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Backfire |
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piper62j 
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Joined: 21-July-2018 Location: 34613 Status: Offline Points: 131 |
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 Topic: BackfirePosted: 13-August-2025 at 5:59AM |
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Hi All.. Need help.. 76 Elite 351M 2bl FMX. 57k miles. Plenty of power until I depress the pedal past 1/2 way. Engine then loses power, bucks and backfires thru the carburetor. Take foot back off pedal and all is well.
Here's what I've done. 2 rebuilt and one new distributor, Motorcraft rotor, cap, plugs and wires. 3 rebuilt oem Motorcraft 2150 carburetors / jets .057 - .060. Manifold vacuum at idle - steady 18". Replaced intake manifold gaskets 4 times.. 2- with valley pan 2- just side gaskets. The spark plugs are clean and dry when inspected. No Cats.. Can drive at most any speed until I depress the pedal past half way.. Sprayed for vacum leaks and none were detected. Is it possible the original intake manifold is cracked inside? Hope someone out there can figure this one out. Thanks
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72FordGTS
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GTS.org Admin Joined: 06-September-2005 Location: Ontario, Canada Status: Offline Points: 6780 |
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Posted: 13-August-2025 at 6:54AM |
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What are you running for ignition? Duraspark? And if so, what are you running for an ignition box? What is the base timing and total timing?
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Vince
1972 Ford GTS Sportsroof - Survivor, One Family car GTS.org Admin |
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Booyah45828
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Posted: 13-August-2025 at 9:09AM |
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To add to Vince,
Who is rebuilding the carbs?
Backfiring through the carb is typically from being too lean. Is the egr currently hooked up? |
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Rockatansky
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Joined: 30-July-2010 Location: On The Road Status: Offline Points: 6398 |
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Posted: 13-August-2025 at 10:44AM |
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sticking intake valve, weak or broken valve spring? also possibly a worn valve guide allowing the valve to close 'off the seat', a bad situation that is a classic set up for a valve head separation and a very bad rest of the day. standard recommendation for any 335 Series engine is to replace the stock multi-groove valves with 1 piece stainless units with single groove locks and appropriate supporting hardware (retainers, lock, springs). has the timing chain ever been replaced?  img credit George Pence
Edited by Rockatansky - 13-August-2025 at 10:50AM |
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72 GT Ute
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Rockatansky
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Posted: 13-August-2025 at 10:55AM |
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is the egr plate in place under the carb? if yes is it heavily carboned or maybe burned through internally?
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72 GT Ute
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78FordLtd2
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Posted: 13-August-2025 at 12:53PM |
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I agree. Pull the carb and the EGR plate off and inspect that plate. It is possible that it is burned out or carboned up. The EGR gasket might be toast as well.
Also check the timing chain for slop. If it's original, swap it out anyway as the factory ines from Ford were plastic coated.
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Rockatansky
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Posted: 13-August-2025 at 3:11PM |
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FWIW the EGR gasket between the plate and the manifold needs to be exhaust duty quality, it's not a regular carb gasket. notice this gasket is listed under Exhaust & Emission |
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72 GT Ute
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piper62j
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Posted: 14-August-2025 at 4:21AM |
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OEM Duraspark at 6 degrees base timing.
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piper62j
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Posted: 14-August-2025 at 4:25AM |
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I have 4 2150s. Two I rebuilt and one off Carb USA and one from National carburetors.
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piper62j
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Posted: 14-August-2025 at 4:26AM |
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Egr plate clean and flat. No cracks or hole.. Ran the engine with egr and without egr.. No difference,
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piper62j
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Posted: 14-August-2025 at 4:28AM |
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Egr plate clean and flat. No holes or cracks. Two new timing chains. First one did stretch (aftermarket junk). Installed new cloyes set.. no slop.
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bamatorino73
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Posted: 14-August-2025 at 8:04AM |
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Hey just a thought. Could it be the cam lost a lobe.
I had similar problem, car ran fine until I got to about 2500 rpm. Like I say just a though.
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cdavis 73 GTS 351c Q laster stripes
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Inkara1
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Posted: 14-August-2025 at 9:27AM |
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What condition are the fuel pump and fuel filter in? Do the issues begin immediately upon surpassing 1/2 throttle, or does it take some seconds until symptoms appear?
Do the backfires happen at any RPM beyond half throttle? (So, like, if you floor it at a stop sign, does it do it immediately, or only once it gets past a certain RPM?
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1972 Gran Torino station wagon, light blue, 351C 2V, prior owner upgraded to 4V (2V heads), originally had a C4 but prior owner changed to C6 for some reason.
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78FordLtd2
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Posted: 14-August-2025 at 12:14PM |
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I'd think if the cam lost a lobe, there would be drivability issues across the board and feel like it's running on 7 cylinders instead of 8. The next thing I'd be inclined to do is a compression test both dry and wet to see if there is a burnt valve...possible but not likely. Doesn't hurt to get that done.
Good point about the fuel. Is the pump adequately supplying the carb with enough fuel causing a misfire instead of a backfire? The pump may be getting weak. Just a thought. Does it still have the factory air pump? I ended up pulling off my vacuum hose off the back of the diverter valve and plugging it as my 351 would backfire out the tailpipes on a hard rev after releasing the throttle. Solved that issue and does not backfire on a hard rev. This was caued by the pump supplying air to the exhaust and igniting the unburned fuel in the pipes under deceleration. Your diverter valve may not be functioning properly. Easy fix.
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78FordLtd2
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Posted: 14-August-2025 at 12:35PM |
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I would also try another coil.
Also double ckeck your plug wires and verify the firing order. A 351M is different from a 351 Windsor. Check the condition and the resistance of the wires as well. Timing? My 351M calls for 14* on the tag. Yours might be happier set there.
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Inkara1
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Posted: 14-August-2025 at 2:32PM |
I'm pretty sure all three "351" engines had the same 13726548 firing order.
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1972 Gran Torino station wagon, light blue, 351C 2V, prior owner upgraded to 4V (2V heads), originally had a C4 but prior owner changed to C6 for some reason.
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Booyah45828
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Posted: 15-August-2025 at 4:03AM |
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Well with all of those carb attempts I'd be reasonably confident in saying that isn't it.
How'd the old plugs look? Does it burn oil? My only other thought is the balancer ring slipped, and that your 6 degrees isn't actually 6. You could go through the process of verifying, or you could add another 6 degrees(for a total of 12) and see how it runs. I found ignition timing for a 351m in 1976 should be 10°@ 600 rpm. If you've got an emissions label go off that. I don't think being 4 degrees off would cause a lean backfire, but it's easy enough to try it and see what you get. |
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CobraJoe
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Posted: 15-August-2025 at 4:09AM |
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....or maybe just try setting total advance @ 34° (32°-36° is what I usually aim for)
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1968 Torino GT F Code
'96 Bronco, '39 Ford Coupe, '57 Fairlane, SOLD 2025 F150 SuperCrew, 2025 Edge Platinum |
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72 RS 351
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Posted: 15-August-2025 at 7:18AM |
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I agree about the balancer, those will really mess up the timing if they slip, plus ruin the engine if it's run high and fast enough. I think those should all be a normal Cleveland balancer, for all of the 385 series engines. Those all had the 13726548 firing order, everything else was the now odd one. |
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Don
73 Ranchero "Sport 72 front end", floor shift/console, planning EFI 7000 rpm 351 stroker 73 Ranchero GT 351C-4V &4R70W for sale later. 92 Lincoln Mark VII SE GTC, OBDII 347/4R70W |
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78FordLtd2 wrote: