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Backfires when accelerator pressed down to take off

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  • Engine: Backfires when accelerator pressed down to take off

    1957 Silver Hawk with a 1958 289 President engine.
    I have owned and been driving the car for about ten years. I have tuned it up a couple of times. It has always ran fine and no problems. It sat for several weeks and I started it today and started driving it. At first everything was fine except it ran a little rough. However, I started to pull away from a stop sign and it died. It started right up but would die when I put it in gear. I found if I revved up the engine it would take off just fine. I did this several times until I got home. I pulled one of the plugs and it looks like new. Looking at the fuel filter it stays constantly full when the engine is running. If I reach under the hood and open the throttle fast by hand, the engine will die or backfire thru the carb. I am not a mechanic so I am not sure about what is causing the problem. My first guess would be the timing but I hate to start messing with the timing unless I am sure that is the problem. I have not checked the points yet but I will tomorrow. They should be OK as they only have a few hundred miles on them. Any suggestions will be appreciated.

  • #2
    This is a really stupid question but did you check to see if your vacuum advance line is still connected on your distributor?


    • #3
      Most likely the accelerator pump in your carb has stopped working and it is not giving the engine a shot of gas as you take off. Especially when cold the engine will either die or will backkfire through the carb. Time for a rebuild.


      • #4
        If you did not turn your distributor, it is not a timing problem. The only ignition timing problem that can happen without turning the distributor is the points rubbing block wearing down, or the vacuum hose falling off.

        Accelerator pump is my guess too. Look into carburetor venturi and see if a good stream of fuel is in both barrels when moving accelerator linkage.
        Frank DuVal

        50 Commander 4 door


        • #5
          Frank Duval, I have not turned the distributor so I guess it is not a timing problem. I checked the points and saw no problem there. I noticed the two arms of the points do not line up perfectly . The two "points " only overlap about half way but I do not think that would cause a problem. I do not have a vacuum hose. The only line going to the distributor is metal tubing. I cannot look into the carb venturi as the opening faces towards the firewall.

          Ross, I had the carb rebuilt by one of the Studebaker Forum members several years ago. Do they fail often?

          Big Dan, the metal tubing is still attached.

          I appreciate all of the input. I checked the spark plugs, all look good. I now only get the backfire through the carb if I am under the hood and open the throttle rapidly several times in a row. That is probably my fault as the cylinders are being flooded with fuel and some of it remains in the cylinder and ignites prematurely on the next stroke.

          My main problem now is it is idling rough. All of the distributor parts were replaced several years ago and the vehicle has only been driven a few hundred miles. All of the parts still look like new.

          Could it be the carb needs adjusted? I only see two adjustment screws on the two barrel carb and I believe they are for air. How do you adjust the fuel?

          Thanks for all of the assistance.


          • Big Dan
            Big Dan commented
            Editing a comment
            Deleted and reposted below
            Last edited by Big Dan; 12-03-2022, 12:59 PM.

        • #6
          The 2 Richness adjustments are Idle Only, FUEL circuit adjustments, try 1 1/2 Turns Out, from gently closed.

          It other words, do Not force them closed Hard.
          Second Generation Stude Driver,
          Proud '54 Starliner Owner
          SDC Member Since 1967


          • Big Dan
            Big Dan commented
            Editing a comment
            "Delicately, and with finesse"

        • #7
          It possible the accelerator pump plunger has given up causing a momentary lean condition when opening the throttle. If the car can't be driven, there could be dirt in the carburetor partially plugging one of the jets, the fuel filter is plugged or the fuel pump is failing. Also keep in mind that a problem in the ignition system will mimic a carburetor problem. There is another thread going about condenser failures which will exhibit the same symptoms due to a weak or no spark so that is something else to look at. Bud


          • #8
            This time of year, they are switching to winter gas blends. And there's more "dirt" in the gas. Also, with the sudden cold we just had there tends to be more water in the gas and it will freeze in the bottom of your carb bowl. With timing being ok this sounds like fuel - water, ice or dirt - if nothing else has been changed. And that ice can ruin your accelerator pump or freeze in passages! And don't discount the possibility of an ice block (or partial block} in your gas tank (line or fuel filter) from ice as well. Especially if you put gas in it and then let it set. And it will stay frozen in there with sub-freezing temp gas even if you have a day above freezing.
            Last edited by Big Dan; 12-03-2022, 03:43 PM.


            • #9
              All good ideas. Years ago I let my 61 sit for several weeks over winter in NC. When I went to start it, I found a couple stuck valves in the right side head. It required pulling it to work them loose. If you cannot find a easier fix, you might want to check the compression or simply do as I did, remove the rocker arm covers to see if a valve or 2 might be hanging open a bit.


              • E. Davis
                E. Davis commented
                Editing a comment
                I bet the sticky valves were the result of ethanol fuel. That crap will ruin and engine if you let it sit until it gets sour which is about 60 days or less without stabil or the like. Even with non-whiskey gas I add a little Marvel Mystery oil and let it run through the system before storage.

            • #10
              Try unscrewing the idle mixture screws, (all the way out, IE; removing them) AFTER you screw them all the way in, seating them gently and counting the turns it takes, usually they are around 2 1/2 turns, and that is clockwise. Get some good carb cleaner spray, like Berrymans, and spray off the 'screws' at the pointed end and wipe them off good. Then put the little tube on the spay can of cleaner and stick it into the hole the screws were in, give it a few squirts, like 2-3 squirts. Replace the mixture screws you took out, set exactly where they were, the ~2 1/2 turns. Start it up and see how it runs, warmed up all the way. Then turn the mixture screws, one at a time, slowly clockwise. When the engine begins to slow, or run rough, back the screw out, counterclockwise, about a half to a full turn, but it should be fine if you adjust them to exactly where they were. Repeat the same on the other mixture screw. It is very common for them (mixture screws and the port they screw into) to get a little crud on/in them. Mine does it once in a while after it sits for a time, 289 with a AFB 4bbl. A quick squirt and it runs fine. And I don't use ethanol gas. I have been building and restoring vehicles for over 50 years, just so you know that I am not a couch mechanic.


              • Big Dan
                Big Dan commented
                Editing a comment
                Make sure you have your safety glassed on or better, a face shield when you do that. I did it once and all the carb cleaner blew back in my face......There were many MANY bad words!

            • #11
              Originally posted by Bud View Post
              Also keep in mind that a problem in the ignition system will mimic a carburetor problem. There is another thread going about condenser failures which will exhibit the same symptoms due to a weak or no spark so that is something else to look at. Bud
              Not to mention an electrical system more than sixty years old, which sits for several months at a time.

              The good news is checking everything is quick, cheap and easy. Start with the battery. Remove both cables and clean the battery terminals and put the charger on it. Clean the inside of both battery terminals. Go to the end connections of both cables and remove the bolt/nut and clean both terminal ends and to what they connect. Clean the wire terminal going to and from the ignition switch, clean the wire terminals to and from the ignition coil, to the distributor. Check the inside of the distributor cap, scrape any corrosion off the terminals. Inspect the distributor rotor; does it have a carbon resistor between the center strap and the tip? These go missing with bad results. Remove and clean the points, reinstall, bending gently to get perfect alignment. Set the point gap and then clean the point faces with a clean piece of cardboard, such as a business card. Clean, file and gap the spark plugs.

              By this time, the battery should be charged, so reinstall the battery cables and with luck, you're good to go.

              jack vines



              • #12
                As mentioned above most likely the accelerator pump or pump linkage. If the carburetor has been rebuilt most likely a standard kit was introduced with a new accelerator pump. The pumps in the modern kits are somewhat spurious. The original factory pumps were made of leather with a stainless spring inside to maintain a level of pressure in the pump housing. The replacement pumps in the kits are just a rubber/neoprene molded pump with no spring to maintain any pressure in the pump housing. The pump assemblies in the modern kits are barely adequate. Linkage can wear over time and may need a slight bend to maintain the required tension, when the throttle is moved the pump should move and immediately deliver fuel, if there is a delay, stumbling and backfiring will occur. Don't adjust anything on the carburetor or timing until this is confirmed.


                • #13
                  I'm going to suggest that your problem is the car sat with older (bad) gas. Why ? Because I just finished coming out of several rabbit holes that all have mentioned above. Last week I filled the tank with 13 gallons of high test (93 octane) just to test the 87 octane that was in the tank. I'm down to about 1/2 a tank and the car is just starting to accelerate and drive like it did earlier this year before it sat for a few months (with a near full tank)as I installed new front fenders and fixed the rusty floors. My symptoms were very poor acceleration at any speed (checked the carb pump = OK, stalling to a stop (adjusted dashpot correctly). When the car required a tow home cause it died in traffic, it was suggested the Prestolite weights were probably bad and with 100+K miles on this 259. I went ahead and had a known SDC guy rebuild the dizzy with all new components, confirming a good vacuum advance system. After that things were somewhat better, but still hesitation at "all" accelerations and occasional stalling at deceleration nearing a stop. This car ran perfectly without problems the past few years on multiple 400+ trips to sub zero missions. I had "exactly" the same symptoms with my 2R6 truck last summer and 93 octane has fixed far. I have rotated my gas stations to rule out the new (same) station (new tanks) issue. I fiddled with timing, carb adjustments, all electricals and this is where I'm at... I will be taking the car on it's 1st 400+ mile trip this week and will know for sure after buying fresh (?) gas along the way...


                  • #14
                    Originally posted by Big Dan View Post
                    This time of year, they are switching to winter gas blends. And there's more "dirt" in the gas.

                    How zat work? Who puts the dirt in winter blends, and what is it used for?
                    Whirling dervish of misinformation.


                    • #15
                      Originally posted by Lark Hunter View Post

                      How zat work? Who puts the dirt in winter blends, and what is it used for?
                      Really? Well, if it must be explained, more "dirt" (particulate matter, contaminants, etc.) in the gas is about "stirring" the tanks. And the extra moisture in the gas is from condensation, and the ice is caused by that extra moisture freezing in freezing temperatures. Thus, the word "freezing."

                      Ever fly a plane?
                      Last edited by Big Dan; 12-07-2022, 12:20 PM.