460 F250 sputter, backfire under load

[Shran]

I guess it is that time of year again where I need to use my F250 and of course there is something wrong with it.

This time it is just running weird. It will sit and idle nice and smooth, and seems to be fine at any RPM when the truck is stopped. However if I am on the highway it just falls on its face at about 40mph (in 3rd gear, 3000rpm or so, right before shifting into 4th) and again at around 65-70 in 4th gear. If I floor it in 3rd or 4th at any speed it will sputter and surge really hard and backfire through the exhaust occasionally.

I have replaced the plugs, wires, cap and rotor within the last 500 miles but that was probably 5 or 6 years ago. I drive it to a campground once a year and once in a while to pick up lumber at Menards but that's about it.

I am going to replace the fuel filter since I haven't ever done that but any other suggestions? It seems fuel related, accelerator pump perhaps? It is virtually a factory stock engine except for a Carter AFB carb and somebody removed the smog and EGR equipment.

 

[RonD]

Accelerator pump, maybe, but that would effect all acceleration from 0MPH and up.

Backfire in exhaust means fuel wasn't burned, or burned completely, and is being ignited when it exits exhaust valves.
Could be spark timing which is effected by engine load, i.e. vacuum advance.
I would pull the vacuum hose and suck on it to make sure it, and the distributor's vacuum advance, is holding pressure.
And recheck base timing.

Have you noticed a drop in MPG, lol, I know on a 460 it is a moot point but sometimes that is a sign of main jets running engine too rich under load, causing the backfires.
And if gas in the tank has been sitting awhile it may need to be to be freshened up, all gas has water in it and as long as it doesn't sit too long it won't settle to the bottom of the tank, when it does then you can get low power and backfires until it is purged out by driving, or siphoning it out.
Filling tank with fresh gas stirs up the old gas and helps spread out the water so it doesn't effect driving as much

 

[Shran]

I will check the vacuum advance hose and timing... good thought.

Gas mileage, I don't know, I kept track once and got around 8mpg. If my math is right on the last tank, I got 4.6mpg but that was including several hours of idling in the driveway to keep the battery charged. I filled it up twice earlier this summer and again yesterday so the old gas factor should be not an issue.

I don't know the history of the carb that's on it and I suspect it has had very little tuning done to it. It has always run pretty rich and always smokes black and runs pretty rough until the choke opens. I have a factory Holley from an '86 F250 460 truck that I am tempted to throw on but it needs a rebuild.

 

[Shran]

I have been messing with this issue all week and seem to be getting nowhere.

Vacuum advance seems to be holding pressure, I don't have a vacuum pump though, just mouth suction. The vacuum hose is in good shape.

Fuel filter replaced, no change.

Carb swapped with a cleaned up Holley 4180, no change.

Timing was actually off slightly, was set at like 15* BTC. I reset it to the factory setting, 8* BTC. Don't think it made much difference though.

So what should I look at? Fuel pump, distributor itself, coil, Duraspark box come to mind. It seems to be really bad in 3rd gear and only 3rd gear, I can pull away from a dead stop going up a hill in 2nd gear and go full throttle all the way to 5-5500 RPM with no hesitation, 1st gear is the same, but once I get about halfway through 3rd it's sputtering hard. Letting off the gas to 1/4 or 1/2 throttle clears it up but any more than that will make it run rough.

 

[alwaysFlOoReD]

Where is the fuel filter located? On my 77 f250 w/460 there was three. Two were aftermarket put inline sometime in its life. There was the stock factory one in the carb inlet that some people don't know about, probably why the others were added. If a fuel filter is plugged you can get to 3rd before using the fuel in the carb up.

Sent from my XT1032 using Tapatalk

 

[RonD]

Now reads like it is Leaning out as alwaysfloored suggested.

The thing with 1st and 2nd gear is that is it fairly quick acceleration and briefer higher RPMs so fuel demand is high but not very long.

When you get to 3rd the demand and load is longer term so you would notice a lack of fuel more at that time.
When you ease off the throttle you reduce the air flow so air/fuel mix gets back to the 14.7:1 ratio needed.

Could be filter issue or in tank screen/sock.

You can "T" in a fuel pressure gauge between pump and carb so you can watch pressure while driving but it is a pain to do that, lol.

 

[Jimmyrig]

Check for vacuum leaks too. Particarly around the base of the carburetor. I've had bad carburetor gaskets act exactly like that. Especially if your getting too much fuel, I had a bad holley a while back that dumped too much fuel and it concealed the vacuum leak and I couldn't hear it over the air coming in through the carb. It would start and idle perfect and was fine until third and fourth. It was jumping from rich to lean and back again.

 

[Andy D]

How old is the gas? I was running on year old gas in the 528e . It had traveled >150 miles last year. I put 10 gallons in last Sunday. Today, I went out expecting to have to coax it to life for a ride down town. It started better than it has in ages

 

[Shran]

I'm pretty sure there is just the one filter right before the carb, plus the sock thing right on the primary float bowl.

I may just have to pull the tank and see if anything is going on in there. My buddy suggested the pressure gauge idea too but the truck has this "hot fuel return" option where even though it is carbureted, it has electric fuel pumps and each tank has a return and supply line going to it so there is a cluster of fuel lines under the hood. I would like to simplify this but it has worked fine up to this point.

So I did a little test yesterday concerning the 1st and 2nd gear thing - I went up a hill and left it in 2nd and held it at about 4500rpm for quite a while... probably the equivalent of a 1/4 mile or so, not even a hiccup. But as soon as I hit 3rd it acted up again. I agree that the lean condition makes sense but wouldn't that apply after a while in any gear at high RPM?

 

[Shran]

The gas should be relatively fresh, I have run about 30 gallons through it in the last few months and I just put 12 gallons of very fresh stuff in.

Carb base gasket should be fine, it is brand new. I did find a vacuum leak at the PCV valve top port but it is now fixed and didn't seem to affect anything either way.

 

[alwaysFlOoReD]

So on my 77 460 truck I had a problem with the mechanical fuel pump and heat soaking. I solved it with a electric fuel pump. I still think your problem is fuel related. Can you rent or buy a fuel pressure tester/gauge and have it hooked up while you're driving? At least that will rule out the pump.

 

[RonD]

"it has electric fuel pumps and each tank has a return and supply line going to it so there is a cluster of fuel lines under the hood"


So you have dual tanks with an electric pump for each tank.
And no mechanical pump.

I assume you have tried both tanks and the problem occurs on either tank.

That should eliminate each pump and the "sock" in the tank as the problem.

So you just have the short fuel line with in line filter at the carb that might be the source of limited fuel flow, which might be but seems like a long shot.

And you swapped carbs so mis-adjusted float level is off the table as well.

Can you or have you pulled 2 or 3 spark plugs to have a look at the tips, to see if they tell you anything?


Also try turning on the head lights and blower to high, see if that makes things worse, just seeing if it might be a voltage issue with spark.


maybe hook up a vacuum gauge in the cab to watch what it does during the problem.

Grasping at straws

 

[Shran]

Right, no mechanical pump, the pump mounting area on the engine block is actually filled in.

Dual tanks, only the rear one works. The selector valve is broken and stuck on rear tank, the front tank was drained years ago by me and I have verified that it's empty and not back feeding or something weird.

I guess my next thing would be to pull the rear tank and check the strainer and pump.

Will also pull a couple plugs.

Load on the battery doesn't seem to have any effect, some of my test driving was at night with the headlights on, and the camper running lights on as well (I have a slide in camper in this truck at the moment.)

The only other thing I recall is one time recently it had a dead battery and I had to jump it. I did accidentally hook up the jumper cables backwards but only for a few seconds. The jumper cables and starter solenoid got pretty hot so fortunately I was able to correct my mistake and it didn't seem to have any ill effects so I didn't give it any more though. I don't suppose this could be related, the ignition was never turned on and I hate to admit it but I've done the same thing several times in the past with this truck and nothing bad happened. I hardly ever drive it so my old shit batteries usually end up in it and inevitably needs a jump once in a while.

 

[Shran]

I fixed it.

It was a weak fuel pump. The pump had no strainer attached to it and must have sucked up debris. Weird, because I could hear it running - I can't recall running across a situation like this before, they always have either worked or been totally dead.

Thanks for the help

 

[alwaysFlOoReD]