Back fire through carb on throttle

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duncan999

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So while I’m rebuilding the fuel system on my Duster I want to run the engine to see if I can at least start it and the power steering works. I rigged up an auxiliary motorcycle tank that holds about 2 cups of gas with a gravity feed to the carb fuel line. If I give the carb a small squirt of starter fluid the engine starts right up, but will die in about 5-10 seconds unless I manually move the throttle linkage. BUT when I move the linkage, even a tiny bit, pretty quickly (a few seconds) the engine will backfire through the carb with a scary flame. It’s even more scary since I'm leaning over the engine with my hand on the throttle linkage. The guy I bought it from drove it onto the roll-back for transport so the timing can’t be that far off, right? While it’s running the engine sounds smooth so I don’t think it’s missing. Next steps? I’m keeping a fire extinguisher next to me with the pin removed while I’m figuring this out. Carb is a Edelbrock 1405 600cfm four barrel with a manual choke.

I should also mention the vacuum advance on my distributor isn’t connected, but that shouldn’t matter at idle, right?
 
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Needle & seat in carb may be stuck closed. They are at the back corner of the carb. Give that area a stout rap with a hammer handle.
 
Back fire through the carb typically indicates ignition firing with an intake valve open. So that could indicate the timing is not correct.
 
Back fire through carb can also be due to leanness. Hence post #2. OP says ' it starts right up' & runs for 5-10 secs, which I doubt it would do if ign timing was out....
 
Bewy said:
Back fire through carb can also be due to leanness. Hence post #2. OP says ' it starts right up' & runs for 5-10 secs, which I doubt it would do if ign timing was out....
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I actually never knew backfire through the carb could be due to leaness, but now that I read about it, it makes total sense. I’m also a little concerned that my gravity feed system doesn’t produce enough pressure to keep the float bowls full. So next things to check will be vacuum leaks, accelerator pump, and fuel pressure.
 
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Bewy said:
Back fire through carb can also be due to leanness. Hence post #2. OP says ' it starts right up' & runs for 5-10 secs, which I doubt it would do if ign timing was out
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He said that was on starter fluid, so it seems to run till the starter fluid disapates.

But I agree just giving it a bit of throttle should not cause a back fire.

Thought.

If the vacuum advance was tied to manifold vacuum would be high at idle but opening the throttle would drop that vacuum and the advance would drop off MAYBE...

The timing was set with manifold vacuum attached and it's getting extremely retarded off idle. So much so that it's firing with an intake still open?????


There is always the possibility of stuck or burnt intake valve
 
Checked a couple of things and tried a couple of things when I got home from work today.
  • Vacuum ports are either capped (timed vacuum port, manifold vacuum port) or connected (PVC). I ordered some tubing for the distributor advance. But could still be leaks.
  • Accelerator pump seems to work (engine off), but I could only see one port.
Tried to start the engine as I did yesterday (gravity tank, starting fluid), but with the choke closed; won’t start. Choke half closed; won’t start. Choke open, started. This time I didn’t touch the throttle. Idle was low and a bit rough but no backfiring. Engine died after a minute. Increased idle quarter turn. Now the engine ran for several minutes and then died. But the RPM’s went up and down and the 2 cup tank was almost empty so I suspect the fuel pressure was too low. Ordered a $20 electric fuel pump that I can run off my gas can and see if that clears up the idle. Then I might try and touch the accelerator .

Conclusion is yesterday’s backfiring was due to lean condition and maybe there are other issues like the accelerator pump.

Engine ran long enough I could check the power steering and it seemed OK. Baby steps.
 
Maybe the auxiliary tank had dirt/debris in it which has now got into the carb & blocked the idle feed?
 
RustyRatRod said:
Hopefully this isn't a new engine where you need to break the camshaft in.
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Ah, no.

IMG_1134.jpeg
 
As long as the needles and seats are good, low fuel pressure won't affect the mixture while it's just sitting idling and free-revving. Edelbrock carbs are pretty simple and easy to work on, I'd just pull it and go through it as standard procedure. I bet it's got some junk inside and a dried-up acc pump plunger.
 
duncan999 said:
Checked a couple of things and tried a couple of things when I got home from work today.
  • Vacuum ports are either capped (timed vacuum port, manifold vacuum port) or connected (PVC). I ordered some tubing for the distributor advance. But could still be leaks.
  • Accelerator pump seems to work (engine off), but I could only see one port.
Tried to start the engine as I did yesterday (gravity tank, starting fluid), but with the choke closed; won’t start. Choke half closed; won’t start. Choke open, started. This time I didn’t touch the throttle. Idle was low and a bit rough but no backfiring. Engine died after a minute. Increased idle quarter turn. Now the engine ran for several minutes and then died. But the RPM’s went up and down and the 2 cup tank was almost empty so I suspect the fuel pressure was too low. Ordered a $20 electric fuel pump that I can run off my gas can and see if that clears up the idle. Then I might try and touch the accelerator .

Conclusion is yesterday’s backfiring was due to lean condition and maybe there are other issues like the accelerator pump.

Engine ran long enough I could check the power steering and it seemed OK. Baby steps.
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Why are you afraid of it? Turn the idle stop screw in 3 or 4 turns, get the damn thing to run and warm up, then adjust it down, and adjust the mixture screws for best idle. Seems like you’re hesitating for some reason. With full fuel bowls it will run for 30 seconds to a minute at low rpm and then die. Get a good source of fuel to it so you can run it longer. And plug off all vacuum stuff for now. You don’t need to worry about that crap. VA can come later in the conversation.
 
With an engine bay that looks like that, I see why you may be replacing the fuel system. Check and or replace parts as necessary, or your next problem is just around the corner. Hot mess.
 
MopaR&D said:
As long as the needles and seats are good, low fuel pressure won't affect the mixture while it's just sitting idling and free-revving. Edelbrock carbs are pretty simple and easy to work on, I'd just pull it and go through it as standard procedure. I bet it's got some junk inside and a dried-up acc pump plunger.
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Definitely possible. This car had been sitting in a driveway for two years at least before I bought it. I’ll add carb disassembly & inspection to the list.
 
TT5.9mag said:
Why are you afraid of it? Turn the idle stop screw in 3 or 4 turns, get the damn thing to run and warm up, then adjust it down, and adjust the mixture screws for best idle. Seems like you’re hesitating for some reason. With full fuel bowls it will run for 30 seconds to a minute at low rpm and then die. Get a good source of fuel to it so you can run it longer. And plug off all vacuum stuff for now. You don’t need to worry about that crap. VA can come later in the conversation.
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Ya I don’t want anymore backfires so I’m slowing down and trying to be more methodical. I did increase the idle and that’s helped. Next step is to hook up an electric fuel pump attached to a gas can. I’m disconnecting the PVC and plugging that hole. All also adjust the idle mixture. If the engine then runs long enough to warm up a bit I’ll try the throttle again. We are aligned .
 
oh and what fire extinguisher do you have 'on standby'? co2 yes, dry powder no (big mess). to be honest a thick rag will put out most carb fires as all you're doing is starving it of oxygen. and a rag doesn't need refilling either, lol. :thumbsup:
neil.
 
CRUZE 418 said:
With an engine bay that looks like that, I see why you may be replacing the fuel system. Check and or replace parts as necessary, or your next problem is just around the corner. Hot mess.
Click to expand...
I was thinking the same. Might be a hot electrical fire...and fuel...and everything else.
 
duncan999 said:
Ya I don’t want anymore backfires so I’m slowing down and trying to be more methodical. I did increase the idle and that’s helped. Next step is to hook up an electric fuel pump attached to a gas can. I’m disconnecting the PVC and plugging that hole. All also adjust the idle mixture. If the engine then runs long enough to warm up a bit I’ll try the throttle again. We are aligned .
Click to expand...
No reason to unhook the pcv. Leave it. As far as ported or manifold vacuum, just cap those for now. As well as any other accessories. But the PCV can stay.
 
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