I just replaced the spark plugs. After finishing, I drove it around for about 15 minutes, it ran well.

It sat for a couple days. I made some adjustments to a manual choke I had installed recently, and tried starting it up. I'm still learning where to set the choke when I start up, so I was fiddling with the choke and cranking it. It was backfiring out of the carb pretty bad. I saw flames shoot out from the top a couple times (the air filter was off because I was adjusting the choke).

I finally got the choke set where it will fire up strong, and seems to run strong when I give it throttle, but when I let off the throttle it won't idle at all, just dies.

I thought maybe I flooded it when I was initially trying to get it to fire up, but even after starting it and revving for a minute, then letting it die, it still backfires when I go to crank it again.

 

The PCV valve hose was disconnected. I must have knocked it loose when I was tinkering with the carb. It's working now.

 

Thanks for letting us know as many don't report back if it was fixed or not

Dave ----

 

Apparently this is not resolved.

I drove the truck yesterday, put maybe 15 miles on it. It ran good except it felt a tad luggy starting from a full stop. When I got home I bumped the timing forward a bit, and turned it off.

This morning I go to start it, and I'm cranking and it's not firing up at all. I tried the choke in various positions and it wouldn't fire, so I pulled the air cleaner off, checked the choke position, which looked about right, and crank it again, and it starts backfiring out of the carb. Not really bad, little burps. It blew the PCV hose off. I'm pretty sure this is what happened before. I don't see any other places there might be a vacuum leak. This thing barely has any hoses.

Could the initial backfiring be because it's flooded?

Any ideas?

 

Once it starts and is running if it was flooded it should clear out.
Now if it has been running say 5 or 10 mins. get no black smoke out the tail pipe and it is still back firing there is an issue with the motor.

Dose this show up more under load / driving than at idle not in gear?
I think your next step is get it up to temp then do a compression test on the motor.
If that dose not show anything I think I would then pull the valve covers and, this may make a mess, run the motor and check all the rockers if they are all moving the same amount.
A flat cam shaft lobe on the exh side will build compression but when running the exh has no where to go because the exh valve is not opening and when the intake valve opens the exh pushes out and you get that little back fire.
Dave ----

edit I see you have a 300 six maybe pull the side cover (easier to pull off) over the valve cover and maybe just cranking the motor you can watch the lifters if they all move the same amount.
Being it is not running should not make that much of a oil mess.

 

I've only had it back fire while cranking to start up. When the issue started the other day, after letting it sit for about 15 minutes and reconnecting the PCV hose, it started up and ran no problem, and I drove it around the next day without issue.

I suspected the issue is related to the carb/choke, spark plugs, or timing, as those are the things I have been fiddling with.

 

I'll try this when I have time.

This all started after I changed the spark plugs. Could that be related?

 

Now the full story comes out
Did you changed 1 plug at a time or pulled all the wires off and the changed the plugs?
If the later but I would recheck any way the firing order.
Make sure the plug wires and on the right plugs in the firing order.
Dave ----

 

I did them one at a time, but I'll check the firing order.

Could a faulty spark plug or wire cause this?

I mentioned in the top post that I had changed the spark plugs. But after initially changing them it started right up and drove fine.

 

Yes it can.
I had a car with new plugs in the motor. The car could not be driven and was started to move when needed every few months.
I wen to start it to move and it idled good as it always did but give it a little throttle and it started back firing out 1 of the tail pipes, could not tell what side by my self.
Because it did not run for months at a time I thought a stuck valve? Car had headers so I wet each tube then started the motor only for a short time.
All but 1 tube was dry, the wet one was not firing. I swapped the plug to the other side of the motor and did the same test and that tube stayed wet.
Again the plugs looked new but that 1 would not fire, I replaced all plugs and it runs great to move it when needed.

So yes a bad plug or wire could keep a hole from firing. The extra fuel not being burned is now burnt from 1 of the other holes like mine was.
I would also do a compression test so we know the motor is in good shape.
Dave ------

 

I had all the old spark plugs, so I swapped them out. It didn't change anything. I can't get it to run at all now with the new or old plugs. I can get it to start but if I stop giving it throttle it dies immediately.

I'm working on getting a compression gauge to do a test.

I noticed that the ignition coil was quite warm after cranking it, even though the engine is cold. Is that normal?

 

I've figured out that I can get it to start right up if I advance the timing a whole bunch, and then it idles good. Once it's warm, I can pull the timing back, and it will start too. But if it's cold, I have trouble getting it to fire up, and when it does it won't idle.

Does that indicate anything?

 

What made you change the timing?
Did you mess with it when doing the plugs?

What are we working on? On phone so I don't see sigs
Dave ----

 

It really sounds like the firing order is @ fault here !!!!!!
Either you got them mixed up so double check first....could also be a dead cylinder...meaning 1 isn't firing....could be firing order/a bad plug or wire or burnt valve etc....check cap for arcing....then check the rotor is pointing to the # 1 spark plug wire @ the cap...If it isn't adjust distributor so it dose...It should start with no back-fire even at an accelerated idle etc. !!!!
then check the timing ...if manually moving only move the distributor approx. 10 degrees only .......& it may make it run better.....but it should not backfire in either position !!!
If you don't readily manually know what 10 degrees is...picture the size of your ignition cap....divide the circumference into 360 degrees & move it that far ...should be roughly around + 1/2 inch of advanced movement !!

Choke ( unless severely flooded already) should not make it backfire...as it warms it should be starting to open....unless it's a manuel choke just open it up as it warms up !! (open choke will not make it backfire)

 

1984 F150 300 i6, Duraspark II conversion

I changed the timing because I was reading around about backfiring through the carb and hard starting, and people say this can be timing related. So I loosened the dist and advanced the timing quite a bit, and it fires right up and idles strong.

I made a small adjustment to timing a few days after installing the new plugs, but this was after my first incident of backfiring.

Last night I couldn't get it started, so I move the dist, advancing the timing by a LOT, it starts right up and idles good. I let it warm up, then I bring the timing down closer to 10-12 degrees BTDC. I turn it off, and try starting again this morning when it's cold, won't start.

Now, it's possible I'm doing something wrong with setting my timing. My truck doesn't have the timing marks on the wheel (harmonic balancer?). I added the marks by pulling out spark plug #1, putting a damp rag in the socket, and turning the fly wheel until the rag pops out, then I stick a screw driver in the socket and turn the engine until I get the screw driver at the highest point, then I marked the wheel right where the timing marks say 'TDC'. This seems to have been working fine for me for months now, so I assume the marks are pretty close to correct.

Again, I haven't had any issues with it starting like this until last week. I replaced the plugs, and then it started up cold no problem. It sat for a few days, after which I made some adjustments to my manual choke setup. After this I tried to start it and this is when my problem started. I assume the issue is not choke related because I'm trying to start it with the choke in every position, choke position doesn't seem to be the cause.

Any idea why it would fire right up with the timing way advanced, like too advanced? Vacuum leak?