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UGGGG....weak 350. Ideas???

Discussion in 'The Garage' started by jtexfisher, Aug 22, 2006.

  1. jtexfisher

    jtexfisher Registered Member

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    New motor blues. Truck is a '79 :burb: Originally 400/350/203,w/31's. 400 was WORN OUT, just flat worn out. Burned so much oil that I kept 5qt jugs handy. The rest leaked out. SOOOOOOOOOOOOOOO......

    Got a new 350 crate motor (12499529 - 350/290 HP) & an Edelbrock performer 3701. 2.5" hi flow cat/muffler/pipe. Single exhaust. Still using the rebuilt qjet from the old motor, but the part number from 'parts is parts' lists for both 350/400 motors. Trans/xfer case rebuilt too.

    Broken in now w/550 or so miles. Base timing @ 12*btdc. Good low end power, no trouble through 40-50 mph. Just don't try to climb a hill at 55 or better in 3rd. It takes 2nd to make any speed on that hill. Is this :burb: just that %^# heavy???:confused:
     
  2. blazinzuk

    blazinzuk Buzzbox voodoo Premium Member GMOTM Winner

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    could be a ignition problem. There is no way a crate 350 would be that weak, something is wrong
     
  3. mudbog42

    mudbog42 1/2 ton status

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    I have the same problem with my new motor (350) going up hills. Let me know if you find out what it is. I am getting my truck back today, I'm having a new Q-jet and headers put on and getting the exhaust(and a new cat put on) redone so we'll see of that helps.
     
  4. big dan

    big dan 1/2 ton status Premium Member

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    I put a crate motor in my truck but it only had 260hp but I had the same problem. after swapping 4.10s in place of my 3.08s, an sm465 for my th350, adding a cam, intake, headers, dizzy, and wires it pulls harder than any vehicle I have ever drove at any speed. the gears and the tranny made a big difference.
     
  5. mudbog42

    mudbog42 1/2 ton status

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    I've got 4:88's and a 700R on mine, so if the carb, headers, and exhaust don't do it I'll probably start looking into the ignition system
     
  6. budweiser_man

    budweiser_man Registered Member

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    where in oregon are u? what do u need exactly? I just pulled the tranny out of my 74 to replace it with a manual. I am in the eugene area.
     
  7. jtexfisher

    jtexfisher Registered Member

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    SM465?? I assume that requires one of them extra pedals on the floor ??
     
  8. jtexfisher

    jtexfisher Registered Member

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    Even worse....I drove the 'burbs brother( :truck: '79 2wd C10) on a parts run last nite. It's got a cheap 'motor warehouse' block w/Edelbrock performer cam/intake, Q-jet etc. Very much like the Suburban. WOW...all the difference in the world. Same hill.....hit it at 55-60, crack the throttle and up it goes. Nice, sweet and solid. Just cuz I'm mad at the Suburban :mad: I'm gona swap carbs today and see what happens.
     
  9. Ben B.

    Ben B. 1/2 ton status

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    Well it all depends on your gearing. If your front/rearend gears are off then it's quite possible that your new motor is operating in an rpm range where it's not making peak power.
     
  10. 1977k5

    1977k5 3/4 ton status Vendor

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    I would bet the carb is not tuned correctly.
     
  11. 1985_K5_Silverado

    1985_K5_Silverado 1/2 ton status

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    What is your axle ratio?
     
  12. mudbog42

    mudbog42 1/2 ton status

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    Have 4:88's in both axles
     
  13. 1985_K5_Silverado

    1985_K5_Silverado 1/2 ton status

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    With that ratio, you ought to be making around 3,000 rpm at 55mph. That 350/290 should be pulling pretty hard at that RPM. Can you say for sure that the ignition system is healthy, that you're getting enough fuel, and that the carb is properly tuned?
     
  14. mudbog42

    mudbog42 1/2 ton status

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    The ignition is going to be my next thing to look at. Other than that I'll find out if its the carb because I'm picking up my truck today. I just had a new carb, headers, and exhaust put on, so I'll see if that'll make any difference. Also the timing is right on and the plugs, wires, cap & rotor are good. So if this makes no difference I'm going to look into a new ignition module and coil
     
  15. jtexfisher

    jtexfisher Registered Member

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    K5... I haven't checked that yet. No stickers/tags etc. to use. I'll talk somone into letting me use their concrete and do the chalk thing to get an idea. Best guess is 3.3ish. (Feels like 1:1:crazy: ). Can't see how they'd be 3.73s or better the way it pulls and revs. Putting a tach on to today to see the revs at various speeds.

    Carb swap was fubar. K10 won't run on C10 carb. Go figgure.:confused:
     
  16. jtexfisher

    jtexfisher Registered Member

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    2250 @ 60 in 3rd w/29" .... 3.42 sounds close. I'm getting an idea..here are the symptoms. I put a few more tools to it this morning. I'm curious to see if any of ya'll come to the same conclusion.

    - weak power, accellerates a bit mushy
    - vacuum 15", wanders 1-2"
    - timed @ 12* btdc
    - connect vacuum advance, idle 1100 rpm
    - put it in gear, drops to 650-700
    - idle speed varies after runs (sometimes up to 1000 in gear)
     
  17. 1985_K5_Silverado

    1985_K5_Silverado 1/2 ton status

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    Have you pulled a plug or two, to check for color and deposits?
     
  18. K5dreamer

    K5dreamer 1/2 ton status

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    there is nothing wrong with the engine, youve swapped to an engine with low compression, small port heads, a tiny cam, and 50 less cubes than you were used to, its just not going to measure up to the original 400.

    going up the hills and such is going to be a matter of torque, i would reccomend a cam, head, intake swap. personally the GMPP cam from the HT383 would be the cam id look at, a set of vortec heads (check the junkyard, you can nab some for like 50 bucks) and a new vortec compatable intake. this combo would provide substancially more power, and would still get rather good gas mileage due to the small cam, and efficient burn in the chambers.

    chances are that the heads your using now are 73cc heads, youll want to check that, the swap to the 63cc vortec heads may cause compression issues.

    do you have the full specs on the motor? such as piston dish, chamber size, carb size, cam specs etc. etc. id be happy to do some numbers crunching for ya if you need it.

    oh oh thats another thing, are you using the carb from the 400 on the 350? if so you may be "over carbing" the engine. chances are youre running a 750 on the 400, with that anemic 350 you probly dont need much more than a 600 or 650 on it. the smaller carb actually feeds the engine better because it maintains a higher airflow velocity that better feeds the engine. the larger 750 carb theoretically flows more air, but at a much lower velocity which actually starves the engine of air.
     
  19. 1985_K5_Silverado

    1985_K5_Silverado 1/2 ton status

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  20. K5dreamer

    K5dreamer 1/2 ton status

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    ok few points noted, the engine calls for 34 degrees of timing, youre only running 12, that would make a big difference.

    you are running the bigger carb from the 400, the specs on the engine call for a 600 cfm unit.

    the cam is a decent choice and you could probly leave that be, its actually larger than i was expecting, although the HT383 cam would still be a nice step up.

    you are running the 76cc chambers on the heads,
    cylender volume is 703.9 cc's,
    8.6cc gasket volume given a .042in gasket,
    given an 8.5:1 compression you get a piston dish of... 9cc's

    so with the 63cc vortecs youd have..... 9.77:1 compression... still very streetable with a good cooling system and a cold air intake. although it would probly be much happier with 89 octain (plus) instead of good ol regular.

    id say fix the timing first, see if that solves your problem. if not go to retuning the Q jet for the new engine demands. its probly running pig rich from feeding the bigger engine.

    other than that, id say swap on the vortec heads and some headers, you could sill use the Q jet with that setup because the better flowing heads and exhaust would make alot more power and match better with the larger carb.
     

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