papat

I just bought an 81 F250 w/ 400 2bbl...it was sitting still for a long time and has ran a little ruff...rats had eaten the vac hoses and so I have tried to put them back where they should be...I have replaced the carb, fuel pump, fuel sending unit, wires, cap & rotor button, plugs, fuel filters, replaced the hard line for the fuel w/ rubber fuel line...so far it is getting better...Sunday almost didn't make it home from church...the backfiring thru the carb is so bad I thought my truck was going to catch on fire...got home and pulled the plugs and the right bank was all black no oil but carbon looking...left side had some but not as much...someone told me today that I should ck the timing but this dist hasn't been moved in I don't know when...he also said that I might have a valve stuck or something like that??? Any help would be appreciated!!!! Papa T

wde3477

My guesses would be stuck intake valves, ignition timing off, or jumped timing chain.

Given that you have only seen the problem on one bank, the age of the vehicle, and the length of sitting idle - would lean toward sticking valves.

Another thought - are you sure the carb is set up right? Float level good, mixture right? And no vacuum leaks?

papat

thanks for the input...the carb is new so I assume it's ok...vac lines I am not sure about...when I first bought this truck and cranked it up I heard some tapping on the right bank...not sure how to ck to see if valves are stuck...if I pull the valve cover and do some shaking on the rocker arms and see if anything is loose is that about all you can do w/out pulling the head off? thanks for your help...papat

wde3477

Tapping is not that unusual on an older truck that's been sitting awhile, but the tapping should clear up in very short order as the lifters pump up.

It is hoped that with the valve spring tensions and close clearances, you won't be able to do much shaking, or else you have other issues.

An inspection of the valve / rocker area should give an indication on what you're up against. If the valve area is all gooky, or you can see physical damage - not good news.

You could pull the coil wire and crank the engine over to see if the rockers and valves are operating smoothly - watch out for oil flow when doing this. A compression test may also help to identify problem cylinders without having to pull the valve covers.

I don't know what the status of new carb setups are nowadays, every one I've bought in the past had to have the mixture screws dialed in after installation.

While I've leaned toward sticking valves as a culprit, don't overlook verification of proper carb setup and elimination of vacuum leaks. Check the easy stuff off the list first.

papat

thanks again...on the vac lines...I look at the schematic but I just don't understand some of the lingo...papat

wde3477

Know what you mean - vacuum diagrams are notorious for using abbreviations, and the arrangement on the diagram is nowhere near the real-world physical arrangement, other than the point-to-point connections. If you need some deciphering, throw them out here and we'll see if we can figure out what they mean.

If you have colored plastic/nylon vacuum lines, those become extremely brittle with age, and are highly suspect as sources of leakage.