Hi all,

F250 1986 351 4b 4x4 manual

Today the truck started running rough/hesitating when accelerating or under load. It starts fine and idles fine, stumbles though when I try to accelerate. wires and plugs are good, I checked the fuel filter at the carb and its clean.

The truck is not a daily driver, I run it about once-twice a week... Should I just try some carb cleaner first?


I've had the truck about two months working out all the bugs including getting both the fuel tanks working. At first I thought the fuel filter was clogged with junk from the tank I just got working... But the fuel filter was clean and it has the same symptoms on both tanks now..

Thanks

 

We need a bit more info. If it is the stock carb I think it'll be the 4180 Holley, and I'll assume that's it. Being a Holley it could be either the power valve or the accelerator pump, both of which are problematic. But, a bad power valve usually manifests itself by the engine being seriously rich. SERIOUSLY RICH, to the point of some time blowing black smoke out of the exhaust. And I wouldn't expect it to stumble on acceleration.

The likely culprit is a bad accelerator pump since it would cause it to stumble during acceleration, although not usually under a steady load.

So, maybe you can fill us in on what the carb is and more specifics on the symptoms.

 

This could also be ignition related... pickup coil in the distributor, ignition coil, ignition module come to mind.

Yes, the accelerator pump not working correctly can result in a hesitation when pressing the gas pedal. I would remove the air cleaner and look down the throat of the carb while fully opening the throttle with my hand... if I see two strong streams of gasoline being injected, the accelerator pump is most likely not the cause of the problem.

 

Hi, it is the stock Holley although not certain of the model number, truck is about as original under the hood as you see..doesn't appear to be running rich, no black smoke and the spark plugs are clean to slightly white .

 

Chris has a good point - it could be ignition. But, I'm still betting carburetion. And, his idea of looking for the accelerator streams is a good one and should tell us if yours is working.

Another possibility is a vacuum leak. These trucks, especially on that is pretty original, have miles of vacuum hoses. As these get old and brittle they tend to crack and leak - especially if moved a bit while changing plugs or plug wires, checking fuel filters, etc. And, a lean condition could easily cause the engine to give the symptoms you outlined - even more than a bad accelerator pump. So, I'd suggest checking each and every vacuum line for crack. And, check the things the vacuum lines hook to, like the check valves, delay valves, temp sensors, etc. They have been known to break off at the hose fitting and cause a huge leak. Further, check the vacuum advance in the distributor. These are known to leak and cause both a vacuum leak and a lack of advance to the spark.

I check these things with my handy-dandy MityVac, but they can also be checked by cleaning the hose off and sucking on the line. You can easily tell if there's a leak by placing your tongue over the line, wherein you'll feel a bit of suction for quite some time if the line doesn't leak. But if it does the suction will go away quickly.

 

I checked to see if two strong streams of fuel are produced with the engine off and they are. I did find two of the vacuum hoses disconnected from the air cleaner , probably my fault from fiddling with the air cleaner (your right, I need a PHD in vacuum hoses for this rig.). I reconnected them. I'm hoping its just bad gas that was in the rear tank I just got working. I'm moving to the gas station to fill up..

 

I believe I found my problem! After going over the carb visually and just about to the point of pulling it off for a overhaul.... I did a Seafoam treatment.... when I pulled off the brake booster line, I noticed on the backside of the line a burnt/melted spot,, probably where the line had come off at some point and fallen onto the hot exhaust manifold,,, there was a extremely small leak at the weakest point which would explain why it would run fine when idling but would bog down under heavy acceleration? maybe the secondaries weren't openning because of this??

Anyway, I'm on the hunt for a new line,, currently there is a 11/32 size on but no one seems to have this,, would 3/8 work?

Thanks to all that have responded!!!

 

All of the above plus check your float levels.

 

That may be the problem, alright. As for the size, if the 3/8" line will seal, and it probably will, then that'll be fine.