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I own a 1980 F-150 4x4 with a 300 cid in line 6 and a 4 speed manual transmission. The truck has 345K on it but the engine has less than 4K on a total rebuild. I inherited “Ole Red” when my Dad passed away in 2016. He bought the truck brand new and took exceptionally good care of it. The old truck runs very good with one exception. Whenever I pull a hill and hold it to the floor, when I let up on the accelerator it back fires. I don’t mean just a little puff, I mean a back fire that sounds like a rifle shot. It opened the muffler from one end to the other just like gutting a fish. It does it nearly every time unless I gradually let off the accelerator a little at a time. The truck starts very well and has very good power. Anyone out there have any ideas on what the issue could be? Mufflers cost $45.00 each.
My first thought is an exhaust leak. From the plural "mufflers" sounds like you might be runnin' headers and duals? Cold air and hot exhaust gasses mixed will cause some noise.
Second thought is a sloppy vacuum advance cam return and/or vacuum advance can starting to wet the bed.
If you're splitting mufflers, is raw gas dumping down the exhaust?
I am thinking it is fuel and a little leak letting air in and BOOM!
I would first check to make sure the choke is open all the way when the motor is up to temp.
Also if the float level is too high can make it run rich.
Then check what Filthy Beast said. Check timing is set to factory spec or a little more advance. Then check if the vacuum advance works as it should.
Are you still running the stock carb/intake & exh. manifold? What about the rest of the exh system, stock or ?
All the vacuum lines still hooked up or was the engine gutted?
Dave ----
Anyone out there have any ideas on what the issue could be? Mufflers cost $45.00 each.
Does the carb have any electronic controls on it? Not familiar with this exact model, but others from this era often have a solenoid to cutoff fuel flow during deceleration.
The other thing that comes to mind is a dashpot on the throttle linkage. This was a little shock absorber thingie, usually disc shaped, designed to slow the closing rate when decelerating. Some had a vacuum input for more refined control. If the throttle moved fully closed at high RPM, the idle ports were now exposed to a very high vacuum, and LOTS of fuel would get sucked out. This would create a very rich mixture that can cause backfiring. Once again, no idea if your carb was so equipped, but it was very common back then.
Another option is to create a rotary muffler dispenser, kind of like the cylinder on a revolver pistol. If you blow out a muffler, rotate the next muffler into position, and be on your way.
The truck has and in-line 6 cylinder with a single exhaust and every thing on it is factory (re-built several times but factory none the less). My Dad was a retired mechanic from "the old school", which was why he kept the old truck. He understood it and could work on it. He had newer ones but would often get confused and frustrated with the electronics. There is a single wire disconnected and hanging near the carb and I'm not sure where it goes. I'll take the breather off and see if I can figure it out. In my opinion, the truck starts and runs too good for it to be a timing issue. It will haul a ton of coal up over a 3 lane hill in 4th gear. It's obviously an issue with raw gas getting into the exhaust, just don't know why. If I decelerate slowly it doesn't back fire. The truck needed a clutch in the summer of 2014. Rather than fight with transmission and transfer (he had no vehicle lift) he took the engine out instead. He put a new clutch on a spare engine that he had rebuilt several years prior. The truck is going on its 3rd engine. Everything on it except the 4 speed transmission has been rebuilt at least once.
Does your truck have a smog pump? A disconnected or malfunctioning diverter or bypass valve could allow air to continue being pumped into the exhaust during deceleration which can cause backfiring too.
KR, I was also thinking dash pot but I don't think they were used on our truck of that year & motor as mine does not have one but the motor is not the stock one for the year of my truck.
Dan, that wire maybe for the choke as I don't know of any other wires in that area.
Post up what color that wire is and a picture if you can as that will help us help you.
Rowdy, I do believe it should have a air pump and most have been seized up and not working. You have a point of it a cause also.
I wish I had a little more input but I have not driven mine from the day I bought it 2 years ago come Dec as I tore it all apart for rebuilding.
We will get this fixed yet.
Dave ----
Does your truck have a smog pump? A disconnected or malfunctioning diverter or bypass valve could allow air to continue being pumped into the exhaust during deceleration which can cause backfiring too.
Ooh, excellent suggestion. Easy to rule out, too. Dan said the engine has a smog pump installed. Just pop off the belt and take it for a test drive. If now good, it was pumping air into the exhaust during deceleration.
If that's the case, I highly recommend fixing the system, not just leaving the belt off to permanently disable it. The air injection system on some trucks sprays cooling air on the backside of the exhaust valves, helping them live longer under harsh conditions.
There are actually 2 disconnected wires coming off the carburetor. The choke is yet a 3rd wire but it is connected and is working. The two wires are the male end in a connector but I cant seem to find where they should connect to. The truck does seem to idle a little too fast and there is no more adjustment left to slow it down any more. The hi-idle kick down works but the base idle is too fast. I am assuming that if these wires were connected that might help as they go to what looks like a little diaphragm on either side of the base of the carburetor. I took a photo but I have no idea how to post it on this site.
Ok on my 81 300 six there is only 1 wire to the carb and that is for the choke.
So you have the idle speed screw out and not touching the stop.
Is the fast idle cam & it's screw holding it on fast idle?
If both of them are not touching the seats try and pull the throttle cable off at the carb and see if it is not lower.
Dave ----
Would like to see a picture.
Take a picture and save it to your computer.
Then go to the advance button and at the top to the right side, the box that looks like a mountain in it "insert image". Once you click it follow the screens.
If this works, here is a picture of my carb. The choke wire is at the top and connected and working. The two near the bottom are not connected and for the life of me, I can't find a connector anywhere near the two wires.
To answer the rest of the questions, The idle adjustment screw is not touching anything and the cable is not hung up. (I even checked under the dash on that end). It won't come back any further. The idle is even higher after setting the choke on a cold start but I can tap the accelerator and it drops down, just not far enough. It has no tach, but when comparing the engine speed to that of my Ranger, I would guess that it is running at a strong 1.5K. (About 2K when on hi-idle but it drops down nicely when I tap the peddle)
