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Loosing power in 4th grear on freeway

Discussion in 'The Hokey Ass Message Board' started by Thor, Mar 1, 2012.

  1. Thor
    Joined: Jan 12, 2010
    Posts: 7


    Hi all,

    Last weekend I drove my truck on my first long distance drive to pick up some Cragar SS wheels. - for a 65 Fairlane...

    120 miles on the freeway. I used over 10 gallons of gas

    The car: 1959 GMC shortbed with original straight six 270, floor starter, 4 speed manual (SM420), original drum brakes all around, no booster, no fuel filter, original gas tank. Stromberg one barrel carburetor.

    The problem: Every time I was going uphill the truck would loose power and start hesitating after about 10-12 seconds. Giving more gas would make it worse. It backfired once.

    The first time it happened I was caught by surprice and the truck died on me. I pulled off to the side and it started right back up and I could continue on the freeway.

    If I eased off the gas a little bit it would make it over the top at slower speed/ less power and then have the normal power after a few seconds.

    The truck drives just fine around the city of San Francisco up and down all the steepest hills with no issues.

    Sorry for the long description here, just trying to provide as much info as possible.

    Thanks for any input on this I really appreciate it!

    Last edited: Mar 1, 2012
  2. 48 Chubby
    Joined: Apr 29, 2008
    Posts: 1,014

    48 Chubby
    Member Emeritus

    These symptoms sound a lot like the carb icing up. I've never had this happen except in a small plane. Ice can build to the point that air becomes restricted enough to run very rich at wide open throttle.
  3. Black Panther
    Joined: Jan 6, 2010
    Posts: 1,843

    Black Panther
    from SoCal

    I would guess fuel issue...weak pump or dirty carb or your points/condensor/spark plug cables are going south...I assume when you go uphill in town youre in low gear and on the freeway you were in high high gear under load is when weak points show...
  4. Black Panther
    Joined: Jan 6, 2010
    Posts: 1,843

    Black Panther
    from SoCal

    But it does sound exactly like a plugged fuel filter...but youre not running one..

  5. It sounds like the vacuum or mechanical advance is no working right.
  6. Engine man
    Joined: Jan 30, 2011
    Posts: 3,480

    Engine man
    from Wisconsin

    Fuel would be my first thought. When you back off the throttle you use less fuel.
  7. gas tank not venting enough?
  8. Thor
    Joined: Jan 12, 2010
    Posts: 7


    The carb is just rebuilt and the mating is good.
    I have brand new plugs and wires.
    The gas tank has the original cap - no lock - and I think it should have plenty of ventilation??

    I plan to take the gas line off right by the pump to see if I have a solid flow first off.
    Last edited: Mar 1, 2012
  9. If it took a few seconds to go bad, and pressing the throttle made it worse; it definitely sounds like a fuel supply problem.

    Why no fuel filter? Really.
  10. Special Ed
    Joined: Nov 1, 2007
    Posts: 7,132

    Special Ed

    Are you certain that you don't have at least a fuel screen at the carburator itself? How much fuel in the tank when symptoms kick in?
  11. rcoffey
    Joined: Dec 13, 2007
    Posts: 161


    accelerator pump in the carb mabey? some times the diaphram is weak will idle or low rpm fine but not so much under load.
  12. mike in tucson
    Joined: Aug 11, 2005
    Posts: 507

    mike in tucson
    from Tucson

    Symptoms of fuel starvation. At higher engine speed, needs more gas.
    Common problems: weak fuel pump, leak in fuel line allowing air to be pulled in instead of gas, kink in fuel line restricting flow, dirty fuel filter, water in gas, water in carb, icing, restricted air filter, or truck is a female.
  13. caseyrod
    Joined: Feb 9, 2008
    Posts: 138


    I agree with mike check fuel rubber lines specially from the gas tank to the hard lines & from the hard lines to the fuel pump all ways go to the simple and cheep solution first.if you have a pin hole in a straw its hard to get the fluids to your mouth, the same principle apply's to your fuel pump.listen its simple enough to check and repair try that first. good luck!
  14. Thor
    Joined: Jan 12, 2010
    Posts: 7


    I am confident the truck if fine up until the carb. There is no screen on the carb and no fuel filter. Not even hidden under the cab. I have three different accelerator pumps and it's brand new. It happens with 1/2 tank or full tank.

    I have a new rubber line from the gas tank to the fuel line and I will put that on this weekend and read up on testing the fuel pump.

    Truck is female and I'll whip her into shape.
  15. Lotek_Racing
    Joined: Sep 6, 2006
    Posts: 690


    Sounds like the sock on the pickup in the tank might be plugged.

    This exactly what happened to me in my '58 Pontiac.

  16. moefuzz
    Joined: Jul 16, 2005
    Posts: 4,950


    Just my humble opinion......

    If there is a lack of fuel supply or fuel pressure than how can it push 10 gallons in such a short time????????

    There is NEITHER "A Fuel Supply" OR "Fuel Pressure Problem

    Thor, check your "new" spark plugs to make sure that they are seated and snug/tight.
    Observed side effects of loose/non seated plugs is lack of power, excessive fuel consumption and exhaust backfire

    If your new plugs have backed out, the engines compression will escape past the spark plug threads and this will be more noticeable in higher gears (high gear = lower rpm + excessive advance With inadequate compression to fully ignite air/fuel mixture).

    The resulting low compression at the cylinder will hinder the ignition
    -the excess/unburnt fuel passes out the exhaust ports and into the mufflers resulting in Backfires.

    The more you push down on the throttle, the more fuel is supplied to the cylinder causing it to feel like it's flooding out while driving.
    -You will see your gas gauge drop rather quickly while barely making highway speeds.

    Backing off the peddle relieves some of the excess fuel problems just as you noticed.

    Does your engine require a spark plug with a metal crush/gasket/washer?
    -These plugs can tend to back out if the washer was not properly crushed/seated under initial installation.
    -Spark plugs should be torqued to a spec especially if they have a metal crush/washer and should be checked and re-torqued after the engine has run and heated.

    When you installed new plugs, did you check each hole to insure that old washers were not stuck to the head causing you to instal new plugs double gasketed?


    Failing that, your timing and ignition is incorrect/weak and/or is not supplying a hot enough spark to ignite the air/fuel mixture causing unburnt gases to enter the muffler/exhaust at which point
    It Will Backfire and use Excessive Fuel just as you have noted.

    check your engine oil/dipstick for gassy smell and change oil if it's contaminated (due to unburnt gas washing down the cylinder walls and getting into crankcase)

    Just my humble opinion


  17. Thor
    Joined: Jan 12, 2010
    Posts: 7


    Thanks everybody for your help!

    Moe, I like the sound of this...

    Does your engine require a spark plug with a metal crush/gasket/washer?

    yes they do and I will check them all. I did not check what torque to use, I just made sure they where tight in place.
    I put the gap a 0.2 or a bit higher.

    The more you push down on the throttle, the more fuel is supplied to the cylinder causing it to feel like it's flooding out while driving.

    This was def how it felt driving and I used 10 gallons which is a lot of fuel.

    - I will go though the tank as well to see if it's plugged in any way.
  18. Dane
    Joined: May 6, 2010
    Posts: 1,351

    from Soquel, CA

    I'd open up the carb and check for garbage. Sounds like it's running low of fuel in the bowl till you back off the go pedal. Make sure there is not a clogged screen in the carb inlet and do a siphon test from the tank to make sure it's not a pickup clogging problem.

    Second would be vacuum advance, but since it climbs SF hills OK maybe not.

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