Carter AFB Backfiring

Discussion in 'Carter' started by Gulfgears, Sep 9, 2017.

  1. Gulfgears

    Gulfgears Gulfgears

    I have a AFB 4055 (off 340) on my 300.
    The timing is set at 2.5 degrees at idle with vacuum disconnected.
    Newer distributor advance canister
    Pulls about 14" of semi steady vacuum
    New (today) accelerator pump piston

    won't idle below 1000
    dies when put in gear
    backfires (with fire) when floored in neutral

    Due to life and medical problems, car hasn't been driven in about eight months, and was running ok before.
    I know the backfire is indicative of timing, right?
    Replaced accelerator pump as it was bad.
    I haven't hooked up the dwell meter to really get into the timing yet.
    Could this thing have jumped timing somehow?

    As always, thanks for any input.
  2. LARRY70GS

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

    Back fire through the carburetor, or out the exhaust? Unless the engine has a cam, 14" of vacuum is low, should be 18-20" for an all stock engine.
  3. wkillgs

    wkillgs Gold Level Contributor

    I've had those symptoms when the carb base gasket went bad. Created a huge vacuum leak to the exhaust passage in the manifold. You need the metal heat shield on those carbs too.
    A backfire can occur if the mixture goes very lean. The above cause could match the symptom.

    Carbon tracks inside the distributor cap is another common cause of backfire. Don't think that's your problem tho.
  4. Gulfgears

    Gulfgears Gulfgears

    Ok, adjusted out the idle screws and it calmed down a little. Still gives a pop sound when floored, but doesn't die out like before. Oh, backfire is thru the carb.
    Goes into gear now without dying.
    Took a fast and dirty vacuum reading off the front of the carb, so that may give a faulty reading. Will look for all the adapters and hook the vacuum up to the hose going to the brake booster.
    I'm really concerned this little turd has jumped timing a tooth or two.
    Engine has about 112K and really doesn't look like it's ever been rebuilt. All the discussions about plastic teeth falling off has me kind of jumpy.
    Will be doing Cruising the Coast next month and that's about a 500 mile trip with all the driving around.
  5. LARRY70GS

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

    Backfire through the carb is lean.
  6. TrunkMonkey

    TrunkMonkey Well-Known Member



    Since it was carb related changes you made, likely that is where the problem is originating.

    Sometimes a change of one component, will result in another that was marginal to show it's ass, but that is an exception to the rule.

    "Last thing you fixed, broke it." (as the follow-on to the prior statement; )"If it ain't broke, fix it 'til it is!"
  7. Aaron65

    Aaron65 Well-Known Member

    Mine has never run well on 2.5* base timing. I disassembled my distributor and added a stop bushing from a Moroso curve kit, and I run 10* initial. I have run up to 15* before, but mine would pop through the carb on 2.5* base timing. I tried it once just for the heck of it, and put it right back.

    With the added initial, mine has about 18 or 19 in. of vacuum in neutral. Just a thought for when you get it running well again.

    Have you checked your accelerator pump shot?
    My AFB equipped car recently had a problem with rough idling and slightly low vacuum (17 in.). I had a piece of trash in one of the primary clusters. I took them out and cleaned them really well, and everything was back to normal again.
  8. Stevem

    Stevem Well-Known Member

    With the motor warmed up if you close off the choke or cup your hands over it and the rpm picks up then you have a vacuum leak.
    Another way to prove that out is with the air cleaner on the motor spray some Carb cleaner around the base of the Carb and all of the vacuum line fittings and see if the idle changes.
    Many times a vacuum hose that's ready to fail givin another 8 months of sitting will do just that!
    Also as posted that 2.5 degrees of timing is a insane minimum, please rework the Dizzy to have at least 8 degrees, and yes! I have had timing chains jump and if fail when shutting a motor down only to find this out when going to restart it a few minutes later!

    3 motors have done this to me over the years , and lucky 2 have done this in my driveway!
  9. Gulfgears

    Gulfgears Gulfgears

    Well, just a follow up to all that has been going on.
    Went to check timing and accidently moved the distributor back a little. Whoa, car started idleing smoother and not acting like such a fool.
    So I twisted a little more and vacuum jumped to a rock steady 18" and engine was really smooth. Checked with the timing light and the marks were about 4" off from the 2.5" tab mark.
    Mark on the harmonic balancer was steady on the timing light.
    Car has all but stopped backfiring (well just a tiny bit off idle), but now it doesn't die when put in gear or when the ac is turned. Drove it and it's never run this smoothly in the eight years I've owned it.
    My conclusion is that the harmonic balancer is toast and needs to be replaced because if it was the timing chain there would be slop in the timing marks (moving around under the light). Plus no high temps even with ac on.

    Next issue is I've lost my kickdown for passing gear. ead the manual and no help there. Can't find any loose wires, and car will work fine if manually downshifted to simulate kicking down. The windshield wipers work, so no blown fuse.
    The kickdown feature was working fine when the car was parked a year ago. If it's a simple fix I'm up for that, but otherwise I'll love with it for today.

    Lastly, I've been dealing with a lot of wandering when going down the road. Found out that three of the bolts holding the gearbox on were loose. Fixed that, but still some wandering. So tried tightening up the set screw on the box, no joy there. Maybe it's time to call Hector and upgrade to a fast ratio?
    Oh, going to take it in for an alignment again, I took it to the supposed old car guru here and he never noticed the loose gear box and left all the front wheel nuts kinda loose! Got a new guy to try out next.

    Oil changed, that went well. Just saying all is not bad, and even with it's problems it is now a blast to drive the car running better.
  10. Aaron65

    Aaron65 Well-Known Member

    When mine stopped kicking down for passing, I had to take the microswitch on the side of the carburetor apart and clean all the contacts. Be careful; it's a tedious job of prying without really prying. Inside, the switch itself is a neat piece of electromechanical engineering; whoever came up with the idea had his stuff together.
  11. LARRY70GS

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

    Advanced timing will bring vacuum up, as well as make the engine run cooler. Does the vacuum advance canister work? Is it connected to manifold vacuum? Are you sure that the distributor in the engine is the correct part number for the year and engine? Put a timing light on it and find out where it is running. You obviously still have a problem since it is still back firing. The advanced timing is masking the real problem.

    Diagnosing a kick down problem is easy with a test light and good hearing.
    Last edited: Sep 11, 2017
  12. Gulfgears

    Gulfgears Gulfgears

    Vacuum canister has been tested with a vacuum pump and works well.
    Vacuum signal is taken from the base of the carb (AFB 4055S)
    Distributor is the one it was born with and is still points style.
    Back firing may be a misnomer as it is more of a hesitation with a tiny poof sound. I think this will go away with correct timing settings.
    My son has been helping me and keeps saying " fuel injection and electronics are much easier to work with". I like things that are mechanical and make sense, otherwise I'd mortgage the house and put in a LS chevy that I'd know nothing about.
  13. LARRY70GS

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

    Your vacuum advance should be operational at idle, connected to manifold vacuum. It should add to the 2.5* setting so that the engine idles in the high teens. That may be part of the problem. The base of the carburetor could be ported or manifold vacuum. With the engine idling, pull the connection off at the carburetor. If you hear a hiss, it is manifold vacuum, if not, it is ported vacuum. Your vacuum advance should be connected to manifold vacuum. If you are still running points, check the dwell. If it is way off, that will affect your timing.
  14. Stevem

    Stevem Well-Known Member

    If your electronic FI shits the bed on you away from home your stuck and facing at least getting another 800 bucks sucked out of your wallet, if your Carb on the other hand starts to act up or even flood out you can fix it right thenand there for 50!
  15. Gulfgears

    Gulfgears Gulfgears

    Time for latest news:
    Checked for TDC with a piston stop. Timing mark on pulley lines up with tab "0". Rotor is somewhat close to the number one position.
    Dwell is 30 degrees.
    This is the nutty part: car won't run worth a darn if set according to marks per Buick manual. Rotate the distributor and it immediately picks up and runs perfectly. Imagine the tab is located at about the 2:00 position, the mark on the balancer is at about the 5:00 position.
    I drove the car about 70 miles today at speeds up to 80mph and it ran like a top. No overheating and no drama.
    I have ruled out jumped timing as the marks wouldn't have lined up at TDC if it had jumped.
    Seven cylinders had 160#, one had 180# and vacuum is steady.

    Is it possible that some time in this cars past the distributor was removed and replaced incorrectly?
    I'm going to keep running it as is until after Crusing the Coast, as everything is just too nice to mess with.

    Lastly, before you spend hours checking circuits and continuity check the wiring diagram. I'm too ashamed to tell all, but my kick down is now working perfectly.

    As always, thanks for the helpful input.
  16. LARRY70GS

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

    If you are looking at the front of the car, and the timing tab is at 2:00, and your balancer mark is at 5:00, that is retarded timing, by quite a bit. I'm sure that is not what you meant to say. If the timing is advanced, alot, the mark could be under the water pump, at 11:00. In any case, you can use the dial back feature on your timing light to tell exactly what the timing is at any point. If you have a standard timing light, make a 30* mark on your balancer and do it that way.

    The distributor can not be in wrong unless the vacuum advance is pointed in the wrong direction, or you run out of travel trying to adjust the timing. It is possible that the distributor has been replaced in the past, and this distributor is different. The distributor part number for the 67 300 is 111158, and 67 340 is 111159. For 1966, the numbers are 111147 and 111144 respectively. What part number do you see?
  17. Gulfgears

    Gulfgears Gulfgears

    I have a dial back light and will see if that helps out any.
    But, the dampner mark was a good 3" lower than the tab when looking at it from the front.
    Damn thing runs so good now, I hate to fix it LOL.
    Also, vacuum advance canister is almost slap against the block now, if that means anything.
  18. LARRY70GS

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

    Gary, that makes no sense to me. If the mark is below the tab, it means the timing is retarded not advanced. Check the dial on the timing light, is it at zero? Are you absolutely sure that you have the light connected to the correct wire? Look at the picture below, up is advance, down below the 0 is retarded timing.

    TimingMarkand Indicator.jpg
    Last edited: Sep 13, 2017
  19. wkillgs

    wkillgs Gold Level Contributor

    Your car car is telling you it's happy, but your instruments say something isn't right!
    Try a different instrument (timing light).

    Have to ask.... is your firing order correct? (1-8-4-3-6-5-7-2), rotor rotates clockwise. And you are checking #1 cylinder on the drivers side?
    I checked 4 web pages and they have mistakes.... a 350 with the distributor in the rear, and Nailheads with #1 cylinder on the drivers side! If you used the wrong reference source to set it up, it could be wrong.

    You can try power timing.... advance until it pings at full throttle, then back it off a little.

    You can check cam timing. At TDC... not compression stroke, but when exhaust is almost closed and intake is just starting to open, The valves should be open about the same amount. Rotate engine back and forth about 30 degrees and you should be able to tell if the TDC mark on the dampener lines up with the centerpoint of your IE valves opening/closing.
  20. Gulfgears

    Gulfgears Gulfgears

    Ok, in the interest of full disclosure, the problem was determined to be caused by human error, mine. I would like to skulk off and let this thread die as many others do, once they have found the problem. But "if your gonna be dumb, you gotta be tough".
    The dial back timing light I was using was inadvertently set at about 15 degrees.
    I took Walt's suggestion and got out an old school timing light, and cripes, the timing was off the north end of the tab, about 20 degrees advanced.
    Fixed the problem. Tomorrow I'll look at the idle and the adjust the carb as necessary.
    One last question, when I was a kid, we used to loosen the distributor and brake torque the engine, then advance the distributor until it started spark knocking. Just curious if we were doing more harm than good.

    Larry, thanks for all your input, I hope you appreciate how much I appreciate the guidance you have provided.

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