I have a 77 F100 4x4.
I bought the truck with the 400M being rebuilt already. It has a mild cam, torker single plane intake and a 750 edelbrock with headers and dual exhaust.

My question is this.
The truck idles fine and runs fine while cruising. the problem is when the accelerator is depressed aggressively it has a bit of a hesitation.

I know this current setup is all wrong for a 4x4.
I would like to go to a dual plane intake and a 650 max carb. does this sound like the right direction in eliminating the hesitation (I'm thinking current problem to much air and or gas)
Are there any other suggestions to check current problem?

I also notice that if I adjust the distributor to take advantage of the full power of the engine it will ping a bit on acceleration. If I back it off a bit to the point of no more pinging, It really hurts the power a lot.
can my problem be within the distributor?

 

Is your vacuum advance working? is your vacuum canister adjustable? which hole is your accelerator pump rod set on? is your vacuum adavance pulling manifold or ported vacuum? all the above can be reason for some hesitation or an off idle stumble.

 

Get it on a dyno and tune it from there.

 

I would suggest either a spacer plate to increase plenum volume, or adjusting on the accelerator pump linkage. Most likely the link needs moved a notch closer to the pivot for a faster pump shot. Should help with the hesitation. The pinging problem sounds like to me that the carb is on the lean side. 750's usually have 71-47 rods I would try a 65-37 (less jet restriction), and possibly a step stiffer metering rod springs from there. Hope this helps.

 

vacuum advance is working, the truck does not have a vacuum canister it never did when I bought it. (still passed smog test in AZ like that)
If I set it the way I thought was correct, i am pulling ported vacuum.

Dyno would be the surefire way to check everything out but All said and done between dyno, and sdjustments, i would be looking at $200-300 bill. I would like to see if I can get help here first.

I was thinking of changing the metering rods because I know the carb is overkill for this setup. i was hoping to get a bit better MPG as well.
if I set the linkage up for a faster shot wouldn't that still possibly dump to much gas quicker? How would the linkage adjustment help?

 

I've ran both a 750 carter/edey, and a holley 750 3310-4 (VS = vacuum secondary). I don't believe they are overkill unless you have too small (less than 210 intake dur @ 0.050") of a cam. The description of your pinging problem is telling me it's on the lean (too much air) side. However, if the hesitation comes with obvious smoke, then you have too much pump shot. If it hesitates without smoke then it's starving. It's one of those things your have to try and see what the motor is trying to tell ya. The spacer will also help with atomization with the AFB carb. (they've a tendency to puddle fuel)

 

There is no smoke at all. I have the carb adjusted at 2.5 turns which from what Ive read, should be correct.

 

tip in hesitation is accelerator pump shot issue.
either the accel pump is dead or the linkage to it is out of adjustment or...
that is the circuit that needs attention

 

2.5 turns outs is where i run my idle mixture too.

 

how can I check the pump and linkage adjustment?

 

Is there someone in your neck of the woods that can tune carbs? I had a local guy tune my 650 Edlebrock for $50. He is really good, he knows his stuff and has the right vacuum tools. We both feel that the truck has about 40 more horses now, its a heavily modified 400. It is very responsive and no hesitation which it was doing before.

Here is the nice thing, my gas mileage went up! It is more efficient now and I don't have to give it as much right foot to do make it go. It went from 7-9 MPG to 10.5-11 MPG. It is one of the best $50 bills I paid!

Unless you really want to do it yourself, I would get it done by someone good and it will be right.

Oh, have you pulled a spark plug and looked for signs of it running lean?

 

The linkage is the bar that runs from the bottom of your throttle lever to the 'rocker' that pushes down on the pump (right front corner facing the front of the carb). the 'rocker' has different holes in it for the linkage. changing holes here will change your pump shot. moving it a hole closer to the pivot of the 'rocker' will increase the shot, and vice versa. be careful with the small clip on the 'rocker' end of the linkage if changing on the engine.

 

wrong section sorry.

 

Like cfrazier77 said, check your spark plugs. They should be "light brown" in color for a good mixture of air and fuel. White plug is too lean (too much air or not enough gas) brown is too much gas and vice versa. This is a good cheap starting point. my02

 

My .02 on the timing advance. I spent a lot of time screwing with distributors and all the advance curves.

Most engines like a lot of timing, in early, but you must be able to lose some of it as rpm advances or the engine load increases. It can be done, you will just have to experiment on your particular setup. If you can afford dyno time, that would certainly be the best, but that isnt always realistic. Like most of us, our time is cheep! Take small steps and be sure not to run if it is pinging, back it down. First thing you need to do is understand the 3 advance systems on vehicles. First is what is set on the crank, thats the initial, but only a small part of the issue. Next is the centrifugal, which is the weights and springs under the point/pickup plate (newer electronics are different). Last is the Vacuum advance portion which deals with advance curves depending on the load on the engine. Obviously, when you add timing with one part, you generally have to take away timing on another.

There are many different opinions on timing, so you will have to form your own---after trial and error. If someone gives you an exact figure, (including me) only use it as a starting point.

My .02 for a starting point is stay fairly low on the crank 10-14 degrees, thats easier on the starter. Get some very light springs and have the centrifugal come all in very early, say ~2500 rpm. The slots in the distributor (as I recall) are in distributor degrees and will be 13 and 15 degrees which translate to 26 and 30 crank degrees. That will put you at a fairly safe power curve of ~38 degrees, which should be good even loaded----maybe slightly less. Lastly, limit the vacuum portion of your advance to ~6 degrees, you will have to add a small peg or strap in the vacuum portion to limit it. That will give you a general no-load curve of ~42-44 degrees for economy only, but you must lose that timing when you load the engine! This is only my suggestion for a starting point. I have (and family) run this setup for 10-15 years on vans and older bronco's, so it will work. Yours will be different! Compression, cams, gearing etc will all affect what you can use for timing.

Sounds like you have a lot of good advice on the carb part of this, so I hope my .02 helps. Your biggest problem is going to be just who to believe, you will, no doubt, get varied opinions. BTW, I do not claim to be an expert, but have been doing this for many years. All my stuff has always run very good and I have never broken anything due to my tuning---certainly no burned pistons etc! LOL