No power from 50 mls and up

Discussion in 'Street/strip 400/430/455' started by colonel, Aug 11, 2021.

  1. colonel

    colonel Speedjunky

    Not sure if this is the right section, if not pls move thread. But it´s engine related and it´a 455 :)


    My new 72 Electra with a 455 starts bucking at around 50 mls per hour and then looses power, no matter how far you push down the gas pedal. Sometimes it bucks at 45, sometimes it goes to 60 and starts bucking then. Kickdown from any speed same thing. Feels like it is not getting any/enough fuel. Yes, tank is still half full. Slow accelaration until like 40-45 mls is fine, but I never got her over 60 mls.

    Had ignition checked, ignition was good. Had idle and CO checked and re-adjusted (was too lean, now 4%), checked vacuum hoses and corrected vacuum to distributor and some other hoses as per FSM. Air filter is ok. Engine idles at around 600, starting behavior is perfect (was fine before). All those changes made no improvement.

    Today I replaced the fuel filter and fuel pump (replaced a most likely factory fuel pump with the skid plate on the arm for an aftermarket one without plate). Car ran great up to 70 mls and I already thought "Ok, great, I got it", and she starts bucking again. On the second run she started bucking at around 50. I do think the accelaration up to this speed is a bit better now, but the main problem still exists.

    Any ideas?
     
  2. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    So now you need to inspect the fuel line from the fuel pump all the way back to the tank. Look for ANY kinks in the metal line, and ANY soft/porous rubber hose near the tank. Then you need to drop the tank, remove the sender and look at the sock fuel strainer.

    If you want to confirm the fuel delivery problem, you can rig up a cheap temporary fuel pressure gauge with rubber hose (from fuel pump to carburetor) and some kind of tee. Tape the gauge to the windshield and go for a drive. Look at the fuel pressure when the symptoms occur.

    https://www.jegs.com/i/JEGS/555/15270/10002/-1

    https://www.jegs.com/i/JEGS/555/41010/10002/-1

    https://www.jegs.com/i/JEGS/555/41057/10002/-1
     
    Last edited: Aug 11, 2021
    Oldskewl59, 1973gs and Smartin like this.
  3. pbr400

    pbr400 68GS400

    How old is the timing chain?
    Patrick
     
  4. colonel

    colonel Speedjunky

    Thanks Larry, will try. I thought about swollen fuel lines?

    @patrick: No idea, why? The PO said, he had a 3 angle valve job done for some reason. The engine was out and painted (so is frame, engine bay, undercarriage and body), looks brand new, don´t know if they re-did the timing chain at the time.
     
  5. pbr400

    pbr400 68GS400

    Sometimes a slack chain will do that-classic sign is stumbling and even spitting through the carb when you kick it to downshift at speed. You can sneek up on a speed but can’t punch it.
    Patrick
     
  6. Schurkey

    Schurkey Silver Level contributor

    Bucking as you describe is very likely a fuel delivery problem. A fuel pressure test on the carb side of the fuel pump is a great idea. You replaced the fuel filter at the carb fuel inlet...right? Not an add-on filter somewhere in the fuel tube?

    Blocked fuel passage(s) in the carb is another possibility.

    With the engine off, you could verify that the Q-Jet power piston is free to move by poking a coffee stir-stick or plastic straw down the vent tube over the float bowl. You should be able to press on the primary metering rod hanger, and feel it sink against spring pressure--and then pop back up as you release pressure on your "special tool". Be sure you're pressing on the metering rod hanger, not pushing on the FLOAT.

    A weak ignition coil could cause this, too. Provides enough spark under light load/low RPM, but falls on it's face at higher load/higher RPM. Test the ignition coil at the coil wire using a spark tester. If you have an OEM resistor wire in the primary ignition circuit, use the "points-style" spark tester. If you've upgraded the ignition system and removed the ballast resistor wire, use a spark tester calibrated for HEI ignitions.

    While I don't think a wiped timing chain would cause "bucking", you could check for slack by aligning the timing mark with "0" on the timing indicator scale, getting to "0" by turning the crank in the normal direction of rotation. Remove distributor cap. Watch rotor as you turn the crank BACKWARDS slowly. As soon as the rotor moves, stop turning the crank. Look at the timing indicator scale. Anything more than 5 degrees of movement is suspect, 10 degrees is my limit for timing chain wear. Of course, this also checks for gear wear between the cam gear and distributor gear--most of the time this isn't a problem. You'll find out about the gear wear when you pull the distributor as you begin the timing chain replacement. If the gear is good...the timing chain isn't.
     
    Last edited: Aug 12, 2021
    knucklebusted likes this.
  7. No Lift

    No Lift Platinum Level Contributor

    Are you saying this bucking happens at WOT around 50 or 70 mph or are you saying you are cruising at part throttle down the highway and it starts to buck when you hit that speed?
     
  8. colonel

    colonel Speedjunky

    Cruising at part throttle, approaching 50 or so mls an hour. But kickdown also, and then at any speed.
     
  9. 1973gs

    1973gs Well-Known Member

    Do as Larry said, inspect the rubber hoses. Back in the early 80's I was working at a Buick dealer. I had a Lesabre that was unable to go up a long hill. Of course, I rebuilt the carburetor. It still wouldn't go up the hill. It must be the fuel pump. So I replaced it. Still had the same problem. I finally figured it out. The rubber hoses over the axle were dry rotted. I couldn't smell any fuel and there were no leaks.
     
  10. colonel

    colonel Speedjunky

    Will do as soon as I get to it.

    The damn thing died on me twice on the interstate today. I took her for a 100 miles round trip, on the way there it ran just fine, up to nearly 80 miles an hour on cruise control. On the way back it started bucking after like 2/3s of the way and wouldn’t take any gas at all until it finally died. Had a hard time getting it started, but then it ran as if nothing ever happened. Until it died again like 10 miles later.
     
  11. colonel

    colonel Speedjunky

    So I checked the hoses today. Everything in the front looks older, but ok. The hard lines look untouched. The two main hoses in the back look replaced, actually brand new. The smaller one (5/16? Return line?) is stiffer than the bigger one (3/8” Goodyear Fuel Line, as by the printing). I can compress the bigger hose with two fingers. Can it collapse at higher speeds, maybe?

    I ordered some new fuel hoses and hope to be able to replace them next week.
     
  12. qak

    qak Well-Known Member

    Losen or remove the fuel tank cap then try, IF the tank vents are plugged that soft Goodyear hose might shut under vacuum with a factory puller pump, probably not with an electric pump.
     
  13. bobc455

    bobc455 Well-Known Member

    I wonder if your coil is overheating. Might test good when cold but then falls apart when hot. Do you feel like it could be a miss?

    -Bob C.
     
  14. colonel

    colonel Speedjunky

    No, not a miss, the power goes away as if you were running out of fuel.

    I will try with a lose tank cap. I got new fuel hoses today and wonder if I can get to the clamp on the feed outlet of the tank unit without having to lower the tank? Maybe with a stub screw driver, but must be pretty tight from what I have seen so far.
     
  15. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    You may have to lower the tank, but not drop it completely. If that is the case, it's entirely doable, unless you have physical constraints.
     
  16. Torqued

    Torqued Active Member

    I had a similar issue with my car. Does your car sound like this 3rd gear ~40 light throttle? In the video I was going about 40 and gradually stepped into the throttle in 3rd a couple of times. Instead of picking up speed the car slows down along with sounding like crap. I assume those are misfires? If I step in quicker the car falls flat without the racket as if it were starved for air or fuel.

    Long story short…check your plugs and wires. After I replaced the plugs (.035 gap) and ensured the coil to cap wire was seated the car runs as it should. You mention the ignition was checked, do you know what they looked at?

    Initially I troubleshot this from the fueling perspective and did not make any improvement after replacing the pump and rebuilding the carb with Cliff’s parts. I t’d in a gauge to confirm the FP was good. Once I had confidence in the fueling, I switched to the electrical. In hindsight I should have simply pulled a plug. I skipped this step since I had less than 100 miles on the set. All of my plugs were very carbon fouled. Fouled Plug I’m not sure if this was due to the .045 gap or that the coil to cap wire may not have been making a good connection.
     
  17. 482

    482 Big Member

    Had these exact symptoms once with a customers car, followed a similar logical path as what's been presented here except that I had replaced the spark plugs previously as part of the tune-up. I couldn't figure out the problem to save myself and it was becoming embarrassing when even the boss came down and started asking what's wrong with you why isn't this job done yesterday. It was driving me crazy, after much grief and pulling the fuel tank because every thing else had been checked and replaced twice I found the problem. One of his kids had been playing in the driveway and stuffed a plastic bread bag down the tank filler spout. It would randomly float around until it would suck on to the tanks fuel line inlet and stop or make the car buck, would start again and run fine after sitting a minute letting the plastic bag float off again for a random amount time. People wonder why mechanics swear.
     
  18. Schurkey

    Schurkey Silver Level contributor

    Fookin' right!
     
    Waterboy likes this.
  19. MOBUICK

    MOBUICK New Member

    Check the resistance wire that goes to the coil. On my 73 century the choke had rusted and hot exhaust was cooking the loom which the wire runs though. Car would start run fine than dead in the water.
     
  20. Buick

    Buick Ramin Ansari

    You said this was a new to you vehicle? A couple of your events seemed to happen on the 2nd run or maybe later in the day? Check for vapor lock. Insulate the fuel lines in the engine compartment or that may run near exhaust from the fuel tank.
     

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