My truck has a very lumpy idle ('79 400) (750 rpm when warm). I don't recall my 351M being this way. I don't know the history of the motor, so I don't know if there are any internal mods. Still has the 2bbl carb, but who knows if it is the original one that came from the factory. All of the emisions controls have been disconnected, non-factory air cleaner (open element), 2 1/4" single exhaust (cat is still there) with glass pack. Runs strong, smooths out around 1000-1100 rpm. Almost feels like a cam issue. Anything other than a cam that would cause this?
Thanks!

 

I bet it is carb. Most cam problems amount to either missing or backfiring out the carb. On the other hand, every 400 I have ever listened to had somewhat of a "rough" idle and cleaned up. Do you smell a lot of unburned gas? What is the timing light telling you?

 

Some unburned gas. Haven't had a light on it yet as I need to go buy one. Haven't needed a light since I sold my '78 F150.

 

I would check for vacuum leaks as well.

 

Checked for vacuum leaks, can't find any. Now that I think about it, timing is most likely the cause as it is starting to "diesel" on shut down. I don't have a manual for this, so what is stock timing on the 400? And has anyone seen better performance and/or economy using a different setting?

 

For a '79 400, the ignition timing would be 8 degrees BTDC (according to my Chiltons Ford repair manual). Also, check your spark plugs. I have seen slightly fouled spark plugs that don't fire all that well that tend to make the engine sound a bit rough. The reason I mentioned the vacuum leaks was that I had a crack in my vacuum modulator line for my tranny, and that engine sounded a bit rough as well. I've never had a timing issue that caused the engine to run rough.

 

"I've never had a timing issue that caused the engine to run rough"

I have at a low idle. I still got money on the carb

 

probably the carb. Should just pony up the dough for an Edelbrock intake and 600 cfm 4v carb and be done with it.

 

rchewitt50
I agree that the carb may be the biggest culprit, but doesn't the dieseling sound like timing? That's the usual cause if I'm not mistaken (I could very well be...). Been spoiled by fuel injection for too long...

 

run-on after killing the ignition (aka dieseling) can be caused by several things:
- running too lean can cause cumbustion chamber heat to be high enuf for detonation without spark. try adjusting your mixture (or different jets) before replacing the carb...
- there's a dashpot (vacuum driven valve) which should cut fuel to the cylinders at shutdown. if it's missing or not getting appropriate vacuum, you'll get run-on.

hope that helps...

 

It helps, but I really have no experience working on carbs. Never had to mess with jets or anything on an automobile. Mikunis, yes, but that's a completely different animal. I should mention that the dieseling just started a few days ago, hadn't been a problem in the last 5000 miles. And it does not do it consistantly, only every few shut-downs.

 

You are correct on timing being related to dieseling, but it can still be the carb. If you didn't change the timing and it starts dieseling then it is most likely vacuum/carb related. If it is the 2150 then it is a pretty simple object to work with if you are not into spending money. If you were already going to do the Edelbrock route then fine, do it. I did, and it worked out great. See this thread also in re: to the valley pan gasket. https://www.ford-trucks.com/forums/s...ght=valley+pan

 

Thanks for the info. Guess I'll have to try and find a Chilton's manual with carb diagrams.

 

Timing can be related to dieseling but it is actually quite rare that it is the sole cause. The main reason for dieseling stems from too high of an idle or a vacuum leak, and from time to time low octane fuel. Some engines have an idle stop solenoid - this solenoid is actually what controls the idle, and you move the solenoid up or down to set the idle speed. When you kill power to the engine, the solenoid retracts, the throttle plates snap shut and no fuel\air can enter the engine.

Unless you have the other common cause which is a vacuum leak. How did you check for vacuum leaks? Spraying with a can of WD-40 is my favorite method. Common hard-to-spot problems include a leaking power brake booster or loose carburetor flange bolts. A rough idle is pretty indicitave of a vacuum leak...at the carburetor or elsewhere.

Many people notice a dieseling problem after advancing the timing, and hence blame dieseling on timing. But advancing timing can increase idle, and it is actually this increased idle and not the advance in timing that causes the diesel. The problem is cured by retaining the new timing setting and lowering the curb idle, which results in the best of both worlds. I know you didn't say that you advanced the timing, I'm just saying in general. This is not to say that timing doesn't cause dieseling, but like I said, it's actually pretty rare.

Why not just check your timing to see where it is at so that you can check that off the list. For your idle mixture screws, with the truck off, turn them all the way in and then back them out one and a half turns each to get in the ballpark. Maybe it's just time for a carb. rebuild anyway? It's the first thing I do when I buy an old vehicle. If this is a truck you have had a while and it just started doing this, it could be time. That's part of the fun!

 

without starting a new thread.

i too have some dieseling after shut down sometimes pretty rough and i know i have a vacuum leak at the intake which will be fixed soon.

plus i get the occasional flame out the carb. now i have no idea what the timing is set at. here are my specs

1979 f250 400M 4-speed stick
edelbroock performer intake (on when i purchased)
edlebrock 750cfm carb #1407 (on when i purchased)
headers unreduced to 3" piping (made from a roll bar) < dont ask i didnt do it, to dual 40 series flowmasters dumped right at the end of the muffler (end of cab)
new plugs, wires, coil, and cap, rotor


i have no clue if there are any other mods ie cam etc.

also what is the proper RPM at idle mine sits around 900-1000rpm


and can you tell if it is cammed without taking the cam out? probly not right?