnum will bog and backfire when the engine is fully warmed up. (Meaning, has been warmed up, driven through city traffic for a while.)

The backfire only happens when I stick my foot in it and all 4 barrels are opened up. With just the 2 barrels it runs fine and only when its really warm will it do this.
When it's cold or just getting warmed up, there is no issue at all.
I'm thinking it's heat since it only does this when cruising through town at stop and go traffic....
I have a electric fan in front of the radiator that pushes air when temps reach 190 deg F and shuts off at 150 deg F The stock fan is always pulling air too.
When I let it sit for a few with engine off to "cool down" even though the temp gauge never goes above 190, it funtions properly again.

Stock 360 engine with 750 Edelbrock manual choke carb. Choke is off.
Edelbrock air gap manifold.
Holly electric fuel pump with regulator. Pressure is about 7 psi.
The hose is rubber until the carb. and isn't touching anything.
The electronic ignition and timing is set properly.
I have have good vacuum.

Again it only does this when warmed up driving in stop and go traffic.
Another thing I think it might be is the carb is just too big for the stock 360. Should I try a 650 Edelbrock carb?

Does anybody have any ideas when my problem is???
Thanks!
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Old 08-22-2010, 07:21 PM   #2
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A 750 is kind of large for a mild 360, but I don't think that cause the problem you are describing. Backfires indicate unburnt fuel, which can be the result of an ignition problem. Make sure your plugs, wires, cap, and rotor are in good shape, and make sure that the shaft in your distributor is not work out (wiggle the rotor and check for play, a touch of wiggle is okay, but more than a touch is a problem). Also check your vacuum connections and make sure there are no leaks.
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Old 08-22-2010, 09:06 PM   #3
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Plugs, wires, coil, cap, rotor, distributor, ballast resistor are all new.
And since I don't have a vacuum leak, I think I'm okay there.
Like I said, everything works fine until it gets really warm.

I do think it's fuel related.
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Old 08-23-2010, 04:30 AM   #4
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Backfires are not commonly fuel-related, though misfires are usually caused by a lean mixture or vacuum leak. Ignition related problems are more often the cause, I remember blowing 2 new mufflers to bits when a condenser wire in my distributor came loose (you can imagine how loud a backfire must be to detonate a muffler).

Anyway, it sounds like you are running an electronic ignition set up since you made no mention of ignition points or a condenser. One thing which can cause the problem you are describing is inadequate power to the electronic ignition system. The old points type ignitions would run with minimal power, but the newer electronic ignitions won't. If your alternator fails, or is not putting out much power, your engine will bog, and you'll get the same misfire problem you are describing.

If the problem is in fact fuel related, your carburetor may need to be re-jetted. The AFB uses 2 sets of jets, one for the primary side, and one for the second. The primary jets use step rods and springs which are vacuum controlled. The position of the step rod in the jet regulates how much fuel gets into your engine. The secondary jets are simple open jets like you would find in a Holley. Using metering rods with smaller tips will allow more fuel into the venturi when you step on the gas, you can access the step rods without taking apart the carburetor, they are held in place by two caps fixed by screws on either side of the carburetor, remove the screw, take off the cap, and remove the step rod with it's springs and piston. If you can't find a step rod with a small enough tip, you'll need to install larger main jets, but this involved getting further into the carburetor. You might start with the secondary jets first, you'll need to take apart your carburetor to get to them and find out what size they are, once you have them, pick up a couple more sets in progressively larger sizes, and see which works best. Adjusting the main jets on the primary side will also help, but it's hard to tell minor differences from the driver's seat. Back when I used to tune cars, dynos were rare, so tuning was done at the track, trying different jets, timing curves, etc. and using the clock at the track to see if these mods made the car any quicker.

Good luck,
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Old 08-23-2010, 05:41 AM   #5
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Yeah. Forgot to mention it's electronic ignition with the orange box and coil is hp also.
Everything in the engine bay is basically new.
I have tried the metering rods and spring set up. I found the ones that runs the best for the carb I have. It runs great until I run it around town in stop and go traffic. That's why I'm thinking heat is the issue and since it's only effecting the secondaries.
But what would change to make the secondarys not open up or blap blap blap when I put it to the floor? It will run with the primaries when this happens.
I guess I'm looking for something other than the obvious.

I'm going to try a 600 sccm carb and maybe getting a larger inner diameter fuel line or route the fuel line differently. Oh yeah, the fuel filter is new too
Back to the ol' drawing board.
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Old 08-23-2010, 05:41 AM
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