Backfiring out carb and wont run right

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davalf

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Another question for the experts.

Fresh rebuild on a 360 with a refurbished Edelbrock 1407 750cfm carb from national carburetors.

With a cold start, the engine will start rather quickly and then sputter out. Restart and pump gas pedal to keep it running and it will spit fuel out the top of the carb and sometimes spits fire. After a couple more restarts, the engine is warm and I can get it to idle. When driving, if I give it just a little of gas, the motor will hesitate and act like it wants to die, if I give little quick shorts bursts, it responds well. I haven't really driven the car because I am still putting it all together.

I have checked the timing which seems to be good. I changed fuel pumps but I am getting the same issue. I am constantly adjusting the carb trying to fix it but the issue persists.

I just cleaned out my fuel tank and installed a clear fuel filter just before the pump. When running, the filter will fill up to just below half. Am I just not getting enough fuel? But I had this same issue while running with 3 feet of new fuel line to a gas can.

Any help is appreciated.

Thanks
 
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Had same problem with my 73 340 Duster and gave up and move to next project til I feel like goin back.
Would love to see what it ends up being in your set up so I can target a specific item.
 
Either you have a timing problem, or a vacuum leak.
I know you chked the timing, but what else has to be timed? (The cam)

Just make sure you double chk the easy stuff first, like firing order on the plug wires and so on.

And yes, make sure your accelerator pump works right first.
I was under the impression that it was totally undrivable.
 
buddy called me after getting a new Edelbrock on his 390 Galaxy. Ran like crap, no idle, etc. turns out the power brake port was wide open in the back and the plug was rolling around the bottom of the box. Sounds like a vacuum leak to me, and check your plug wires, could be 180 out. I had a 351C that actually started 180 out!
 
In no particular order

vacuum leak

carb AFU IE gummy, floats too high / flooding, dirt in needles seats/ pump problems

timing / ignition. Use a piston stop. Check your timing mark. Check that the advance mechanism is working and the timing is where you think

CHECK THE PLUG WIRES

Valve hung open, IE bent valve, damaged valve gear mis adjusted valve, etc
 
Check for a vacuum leak. This is the first thing that comes to mind with the symptoms you describe.
 
Sure sounds like vacuum or timing thing (spark or cam) to me. Also, on your initial start....did not look it up, but does the carb have a choke? If it is a vacuum leak and the choke is set correct, then you are way too lean and will not run when cold. I guess same would hold true with the timing thing somewhat......just throwing out some pennies.
 
Couple more pennnies...I don't have a choke and have to run for about 90 seconds to idle down. It then gets crisp on the throttle. I am at 24 initial on timing, but pretty hefty bump stick, .614 with 256 @ .050. If you don't find a vacuum problem, advanced your initial to where your starter is just starting to slow turn, then tweak it back about a degree or two, you should be in the park. Just remember, you don't want to end up with too much total advance, somewhere in the 32-36 degrees range.
 
Sounds to me like something not right in the carb.
 
I would recheck float levels, making sure the needles and seats do their job to stop fuel from dumping in. Adjust accelerator pump and get initial timing where it wants to be like aengingguy described. My 440 wants 20-21' initial, cams with 110' lobe separation want more initial, I then had to recurve distributer down to 10-12' mechanical to give me the 32-34 total I needed. Before reworking timing I had same problems as you. Do one thing at a time so you know what's working. In my case, my stock six pack set up was way lean for the .040 over 440 with match port eddy heads and aggressive cam. (Backfire through carb and spit all over). Timing dial in, big bump in jetting, vacuum secondaries and floats set correctly, and acc pump shot increase and she's starting to run like a raped ape. It's been a long slow road for me since I'm not a professional mechanic, a lot of reading and research but when it starts to come together it all makes sense. Good luck
 
I would say timing.

Second I would make sure that the timing chain isn't a tooth or two off.
 
those 750's are rich out of the box. I had the same issue until I jetted it down , and now it runs great

just throwin' it out there
 
73AbodEE said:
those 750's are rich out of the box. I had the same issue until I jetted it down , and now it runs great

just throwin' it out there
Click to expand...

That was my first thought when I saw it spit un-burnt fuel out of the carb. My neighbor, who has more experience than I do, said it was too lean.
 
I am going to say timing. Backfiring on a fresh engine is usually timing. What type of rocker arms, adjustable or stock nonadjustable?
 
Hmmm ..... It took 12 posts for someone to offer up the mandatory "timing chain"..

You guys are getting slow.. lol
 
inertia said:
Hmmm ..... It took 12 posts for someone to offer up the mandatory "timing chain"..

You guys are getting slow.. lol
Click to expand...

he does state "fresh rebuild" so I threw that out the window assuming they know what they are doing. but I could be wrong. I have the same exact carb that would backfire and spit when I would first fire the engine, it would also hiss and try to spit when I opened the secondaries. I put another smaller carb on without touching anything else, it never once backfired and ran tits. I jetted the 750 down, and it is running excellent, I still have not touched anything other than the carb.

without knowing where his timing is set , its all speculation
 
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