Car sputters under high torque at upper RPMs

Discussion in 'Troubleshooting & Diagnosis' started by Speed Happy '77, Apr 18, 2009.

  1. Speed Happy '77

    Speed Happy '77 Veteran Member

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    So here's the problem: While I was driving a month and a half ago, my car started sputtering at 3,200 RPM in fourth trying to pass a guy. Thought I'd check it out again, so I passed another guy to figure it out, and it started sputtering at about 3,000 RPM. Anyway, over the next week, I kept driving it till it did it at 2,200 going uphill.
    I cleaned out the whole fuel system, used the fuel system cleaner, ran an electronic fuel pump just under the carb to check for flow... worked fine. Replaced the old mechanical pump. I had the problem again with a lot more weight in the car, this time at around 4,000 RPM starting in second gear. I dropped it into third fast and fourth just as quick and it stopped, but I would like to figure out what this problem is. I've been tuning and tinkering all winter for a summer road trip and don't want to have this spring on me in bum-town nowhere. If it does, I'd like to save on labor costs at least by knowing what the problem is and being able to point it out.
    It's a '77 LT with a 305 4v (QJ, 600 or 650). As far as I know, this only happens with the secondaries open. The only thoughts I've had are the fuel pressure drops out with the secondaries, so maybe a worn cam lobe that doesn't cause trouble at low pressure or a clog that comes up in the fuel tank sock with higher fuel draw?
    Please let me know! Thanks,
    - JD
     
  2. mjoc

    mjoc Veteran Member

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    sounds like you may need to check the ignition system as well as maybe a carb rebuild.

    Mike
     
  3. gordonquixote

    gordonquixote Veteran Member Lifetime Gold Member

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    -Change the fuel filter?
    -Check for good ignintion ground?
    -Plug wire with a burned spot grounding out?
    -Distributer not advancing?
     
  4. tom3

    tom3 Veteran Member

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    Valve springs?
     
  5. muscl car

    muscl car Veteran Member Lifetime Gold Member

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    could be a number of things starting to fail like .......

    HEI module

    fuel pump

    coil

    weak valve springs

    HEI advance weights sticking
     
  6. Problemchild

    Problemchild New Member

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    This is probably not the problem but are the flaps that run off the engine vacume to open up the secondary barrels sticking? I had an old Quadrajet that did that to me once...
     
  7. Problemchild

    Problemchild New Member

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    I'm liking the "distributor not advancing" theory though...
     
  8. Speed Happy '77

    Speed Happy '77 Veteran Member

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    I run it on premium, advanced to the edge.

    I have, so far:
    Replaced plugs with non-fouling E3 plugs.
    Replaced plug wires, including wire clusters (to avoid wire touch, voltage loss)
    Replaced the fuel pump (with mechanical, but I ran an electrical to check flow first)
    Replaced the cap and rotor

    The distributor is old, so I could check that, but I was getting good spark when I tested the new plugs/wires/cap/rotor at WOT and it will do higher RPM in lower gears, so I wasn't sure if that could be the distributor since it seems to be affected by actual speed, not just RPM past a certain point. The coil is also relatively new (still shiny, as a matter of fact)

    Valve springs sounds a little more likely, it seems? What should I do if they are bad? Replace them with stock, or better than stock? I have a few hundred bucks to do this, so if I'm taking off heads and can find parts on CL, what else should I do?
     
  9. Speed Happy '77

    Speed Happy '77 Veteran Member

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    I found some stuff on CL. Would it work? And what would it do for the engine power?

    small block shevy tripple valve springs. Manley 221443-16 NexTex springs 1.580" double w/ damper inst. 235 lbs at 1.950 open 610 lbs at 1.250" coilbind 1.170"
    batch 6-30-08. $100.00


    Also, for $145:
    [​IMG]
    [​IMG]

    Lastly, headers? How much work would they be for a 305 and would these do the job right?
    They're $100
    Brand: Hedman Hedders
    Product Line: Hedman Street Hedders
    Part Type: Headers
    Part Number: HED-68460
    Header Style: Full-length
    Header Material: Steel
    Primary Tube Diameter (in): 1 5/8 in.
    Collector Attachment: Ball and socket flange
    Collector Diameter: 3.000 in.
    Tuned: No
    Primary Tube Gauge: 18-gauge
    Flange Style: Standard
    Flange Thickness (in): 1/4 in.
    Bolts/Studs Included: Yes
    Gaskets Included: Yes
    Reducers Included: Yes
     
  10. 79supergasdriver

    79supergasdriver A Scholar and a Gentleman

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    Lots of threads dedicated to the art and science of selecting proper springs. Bottom line: all stock engine and cam=all stock springs. Those rockers are nice peices; I know a few guys at the local strip that swear by them. Check around though; you may find them cheaper. Also, headers are nice on an old 305. The wieght savings switching from the cast iron and bulky full exhaust makes it worth the minimal effort and expense. Ensure they support your end goals so you don't end up buying two sets for different engines. Lots of threads dedicated to proper header selection/tuning as well.

    Either way, I don't think either of these things will solve your popping issue. Sounds like you're going lean at high RPM under load. This happened to me last season. I spent every weekend replacing ignition parts and chasing this issue...turns out the mechanical fuel pump was slowly collecting goo inside, and the engine would starve at WOT as RPM climbed. The dirtier the pump got, the earlier in the RPM band it would start popping. The only way I figured it out was to take the line that comes off the pump to carb into a container and crank the engine, watching for steady solid flow. It looked like an old man squeezing out kidney stones. Replaced the pump and ran my fastest times ever!
     

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