I've been having hesitation issue since I put a different 390 in my truck back in August.

I had hesitation in at all RPM's and it drove me nuts! Back in the fall I put a new accelerator pump in. The old one was stretched out. However that didn't cure all my hesitation. It did help with my high RPM's though. So I'm good there.

I still have hesitation in my low RPM's. I only have it when I'm going from a stop. Like if I'm stopping at a red light or a stop sign, the engine acts like it wants to stall then kinda jumps forward and then all is ok- if I'm moving I don't have any proble.

Also- if I'm in park/idling and i give it some gas, it will do the same thing- it'll act like it's gonna stall then the RPM's will rise quickly. Sometimes it will backfire through the carb (thats only usually when I'm rapidly tapping the gas pedal when i'm trying to start the truck in the morning). It has backfired through the carb while driving before, too! Only like 4 or 5 times in the past 6 months.

I also checked my timing this afternoon- seems good. I tried a few different settings and drove around the block- still hesitates.

What's next guys? Think it's the carb? I don't wanna rebuild it because in the next couples months I'm putting a stock 4 bbl intake and Holley carb on, so fixing my current Motorcraft 2 bbl isn't needed.

Any tips/suggestions/help?

Thanks a bunch.

Oh yeah- I checked for vacuum leaks but didn't find anything, but I'll be honest- I don't exactly know what to specifically look for.

 

What do the plugs look like ? What do you know about this motor ?

 

Was told it had 90k miles and that it was rebuilt "about 5 or 6 years ago". The previous owner is a retired mechanic. He used this engine in his old '74 f350 tow truck. To be quite honest, the engine feels sluggish- I doubt its working up to its potential. I need to get a vacuum gauge and learn how to use it.

Plugs are dirty, I bought some new Autolites. Don't wanna put them in til I get wires to go with them.

edit- also wanted to throw this in. Not sure if it was actually rebuilt. At the time I just needed an FE engine that was running as I needed to drive my truck. I can tell at some point he took the 2 bbl intake off- I can see the red sealer around the intake. I asked why he didn't use a 4 bbl for the truck and he said he prefered a 2 bbl for hauling cars, that he didn't care for any 4 bbl.

 

The usual methods of checking for an (intake) vacuum leak won't work on a FE for obvious reasons, the intake gasket is under the valve covers and bathed in oil, one way to check is to remove the pcv valve and breather/oil fill cap on either valve cover and place duct tape over the openings, start the motor and see if the duct tape becomes concave i.e. gets sucked inward, if so you have a vacuum leak.

 

Montana_highboy- I'll do the duct tape trick. I have to work during day light hours, so I probably won't get to do it until Friday morning at the soonest. I will report back here with my results, thanks!

Anyone else wanna chime in?

 

Hmm, does backfiring through the carb mean my timing is off?

edit- or can it mean my carb is running lean?

Or both??

 

Bump for morning crowd

 

Backfiring through the carb indicates a lean condition or that the timing is off. So, yes to both of your questions. Since you have a bogging problem, I'd point toward a lean condition. Can you actually see the accelerator pump shoot fuel in the primaries? I'd also throw a vacuum gauge on it and see what it tells you (a needle is worth a thousand words).

 

Since you are going to put a new carb on it soon the best things at this point is to be sure your timing it good, your current carb is adjusted properly (not to lean) and you have no vacuum leaks (like fmc400 says)

As an aside, for years I had the same hesitation problem and when I finally did replace the original carb with a new properly adjusted one (along with proper timing, etc.) all problems went away. Just a thought.. Oh, and I would go ahead and put the new plugs in now or at least clean and re-gap your old ones. It's not hard and it can't hurt! What about your wires?
Good luck!
Dak

 

Fmc400- yeah, gas is getting to primaries. I can't find a vacuum gauge anywhere. I'll just get one from Harbor Freight. Lean?- Just turn the air mixture screws out a little to open them?

Dakonthemountain- I'll triple check the timing. I have the plugs but not the wires. I'll grab them asap and do the plugs and wires at the same time.

 

Question- what should I be timing to? Never hurts to ask right? Haha

 

On a FE i like to run 10-12* initial, 26* mechanical (13L advance slot) for a 36-38* total.

 

Between all the rain, having to work, and having a sick wife to take care of, I'll never get around to working on the truck! Go away rain! I have off tomorrow, hopefully I can work on the truck.