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360 backfiring through carb

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FreeMoCo2009 View Drop Down
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    Posted: Aug/11/2022 at 6:31pm
Hello everyone!

At a bit of a loss and have tried researching this one, but can't seem to get a solid answer. I have a 360 in my 1970 Javelin that we recently got running. Me and some family friends managed to break in the cam with no issue, but I discovered that I hadn't installed the intake manifold pan correctly and had to redo it (burning oil in 2 cylinders, not enough to prevent the cam breaking in though). Redid the manifold, changed the oil, got it all back together, everything seemed okay.

Car had issues starting; it wouldn't idle unless it was already warmed up, and to do that, I had to crank down the idle position screw until it was turning at 1500-2000 RPM right at the start. Once warmed up, I backed off the screw, and everything was okay again. Got the engine to idle at 850-900 RPM as per the TSM, throttle response was sharp, engine sounded wonderful. Unlike the night we did the break-in, the power brakes seemed to work a lot better (booster felt strong, pedal went right to the floor). Shut it down to top off trans fluid (trans was already mostly full, but it needed a few extra quarts, and the dipstick was too hot to pull out next to the headers). Went to fire up the engine and nothing seemed to work right; it went to about 300-500 RPM idle, and whenever I bumped throttle, ever so slightly, it would backfire out through the carb. I backed the timing down, upped the idle speed screw again, but couldn't get it to work again, same issue as before. More fireballs, more concern from me. I tried a few times before I eventually quit, out of fear of potentially setting things in the engine bay on fire, or doing damage to the engine as a whole.

Admittedly, I'm new to building cars and fiddling with carburetors, so I'm at a bit of a loss. I'd like to avoid any potential damage, so the Javelin remains grounded. Other forums and posts on here seem to indicate that it could be a number of things; I don't think I have any more vacuum leaks, but aren't completely sure how to safely check, and almost all of the parts on the engine are new. Motor has good spark and the carb seems to throw fuel just fine, but I'm confused on where to proceed.

Specs and numbers for assistance:
360 V8 using 93 octane gasoline
Edelbrock AVS-2, 4-barrel carb, 650-CFM (I think, need to double check).
Edelbrock Performer AMC intake manifold with adapter plate for spread-bore carb to square-bore intake (non-airgap)
Stock internals; low compression 1970 rotating assembly with cast crank, rods and dished pistons (was a 2-barrel motor originally)
Heads: Original cast iron AMC heads, 322 code (off of a 1974 360 I believe)
Cam: Comp Cams A8 260H-10, hydraulic flat tappet, 260 duration on both exhaust and intake, 212 duration @.050 on both lobes, .447 valve lift on both, 110 degree lobe separation
Ignition: Davis Unified coil-on-distributor unit feeding high-voltage rated Summit Racing plug wires and NGK platinum spark plugs.

I will have to double check the timing at this point, as I was moving it all over the place, backing it up to avoid potential fireballs out the carburetor. Also, in case it isn't already clear, motor runs really low compression due to the parts used lol. Also running plenty of accessories (power brakes, power steering, automatic trans), so I opted for a not-so-radical cam.

Any and all help would be appreciated. This is still very much a learning process for me, and the car has been very good to me so far. Just hoping it stays that way!

Thanks in advance!
Cheers!
-Brad L.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BassBoat Quote  Post ReplyReply Direct Link To This Post Posted: Aug/11/2022 at 7:05pm
Popping through the carb is usually a bad accelerator pump.  On the edelbrock you can select a different hole for the actuator rod and get a little more aggressive pump action.  I'm a little rusty on Carter/Edelbrock but my recollection is there are check valves in the accelerator circuit and they were a problem for me.  Other issues that can cause the popping are excessive lean conditions or timing way off or bad spark plug wires.  Sometimes even having the spark plug wires run parallel can cause a cross fire, spark plug wires should cross, not run parallel even though it looks neater if they are parallel.

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Post Options Post Options   Thanks (1) Thanks(1)   Quote Heavy 488 Quote  Post ReplyReply Direct Link To This Post Posted: Aug/11/2022 at 7:58pm
Were the plug wires removed?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mstrcrftr Quote  Post ReplyReply Direct Link To This Post Posted: Aug/11/2022 at 8:39pm
did you by chance happen to leave off a vaccum line?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mmaher94087 Quote  Post ReplyReply Direct Link To This Post Posted: Aug/11/2022 at 9:11pm
Timing chain problem.  It' probably off by one tooth.
Mike
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 73hornut Quote  Post ReplyReply Direct Link To This Post Posted: Aug/11/2022 at 9:45pm
Pull the distributor and check the gears.
71 Javelin
74 Gremlin
79 Spirit AMX
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Post Options Post Options   Thanks (0) Thanks(0)   Quote rgsauger Quote  Post ReplyReply Direct Link To This Post Posted: Aug/11/2022 at 11:11pm
^^^. Easy and quick.  Yank the dizzy and take a look.  
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Gench Quote  Post ReplyReply Direct Link To This Post Posted: Aug/11/2022 at 11:17pm
Based on all the idle screw adjustments, I’d say vacuum leak. Since it changes when warm, I’d guess you have a constant leak with added leak or timing from a temp vacuum control on intake?

Hard part is it ran fine till you added fluid. Lots of good ideas.


Edited by Gench - Aug/11/2022 at 11:23pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote troutwilly Quote  Post ReplyReply Direct Link To This Post Posted: Aug/11/2022 at 11:30pm
All of the above, and:
So you've had a backfire through the carburetor.  Had that happen to me.  It puts pressure on all the vacuum lines so check and make sure all the unused ports have their caps on them.  The unused small port on my brake booster manifold fitting had its cap blown off, I'm (pretty) sure it was once there, and once replaced all was fine.  Also check the brake booster and, if you have one, the vacuum advance canister to make sure they can hold a vacuum.

Also, how much disassembly did you do when removing/replacing the intake manifold?  Did you remove the distributor or just remove the cap?  I suggest checking the advance with a timing light, not just turning it down because you think it might be too far advanced.  You can probably have at least 12-14 degrees initial timing on that motor. And if the timing mark is jumping around suspect the distributor gear or timing chain.  Even look under the cap for missing electrodes, especially the center one.  Disclaimer--I plead ignorance on DUIs.

Good luck and keep us posted.
Bill O.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote troutwilly Quote  Post ReplyReply Direct Link To This Post Posted: Aug/11/2022 at 11:33pm
Just thought of another simple check.
When you did he intake manifold you probably had to get the wires out of the way.  Are you sure you got the plug wires back on in the correct order?
Bill O.
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