hi guys my dad and i need some help. we recently changed motors out of my dads 78 f250 we took out a 400 mod. and put in another 400 mod. the other motor came out of another truck but i not sure what year the motor is. we didnt rebuild it just pulled the top end off down to heads and put new gaskets in it. we fired her up and it idle's like a dream but when you give it less then 1/4 throttle its like the truck just dies if you hurry up and let off the throttle and give it more then 1/4 throttle it reacts just like it should if you dont the motor just quits. we though it might be an intake problem so we switched out intakes that didnt work. had an extra carb sitting there from my 400 mod so we switch carbs and that didnt work ether we checked timing and it idles fine and its got a manual fuel pump and seams to be keeping the fuel filter full does anybody have an idea whats going on or were we should look cause were completly stumped at this point sorry about the long post thanks for reading


nate

 

Is it backfiring or anything of that nature? I would suggest a vacuum leak due to what work you have done on the engine, however that typically would cause a backfire out of the carb or tailpipes. Try hooking up a vacuum gauge and see what it reads

 

no no back firing at all it just acts like you shut the key off and as soon as you let off the throttle it goes back to idleing. were using the same distubuter cap and rotor off the other motor. thats what i thought to was a vac. leak but we've checked and checked again.

 

Well from you description it points to a carb problem IMO, have you pulled it apart and cleaned the jets? When you blurp the throttle fast the acc pump in squirting the fuel, however a slow push on it relies mostly on the jets. What carb are you running? And does this happen when the choke is on?

 

If you completely ruled out vacuum leaks i would suspect the carb as well.

 

Emd160- I'm having the samr problem. I replaced my accelertor pump and fuel filter. I'm still having the hesitation issue. All I can think to do now is to rebuild the whole carb. Just haven't had a chance to do it yet.

 

Two carbs doing the exact same thing?????

Is there any EGR on this truck? Seems that there is a lean condition, and this may lead to faulty EGR, or something related to the emissions control.

 

no i dont think its carb we put a brand new holley on it and it did it then we pulled it off and put the eldbrock carb that was on the other motor on it and it did the same thing i not sure about the egr i check with dad and get back to ya thanks for all the advice so far guys


i stand corrected zak8tac dad said tonight its back fireing through the carb it wasnt doing that before so i go down and steal it while he is at work and go over it again for leaks i dont think its the intake cause we changed intakes i could see us messing one up and having it leak but not two
75f350 it does not have any egr on it

 

That's indicative of the air/fuel mixture being leaned out, i'd check again for vacuum leaks.

 

Ill always lean towards a vacuum leak, however, I do believe that there could be a problem with the timing.
Can you verify that the timing is advancing? If the timing is too far retarded, and the throttle blades are open too far you will get a pop, and the throttle response will be poor at best. May not even run when the throttle is opened any further.

Another thing has entered my mind, and this might be some sort of binding on the throttle shaft itself. Is it pocssible that the kickdown or something is holding the secondary throttle blades open slightly?

Vacuum leaks, poor timing, or little to no timing or advance at all, will also cause this.
Also opened secondary throttle blades will cause this problem. If you have a spacer or something that can cause any type of interferance, you might have found the culprit.

All of the vacuum lines are correct right? How about the heat crossover? Any chance of an exhaust leak right into the intake?

 

i have no idea if its advancing i rebuilt them but havent worked on them much how would i check this (i would ask dad but he pretty busy and kinda gave up on it but i refuse to) i pretty sure the vac hose are right but i check them again though along with the other stuff you said about dad did say something about he though it might be the timing chain and gears

 

Timing chain is off one tooth. Gotta be...........
Could be the dizzy, but, if there is a chance that you are your dad believes that there is a problem with the cam timing, then you are probably right.
This will cause your symptoms.

 

i bet it is, i thought it run a little bit rough at idle if it was. You know 75f350 dad probally thinks that to but isnt telling me cause he knows i just get the truck and rip it down and fix it. He tells me not to be wasting my time on his truck trying to fix it to go do other things i want to do but i rather work on the old ford then do just about anything else. thanks for all the help guys i love this site i going to check over the truck tomorrow and maybe rip into it sunday merry christmas everyone

 

well the timing gears were right on and after talking to dad more he is not sure what year the motor is and thinks he got the wrong timing gear set cause napa list 4 differnt ones so anyone have any suggestions

 

One theory I have, do you have the vacuum advance hooked up to the wrong port on the carb? I did this and had the same symptoms. I wonder if that is your issue?