Hi All, I've got a '78 F150, 351M & recently have a problem that "might" have started months ago. Upon initial fire up, there's always been a cloud of white smoke due to oil getting past the valve guides, but once the truck is running, it's been a strong, dependable vehicle.

About 6 months ago, I had to floor it unexpectedly & a huge black cloud blew out the exhaust. The "normal" white smoke no longer appeared upon initial fire up & the performance was getting worse & worse. Changed the air filter, PCV, crankcase vent filter, oil, oil filter, plugs, turned out the air mixture screws 2 full turns each & everything seemed fine after that.

About 3 weeks ago, I noticed the performance progressively getting bad again. Engine would "shudder" a little, black smoke had returned, acceleration had dropped dramatically to where I can barely get to 20 mph on even slight inclines.

Checked & cleaned the plugs (they were fouled, dry black on gap, black & wet around base of plugs) Changed the air filter, PCV, crankcase vent filter, fuel filter & got no noticeable change. Turned out the air mixture screws 2 MORE full turns & then everything seemed fine again. Two days later, the performance is again starting to bog down & getting progressively worse again.

Am I dealing with an ignition problem ? Carbueration problem ? Both ? It fires right up, but runs extremely bad.

Any thoughts & advice would be much appreciated. Thanx in advance.

 

I had same problem. It was the floats in the carb.

 

carburetor is way rich, resulting in poor performance.

 

Check the cap and rotor on the distributor.

 

Black smoke usually means too much fuel/ not enough air to completely burn it. Sounds like your carb is out of whack.

 

But if you are not getting a good spark your not going to burn all the gas, and it will smoke.

 

Thanx very much, All.....Going to check the air gap in the distributor & someone locally told me to check to make sure the bearings inside there aren't loose. Also will be putting in some hotter plugs while I'm at it.

There's now some seepage near the base of the carb & the auto parts guy told me that it's probably dumping too much fuel as well as filling up too much at times (floats ?), so I've bought the carb rebuild kit & will keep my fingers crossed.

Thanx again!

 

Are you running the stock Coil and wires?

I would get a higher output coil and lower resistance wires, run a higher octane gas, change the spark plugs, have the ignition module tested, and if needed replaced with a Motorcraft unit.

 

Yes, mattsbox99, it's the stock coil & plug wires. Fires right up everytime, but while driving it's bogging down REAL bad.

Recently changed the ignition module as there was an intermittent problem, sometimes it would fire, sometimes it would fire & immediately die. With the new ignition module, it's been fine.

Will definitely go with the hotter plugs & rebuild the carb (the carb is looking pretty funky, LOL!). If that doesn't cure this problem, then I fully agree that I'll have to go deeper into the ignition system. Thanx!

 

I think I paid $16 for the higher output coil, which actually helped my gas mileage, and my plugs are gapped a little on the far side, .046". I use splitfire plugs, the copper ones, not the iridium stuff.

My truck did the same thing as yours did, my problem was solved with higher octane gas and a carb rebuild.

 

Sounds good. Will keep the higher output coil in mind, but I think the biggest problem is the carb really needs some attention (rebuild).

First time this happened & the plugs were fouled, I was low on dough & just threw in some new cheapo plugs. They worked O.K. until this 2nd time (roughly 6 months later). Got the hotter plugs, copper, but waiting until I get the carb rebuilt before putting the new plugs in.

Just hoping that the carb rebuild/hotter plugs do the trick & it sounds like it might. If it's not "quite" up to snuff after that, will check out the higher output coil & higher octane gas.

Thanx again!

 

Check the float level and the fuel pressure and the needle and seat. High fuel level in the float bowl will cause a rich mixture.

Any of the things mentioned above can cause a high fuel level/rich mixture.

 

Thanx Guys........Well, it finally went down for the count tonight, LOL! No kiddin'. Bought the rebuild kit, it stalled on the way home, fired up, made it home, went to make a short drive tonight & it stalled again. Now it's cranking but not catching one bit. Got a push home.

I hope the rebuild kit is enough to make it right & hoping that it's not entirely trashed. The high float level could explain the seepage at the base of the carb I guess, plus maybe it's so messed up now, that the overly-rich mixture is causing it to act in a permanently "flooded" condition & can't fire at all, especially with the plugs probably being totally fouled out.

Put a new fuel pump in last year & when I changed the fuel filter a few days ago, it was pumping fuel when I turned the key, so I'm assuming that the fuel pressure is O.K. If not, I've got a new fuel pump stashed from last year's round of "fun", LOL!

Just a matter of gritting my teeth & doing the carb rebuild, changing the plugs & seeing where it's at. Will try to check in now & then if I get a chance.

Thanx again to All very much....

 

sorry to hear the bad news, hopefully we can armchair this one. Last time I heard something like this, it ended up being a timing chain. I hope yours isn't that bad.

 

If the carb rebuild fixes it, you should probably change your oil before you do any real driving. You can almost bet that all that excess gas has created a a fuel wash, and your oil is probably too diluted to stick to anything.