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Truck loses all power at full throttle??

Discussion in 'The Garage' started by 74Chev, Feb 5, 2004.

  1. 74Chev

    74Chev 1/2 ton status

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    I have been trying to diagnose this problem for 8 months and have even had my friend at a repair shop look at it and hook it up to a scope with no success or change to this problem. I have changed the distributor, new holley, new q jet, new plugs twice and new plug wires twice. Replaced fuel pump fuel line and filter. All vacuum hoses are new. The truck starts and idles great. Runs good until 3/4 throttle. Open secondaries all the way and the engine starts bucking and acts like it is running on 4 cylinders. No one has been able to figure this out. Back off the throttle a bit and the motor snaps right back into shape. This truck I have owned for 15 years may be gone soon if I can't figure this problem out. /forums/images/graemlins/angryfire.gif /forums/images/graemlins/angryfire.gif /forums/images/graemlins/angryfire.gif
     
  2. bshewmaker

    bshewmaker Registered Member

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    is this your 74? Whats the engine like? stock, radical?
     
  3. rjfguitar

    rjfguitar 3/4 ton status GMOTM Winner

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    Is it flooding out or leaning out at WOT?
     
  4. bigredblzr

    bigredblzr 1/2 ton status

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    Is it a FORD engine? /forums/images/graemlins/grin.gif
     
  5. 74Chev

    74Chev 1/2 ton status

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    Sorry forgot that info. It is a 350, 9.0:1 compression, 262 cam is about 1 yr old, edelbrock intake. Engine is mild has very good vacuum at idle. It is the 74 pickup.
    Thanks
     
  6. 89K1500Chevy

    89K1500Chevy 1/2 ton status

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    Most likely sounds like a internal problem. almost like the timing is off b/c you can have the timing off and have it idle perfectly but when you get on it backfire and run like sh*t. Take #1 piston to TDC and reset timing.
     
  7. the cowboy

    the cowboy Registered Member

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    Wow, that is a good one.... /forums/images/graemlins/thinking.gif
    Since you changed out the carb, I'm gonna guess something is amiss in the valvetrain. Have the valves been timed?

    I had a holley in a truck that did the same thing once. Secondary jets were fouled up.

    It would help to know the specs and miles on the motor.
     
  8. 74Chev

    74Chev 1/2 ton status

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    Yeah that is what i thought initially. I checked all that even pulled out my timing set and put in a new one same problem still. The only think I havent checked is the valves. They are hydraulic lifters so shouldn't really need to much adjustment I thought??? /forums/images/graemlins/confused.gif
     
  9. GMCLegacy

    GMCLegacy 1/2 ton status

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    what carb are you running?
     
  10. 74Chev

    74Chev 1/2 ton status

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    It is a brand new stage 2 JET quadrajet.
     
  11. the cowboy

    the cowboy Registered Member

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    Yer probably right. That happens, but it is rare.
    If the timing is on and there is no backfore, I would recomend hooking up a vacuum gauge and see what happens there. The "running on 4" symptom sounds promising.

    What tranny is in this thing?
     
  12. the cowboy

    the cowboy Registered Member

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    What tranny is in this thing?

    [/ QUOTE ]

    You might check the vacuum moduler on that th350 to see if it is "stealing" your vacuum under a load (when you get on it).
     
  13. the cowboy

    the cowboy Registered Member

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    What tranny is in this thing?

    [/ QUOTE ]

    You might check the vacuum moduler on that th350 to see if it is "stealing" your vacuum under a load (when you get on it).
     
  14. 74Chev

    74Chev 1/2 ton status

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    It is real weird. You can floor it and the problem starts, back off just a little and it goes away. Tranny is TH350, modulator replaced about 3 years ago due to sucking tranny fluid into manifold. This s%*t is really frustrating me. One other thing. I had an old piece of junk q jet on there and it seemed to make the problem almost go away. Is it possible that this new carb is miscalibrated or malfunctioning. I returned the Truck avenger Holley I had and put this new JET carb on becasue the Holley compounded this problem 10 fold. It blew black smoke and would pop and almost sound like a slight continuous backfire when climbing hills./forums/images/graemlins/confused.gif /forums/images/graemlins/frown.gif
     
  15. GMCLegacy

    GMCLegacy 1/2 ton status

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    is it a case of bad quad-ra-bog? could be too rich when the secondaries open.
     
  16. CyberSniper

    CyberSniper 1/2 ton status Premium Member

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    What's it got for secondary metering rods and hanger?

    I'd probably run an "H" to "K" hanger and DR or CP rods.
     
  17. the cowboy

    the cowboy Registered Member

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    This is a REALLY good one..... /forums/images/graemlins/thinking.gif
    My carb experience is with holley and edelbrock. Those Q-jets have a rep for being good 'bolt it in & go' replacements, especially new. If changing the carb changed the problem, then it is most likely a carb issue.
    I will talk to a guy tomorrow that really knows Q-jets. let me know what size the primary/secondary jets are and I will get back to you.
     
  18. 74Chev

    74Chev 1/2 ton status

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    I haven't checked the hanger or rods becasue I figured this thing would run good on my motor. I have lots of sets of hangers and rods co I could change them but i din't figure that was the problem, it doesn't just run bad it runs reallllly bad. I don't think it is quadrabog, because as it transitions to WOT the problem starts and stays the same for as long as you hold the throttle open, several minutes if climbing a hill pulling a trailer. When it is doing this the motor won't pull any more R's.
     
  19. 89K1500Chevy

    89K1500Chevy 1/2 ton status

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    what exhaust is on this truck im guessing just straight pipes but sounds almost like a backed up cat or some form of backed up exhaust. check ur pipes.
     
  20. CyberSniper

    CyberSniper 1/2 ton status Premium Member

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    To me, it sounds like a fuel problem. Since the carburetor is "new" it probably doesn't have any plugged passageways. Since you changed the pump, the lines, and the filter fuel is probably getting to the needle and seat. Since it'll tip in to the secondaries it doesn't sound like a float level problem. If the float is set way too low the accelerator pump will go dry before the normal 3-4 squirts.

    That leaves the secondary metering rods and hanger. A lot of remanufacturers will stick any old rods and hanger in a carburetor. Not many people drive on the secondaries and as long as it is "good enough" for a 170hp motor they don't care.

    Chances are, you'll find an "O" hanger with a pair of rods with a long taper and a fat end on them.
     

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