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Cummins PT fuel pumps

Discussion in 'Trucks' started by Ruger_556, Apr 13, 2019.

  1. Ruger_556

    Ruger_556 Well-Known Member

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    I don't work on old stuff much, is the air compressor and fuel pump timed on a Big Cam Cummins with a PT pump? Also are the pumps interchangeable, say I have an NTC 400 will the pump work on a 350? I know nothing about these old engines and the service manual I have doesn't say much about the fuel system.
     
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  2. trucker1

    trucker1 Well-Known Member

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    No timing for the pump to the engine, it just pumps fuel for the engine and timing is done thru the camshaft with rockers for the injectors, next to the rockers for the valves. You can probably put a 400 pump on a 350, but there are small changes inside the pump, a "button" that meters the amount of fuel that goes thru to the injectors, and the injectors would be different, but not that much. years ago I rebuilt a 300, put on a 350 pump and injectors. There is also an offset cam key, but did not change the key in mine, and it ran fine like that for years.
    George
     
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  3. kshansen

    kshansen Senior Member

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    Can't talk to big Cams but the small cams all did have the air compressor "timed" to the engine but NOT so it would run right. Sounds odd but.....

    The compressor drive gear has timing mark on it as the marks on the accessory pulley on the front of the engine has the marks you use when setting injectors and valves.

    Some of the older engines had the crankshaft for the compressor that was one piece with the shaft that the accessory pulley mounted on. Newer engines had the two shafts splined together making it much easier to replace a bad compressor.

    On those older engine with the one piece accessory shaft/compressor crankshaft if you were replacing a Cummins single cylinder compressor it was common to get a reman compressor with no crankshaft. You could actually work the rod for the compressor over the end of the crank without removing from engine.

    If the crank was bad in the compressor and you needed to replace it first you would remove the accessory drive pulley. Behind it in the front cover of the engine there was a 3/8 pipe plug that you needed a, I believe, 5/16 square drive socket to remove. Then you had to watch in that small hole while turning the engine over until you could see the timing marks on the compressor drive gear and the cam gear lined up. Then remove the old compressor and install the new one making sure the marks line up again.

    Or if you were a real a$$hole and not going to be doing a tune-up just forget about the timing marks and yank out the old one and stick new one in and let the next sucker pull his hair out while trying to set valves and injectors.
     
  4. Shimmy1

    Shimmy1 Senior Member

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    Are you speaking from experience as the recipient, or the perpetrator??
     
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  5. trucker1

    trucker1 Well-Known Member

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    Thanks for catching me on that kshansen. Its been about 25 years since I had the compressor and fuel pump off my big cam, and you are right. The big cam also has to have the accessory drive timed off the camshaft, just like the old small cams. I never had the front off of mine before. But I broke down and looked at my service manual and it gives a detailed description. After the accessory drive is timed, you also have to time the compressor to the accessory drive. But the fuel pump just bolts on to the back of it, no timing there.
    George
     
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  6. RZucker

    RZucker Senior Member

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    FWIW, my N-14 manual states that "timing single cylinder compressors is no longer required". Something change on N-14's?
     
  7. funwithfuel

    funwithfuel Senior Member

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    But if you install a holset single lung compressor, you'll notice a knock . Something about the harmonics. Time it up, knock goes away.
     
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  8. Truck Shop

    Truck Shop Senior Member

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    Just a damn cummins,lol
     
  9. DoyleX

    DoyleX Senior Member

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    A big cam makes 2hp per pound of rail pressure. The only difference inside between a 350 and 400 is the fuel timing. You could pull the cam followers and put in another timing gasket sliding your timing back a bit. Or just give the screw in your throttle shaft a eighth turn in cutting the rail pressure.

    A old friend of mine ran cummins in a 80 trk fleet for 50yrs. In the small/big cam days he could look at the liners and tell you what hp they were at (350 or 400) All the 400's had counterbore problems. That was corrected with the lower press ft liner. It had a 3/8 lower thinned lip that protruded into the block. As the injector fires the liner distorts. This thicker liner lip solved about all the liner problems besides cavitation.
     
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  10. kshansen

    kshansen Senior Member

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    Think I only got caught by that one time when setting valve and injectors and things just plain did not seem to be making sense!
     
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  11. RZucker

    RZucker Senior Member

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    I got an injector push tube or two the first time I ran into that. I should have felt it barring the engine over. :rolleyes:
     
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  12. Truck Shop

    Truck Shop Senior Member

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  13. DMiller

    DMiller Senior Member

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    PT is Pressure Time. Timing is more important than pressure, do not change cam timing not going to get the response you look for. As to the Compressors, the old shop manuals had a Off Timing set for the one lung compressors that was to avoid a harmonic, on the two cylinder units still want off time so one cylinder is not hammering as the engine is coming on a power stroke but not as critical.
    When the BC series came out and the Old Hands tried their old school tricks to add fuel pressure and fuel volume not much more than smoking occurred. Takes a little more than that as to adjusting the restrictor insert in the throttle shaft, button change, changing injectors to a different rate for bypass, changing cam timing and injection point timing(offset key and cam box to engine). Stingers were still the preference of a lot of guys, then came the Blanked Governor spring cover and break away cover bolts, made it difficult to adjust idle speed but stopped the twits from over speeding engines.
     
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  14. Truck Shop

    Truck Shop Senior Member

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    10 orifice injectors are the way to go when adding more fuel by changing the button. A #22 button changed to a #15 button will over fuel 10%. Which adds 10% more stress to the
    injector lobe on the cam, #5 button 25%. If a person can find a shop that can build ten orifice injectors-those injectors will drop the rail pressure back to tolerable level.
    The NTA 420 had 10 orifice injectors from factory. The 10 orifice injectors in my BC III were custom built along with a KT 450 pump with a 1" gear and a reworked 444 turbo.
    It was a very strong BC III but the real power came from the injectors.
     
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  15. DMiller

    DMiller Senior Member

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    Should purr like a mountain lion on a scent
     
  16. td25c

    td25c Senior Member

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    No timing on the pump or compressor .

    Yes, the fuel pumps will interchange given the engine specifications .

    Set the low idle at 750 to 800 RPM and the " air compressor knock " go's away . :)
     
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  17. DMiller

    DMiller Senior Member

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    Only minimal differences on pumps engine to engine in the older Cummins, will not get a 400 out of a 350 swapping pumps, minimal to no changes on delivered power. There is a CPL number stamped into the Eng ID Tag, also on the pump on a data plate top just rear of the tach drive. CPL will give Cummins the information needed to set up stock. A good pump shop can set one up for a little faster RPM, a little higher fuel pressure and reduce the restriction thru the throttle shaft, if Aneroid equipped can set that a tad hotter but limited as to how far they can do so under EPA crap.
     
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  18. Ruger_556

    Ruger_556 Well-Known Member

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    Thanks Guys!
     
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