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turning up a b model cat motor changing afc diaphram

Discussion in 'Trucks [ Eighteen Wheelers ]' started by log trucker, Jan 26, 2012.

Page 1 of 4
  1. Jan 26, 2012 #1
    log trucker

    log trucker Light Load Member

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    Dec 19, 2011
    marshfield wi
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    Im going to change the afc diaphram do I need to take off the whole afc or can I just remove the first 2 sections to replace diaphram? Then the plunger in there I was told to turn that in until can just lock the nut down what exactly does that plunger do and what do I gain by screwing it in ?? Then I'm going to turn the fuel and torque screws out 2 turns,or should I wait to see if the diaphram helps,need more power this engine no power on a hill. I thought it was a 425 HP but tag on motor says 350 could of swore previous owner said 425. Engine numbers are 7fb90778 arrangement 4w3770
     
    Last edited: Jan 26, 2012
    log trucker, Jan 26, 2012
    log trucker, Jan 26, 2012
    #1
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  3. Jan 26, 2012 #2
    bender

    bender Road Train Member

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    Don't Kid Yourself
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    The purpose of the AFC is to sense manifold pressure (boost) and delay fuel delivery until manifold pressure is achieved. This helps in running a clean stack but throttle response suffers. Adjusting the AFC as described will fuel the engine early without requiring manifold pressure. You will gain instant throttle response and bellowing of black smoke out the stack. Driveability will be improved but overall power such as for pulling grades would remain the same.
     
    bender, Jan 26, 2012
    bender, Jan 26, 2012
    #2
  4. Jan 26, 2012 #3
    log trucker

    log trucker Light Load Member

    161
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    Dec 19, 2011
    marshfield wi
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    It seems to lack get up and go more on the hills just going up small grade be doing 65 and by time I round the hill I'm down a gear and doing 52 it pushes about 18 to 20 pounds boost, from what I understand even a 350 HP motor should pull better. The parts guys at the dealer said they use to change a lot of those diaphram's. Is it possible that diaphram is bad. Would I be better off leaving the afc setting and turning out the fuel screws a couple turns ?
     
    log trucker, Jan 26, 2012
    log trucker, Jan 26, 2012
    #3
  5. Jan 26, 2012 #4
    bender

    bender Road Train Member

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    You can test the integrity of the AFC diaphram with a hand vacuum pump to your boost signal line. If it holds vacuum, then it's not the problem. Ultimately you would see 25 psi boost in a hard pull. Adjusting the AFC out of the picture would eliminate the need to change it if it isn't working properly. The black smoke produced from eliminating the delay can be a nuisance. If you decide to make fuel pump adjustments, smaller adjustments such as 1/4 turn at a time until desired performance is achieved could be the wiser choice.
     
    bender, Jan 26, 2012
    bender, Jan 26, 2012
    #4
    log trucker Thanks this.
  7. Jan 26, 2012 #5
    Les2

    Les2 Road Train Member

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    If you change the AFRC take it off to do it and be careful not to lose any springs! When you put it back in, the end of it, which looks like a nail head, has to go over a fork. Easy way to check is put it in place and pull back out a little to see if it is hooked.

    As for the screw in the end of it, its just for throttle response. The more you screw it in the better it will be but the more smoke it will blow. I got a video of what one does without the AFRC...:biggrin_2556:

    Have you owned this truck a while and are you comfortable with the durability of the engine?? If so, turn the screws out 5 turns each. It'll run then but you will have to watch turbo and water temps just to be on the safe side.
     
    Les2, Jan 26, 2012
    Les2, Jan 26, 2012
    #5
    log trucker Thanks this.
  8. Jan 26, 2012 #6
    little cat 500

    little cat 500 Road Train Member

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    we have a 425 with a 350 pump on it but i put it there just because thats what i had laying around i tured the screws out 6 turns did have the diaphram screw in all the way but it smoked like a freight train i just turned the screw on the diaphram back out a turn at a time till it cut the smoke way back and had good throttle responce
     
    little cat 500, Jan 26, 2012
    little cat 500, Jan 26, 2012
    #6
  9. Jan 26, 2012 #7
    Les2

    Les2 Road Train Member

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    This wasn't with the AFRC blocked off but with the screw turned in almost all the way......

    Turn volume off.

    [​IMG]
     
    Les2, Jan 26, 2012
    Les2, Jan 26, 2012
    #7
  10. Jan 26, 2012 #8
    07-379Pete

    07-379Pete Crusty Commando-Pete

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    Ever text and drive? Just askin.:biggrin_25523:
     
    07-379Pete, Jan 26, 2012
    07-379Pete, Jan 26, 2012
    #8
  11. Jan 26, 2012 #9
    Les2

    Les2 Road Train Member

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    Who me? Nah!

    That is a crappy cell phone video though!
     
    Les2, Jan 26, 2012
    Les2, Jan 26, 2012
    #9
  12. Jan 26, 2012 #10
    log trucker

    log trucker Light Load Member

    161
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    Dec 19, 2011
    marshfield wi
    0
    I know the head was redone a while back and was told by a shop that use to work on it that it had been rebuilt awhile ago by another shop. The guy I bought it from was the son of the owner he only used it off and on for last 4yrs or so mostly for firewood after his dad passed. It was always worked on by 2 shops I was at one getting a pin for pup trailer and asked they claim to remember his dad having it rebuilt. It has good oil pressure on cold start I idle it up about 900rpm it runs 70 to 75 pounds oil pressure warm it is 40 or so warm running down the road it runs 60 to 65 pounds. I thought that was good. No blow by out the breather tube or at least not much. Just is doggy on the hills I think it should be better. I will vacuum check the afc and maybe just do some small adjustments on the fuel screws. I don't need mega power just wana pull the hills better. I may have little more boost than 20 on a pull I just used a old gauge I had it reads a little low. Do the 350 cats just run kinda doggy ?? Are the 425 cats that much better ??
     
    log trucker, Jan 26, 2012
    log trucker, Jan 26, 2012
    #10
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