Quadrajet stalls at high speed

Discussion in 'The Venerable Q-Jet' started by Buick Riviera 7, May 17, 2018.

  1. Buick Riviera 7

    Buick Riviera 7 Active Member

    I have a freshly completely rebuilt Quadrajet 4MV 7040247 on a stock 455ci and it runs good until 60 mph then stalls - as if starved for gas. It also runs good under WOT until third gear, then stalls. Electronic ignition, complete tune up, new electronic fuel pump - 40 gph, fuel line, vacuum lines and oil pressure sending unit on transmission. Vacuum advance on cap is taped off. Could it be another vacuum problem?
     
    Last edited: May 18, 2018
  2. flynbuick

    flynbuick Super Moderator Staff Member

    Are your fuel tank vents and fuel pick up sock clear? I suppose there is no debris in the filter located in the neck of the Q jet?
     
  3. Buick Riviera 7

    Buick Riviera 7 Active Member

    Thank you, Jim. I have clean new in-line filter and carb neck filter, clean sock on electronic fuel pump in tank. There is only one tube coming out of the top of the fuel sending unit on this 1970 Riviera, not sure where a vent would be located. The original gas cap is "non-vented".
     
    Last edited: May 17, 2018
  4. pbr400

    pbr400 68GS400

    Try driving it with the gas cap loose.
     
  5. Buick Riviera 7

    Buick Riviera 7 Active Member

    Will do, thanks again!
     
  6. telriv

    telriv Well-Known Member

    A '70 Riv. should have an electric pump.
     
  7. Cliff R

    Cliff R Well-Known Member

    Fuel delivery issue, probably not a carb problem unless someone rebuilt the carb at some point and put a small N/S assembly in it.

    Even a mild 455 will need the high flow N/S assembly and 40gph is marginal to feed one making decent power, IMHO......Cliff
     
  8. Buick Riviera 7

    Buick Riviera 7 Active Member

    Thanks, Cliff! I will check with the shop that rebuilt the carb on the N/S assembly. I confirmed with OPGI and others that a fuel pump with 38 gph would be sufficient and going too large would cause other problems. But I will keep this in mind.
    Just waiting for rain to stop here to drive it with a loose gas cap, as previously suggested.
     
  9. gsgnnut

    gsgnnut Well-Known Member

    sounds like a fuel delivery problem. have had problems with dirt on a needle and seat that did that. Also Had a kinked fuel line issue that did the same. Ask ken of everyday performance. if its carb related he will know all the possibilities.
     
    techg8 likes this.
  10. Buick Riviera 7

    Buick Riviera 7 Active Member

    Thank you for the suggestions! I will take a look at these.
     
  11. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Does it actually stall out at 60 MPH, or at WOT in high gear? Or does it nose over?
     
  12. Buick Riviera 7

    Buick Riviera 7 Active Member

    Hello Larry,
    It starts and drives fine up to about 65 MPH. Driving on the highway and getting up to 65mph it will then stalls out - like it is not getting gas. I feather the gas peddle a few times and it will start moving again, but below 65mph.
    From a dead stop and WOT it will go just fine until third gear, then stalls out like its not getting gas. It does nose over.
     
    Last edited: May 22, 2018 at 8:05 AM
  13. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    OK, when you say stalls out, that means to me that the engine quits completely and has to be cranked and started again. What you're describing is the carburetor float bowl going close to empty and the car noses over. It doesn't stall, it just leans out to the point where it completely stops pulling, but the engine continues to run. If you put a fuel pressure gauge in line, I'm sure you would see the pressure nose dive to 0 when it does this. You have a fuel delivery problem. Either the pump is not capable of supplying enough fuel, or you have a restriction somewhere. It could also be a problem within the carburetor as well.
     
  14. Buick Riviera 7

    Buick Riviera 7 Active Member

    Thanks Larry for the detailed explanation! I appreciate your time and will try the fuel pressure gauge.
     
  15. Briz

    Briz Platinum Level Contributor

    I was having similar issues with the Riv. chased it round and round. Found a smashed flat spot in the steel fuel line back near the R rear tire where a rock or something had hit it. Just for grins I replaced all the 3/8 line with 1/2" and installed a RobbMc pump. Have not had that issues again.
     
  16. Buick Riviera 7

    Buick Riviera 7 Active Member

    Briz,
    Thank you for your input! I will check the complete fuel line for dents and follow what you did by just replacing it.
     
  17. Buick Riviera 7

    Buick Riviera 7 Active Member

    Hello, Ran the car without the gas cap and still have same problem. Checked the fuel line - no kinks or dents. Fuel pressure at idle is 7lbs. Fuel filters are clean.
    The shop that rebuilt the carb said to remove the vacuum lines and plug the vacuum advance. I taped over the vacuum advance on the cap and put the hose over it to hold the tape in place and look stock. After taking a test drive the hose and tape were blown off! I put on new tape and again it was blown off. With the vacuum line on, off or the vacuum advance taped up, I still have the same issue of the car leaning out at 65 mph or under WOT going into third gear or about 45 mph.
    Are there vacuum hoses that should be plugged or removed when taping up the vacuum advance?
     
  18. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Vacuum advance has nothing to do with it. At WOT, vacuum is near zero. Vacuum advance doesn't function without vacuum. Fuel pressure at idle also means nothing. What is the fuel pressure with your foot in it?
     
  19. Buick Riviera 7

    Buick Riviera 7 Active Member

    Thanks again Larry! Do I run the gauge and tape it to the windshield and drive the car? Should I connect the vacuum advance hose and NOT tape it up?
     
  20. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Yes, you need to see what happens to the fuel pressure when you drive the car. Leave the vacuum advance connected, it is not the cause of your problem.
     

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