Hi All,
Have a 1973 F250, 360, aftermarket but tired 4 barrel carb, which was delivered to my shop a couple weeks ago. Sadly, the transport company found it fit to mess with the truck. It was running well before it was transported to my shop. But upon arrival, the battery was dead. Went to open the hood, found the transport company had removed the hood springs and bent up the hood hinges. After finally getting the hood open and putting in a new battery, got her fired up.
Unfortunately, the thing starts backfiring almost immediately "pop-pop-pop pop-pop!" and then will die. When I get it started again, the same thing will happen, but if I push the accelerator or shift into gear, the backfiring is even worse and the truck dies even faster.
I have checked and replaced the spark plug wires, making sure they are routed correctly. I have changed the plugs as well as the coil. I put fresh gas in the tank. None of these steps have had any effect whatsoever. I will change out the fuel filter later today, but I doubt that is the issue. I opened the distributor cap and the cap and rotor look almost new with almost no wear on the rotor. I do not think it is a timing issue as
1) the truck ran fine before being dropped off, and
2) I see no signs that the distributor was loosened and timing adjusted

Has anyone ran into this before? Any ideas. I'm thinking maybe contaminated fuel at this point.

Thanks,

Keith

 

backfiring is from what i know, the ignition of gas fumes at the wrong time, whether this is in the exhaust pipe or in the cylinder can tell you what the problem is
if the gas is extremely rich it can build up in the exhaust header and ignite but that's most likely not the problem
if it was a rich mixture your spark plugs they would be wet or carbon fouled from over rich mixture

other options for cause of backfireing can be ignition timing, which is easy to check and i would say check that first
check the spark plugs for signs of damage from something being wrong
other problem could be valve timing or a valve stuck open maybe? (i'm not good with valves and heads so i don't know if that's possible)

honestly i don't know what the transport company could have don't to mess it up that bad

other people will chime in with more info and correct me if im wrong

 

Thanks 78Fords.
i did pull and replace all the plugs. None of them were wet, but they did have a lot of carbon build-up. I do not think they had been changed for a very, very long time. But the backfiring thru the carb continues after the spark plug and spark plug wires change.
While I have not ruled out a stuck valve, I find it unlikely that in the space of 7 days (from the time I bought the truck to the time it arrived at my shop) that a valve became stuck. Just doesn't seem likely. I suppose it's possible that the transport company loosened the distributor and messed with the timing, though I do not see any evidence of that. Some people have suggested perhaps they dumped something in the gas....I suppose that's possible, too. I will continue to troubleshoot......

 

Backfiring through the carb could be a lean condition.

However, is this a points & condenser ignition system? Make sure the condenser is not one of the el-cheapo chinesium POS condensers. They will fail in seconds. Try an old school dusty one from your junkbox. Clean the point faces with some fine sandpaper for grins.

 

i thought it was exhaust backfire, carb backfire is usually a ignition/ carb adjustment issue

 

I've also had a bad condenser cause backfire but I wonder if between purchase and delivery if the firing order was messed with. When you changed wires, did you double check the order or do a wire for wire swap?

Michael

 

All good answers and with what the transport Co. did to the hood anything could of been messed with so all needs to be checked.
When you had the plugs out did you take a compression test?
Check firing order just 2 wires moved could cause this.

Did you find any tire rubber on the rear wheel body panels? Doing burn outs can cause the first thing listed below.
Now I have seen exh valve not opening from a push rod that fell out or bad / flat cam lobe cause a back fire out the carb. The exh has no place to go so when intake valve opens out it comes!
If you can keep it running long enough pull each plug wire and see if the back firing goes away then that is the cly you need to look into.

BTW I have had back fire out the tail pipe anything above idle and the cause was a bad plug, first time I ever seen that!
Dave ----

 

Bent hood hinges reminds me of the time the used car lot I worked for bought a Falcon at auction. The hood came open at around 40-45 MPH. Wasn't good for the hood hinges at all. If it sat with the hood open in the rain, a valve could be stuck.

Michael

 

First off, before it gets too late, I would file a lawsuit against this Company...or at least look into your legal options. You handed them a running, driving vehicle and a week later the vehicle won't run and some damage has been done whilst the vehicle was in their possession.

If you're of this mind, do it now and don't play with the vehicle until an adjuster sees it....

If not, well...........

 

If they messed with your hood, I wouldnt put it past them to have swapped some plug wires just to get a chuckle out of it.... Double check your firing order .....

 

 

And just so we're all on the same page, how do you know the engine was running before? Was it your truck and you simply had it shipped to your shop in another area? Or did you buy, or have a customer ship it in and you saw in a video that it was all hunky dory under the hood?

And is the engine older, or has it recently been rebuilt? If a new cam was installed, was it broken in properly, and is high-zinc oil being used?
Just wondering if some lobes have gone flat in the meantime. Same thing someone said about pushrods. Seen it way too many times in recent years for some reason. Way more than I ever heard of it way back when.
If someone was playing a little too hard with it (the suggestion to look for rubber debris in the wheel wells is a good one) while it was being transported, there could be a valvetrain issue.

The rotor is turning and looks good, but did you check the actual timing? Something could have changed, but not sure yet.
What type of ignition? If someone did a Duraspark swap from way back when and used one of the modules with a start-retard function then there may be voltage at the start wire causing at least a minor timing change.
Whatever is going on though, sounds like a much larger timing issue than just a little bit of difference in ignition timing. I'd lean more towards the super-lean condition. I'm guessing you looked around to make sure that a vacuum booster hose or PCV hose was not dislodged and causing a huge vacuum leak?

Just spitballing a bit and throwing out a few things and times-twoing what some of the others have already said.
Good luck!

Paul