By on June 29, 2018

TTAC Reader LandArk 1967 Impala EngineTTAC Commentator Land Ark writes:

Hi Sajeev, it’s me again. I am nearing my wit’s end.

My Impala has been out of commission for several months now and as I’ve been telling my friends, the Land Ark and I are not currently speaking to each other. What got us to this stage of our relationship is my seemingly brilliant idea of upgrading my fuel system. I had been concerned about the E10 used in gas stations currently and fearful that we’d soon be provided E15 that would likely start accelerating the degradation of my fuel lines and carburetor bits. I’d purchased a replacement gas tank and sender a few years ago and I found a guy who I thought I could trust to replace my tank and all my fuel lines. I put in everything that could potentially survive E85. I was willing to pay for quality and I did. Everything was replaced but the carb.

I picked it up after the work was done and drove it about 30 miles home. It ran perfectly. Once I was close to home I stopped at a gas station to fill up. As I filled the tank I noticed it was pouring out on the ground. The pressure of the gas being pumped in pushed the sender cap off the top of the tank. It wouldn’t run unless I turned the carb all the way up and then it ran horribly but enough to get me home. I called my guy and he raced over and spent the rest of the night with me pulling the tank back down and reseating the cap. It seemed the gasket wasn’t positioned properly the first time so we got it to sit in place and put back together. I was able to fill the tank and all seemed well.

Meanwhile my garage reeked of gas fumes. I searched for leaks and found nothing. I feared the sender was again unseated so I pulled the tank down again. It was in place and everything in that area was dry and did not smell. I eventually realized the rubber hose for the vent wasn’t really sealed onto the vent tube. On my car there is a vent tube that runs into the trunk, does a 180 and exits back out the floor again. I got some smaller clamps and tightened it up. This cured the fuel smell.

It’s been a while but I believe it ran fine after that. One morning I went to get in and go to a car show but it wouldn’t start. It seemed to not be getting fuel and then it was flooded. At least I think it was flooded. It acted like it was and when I pulled the carb off I’m pretty sure the intake valley was full of gas. I emptied all the gas out of the carb and put it all back together. I checked the floats and nothing seemed to be sticking. There was gas in the oil, it didn’t seem to be a ton of it, but there was a strong fuel smell. Once I changed the oil and put the carb back together I was able to get it running. It didn’t sound bad, but not 100%. I could rev it and it would sound a bit off, but it held rpm. It wasn’t until I took it out for a test drive that it was obvious something was wrong. I’m not sure what to describe what was happening. I would guess detonation or pinging under load. I accelerate and I could almost feel the pedal push back as a popping or puffing noise came from the motor. I have tinkered around with the air/fuel mixture and it doesn’t eliminate the issue. And the more I drove it, it would start to backfire. I checked all the vacuum lines and they are all in place and secure. I had also stripped the threads on the left rear of the intake manifold pulling the carb on and off so I thought maybe it was just not sealing properly, but I have since corrected that (sort of) and the seal should be good.

It all seems to be centered around the left rear cylinders. It only backfires on the driver’s side and the puffing seems to be coming from the driver’s side of the motor. It still idles… ok, but not great. It really only seems to have major issues under load. The motor was installed about a decade ago, it’s believed to be a 350 Pontiac of unknown origin. It has a Voodoo cam in it (which I hate and want to remove).

I also did a compression test which had some interesting results.
Driver’s side:
1 = 135
3 = 135
5 = 118
7 = 135
Passenger side:
2 = 140
4 = 130
6 = 118
8 = 128
The plugs on cylinders 5 and 7 turn black after running for a few minutes. The others two on that side are clean. I’ve changed all the plugs. and swapped plug wires. I did not remove the distributor but I checked the cap and it looks fine.

I’ve attached some photos in the hopes that I am simply missing something simple and one of the B&B might spot it. I am willing to try anything someone suggests. My fear is that while it was flooded I tried starting it and fouled a cylinder, but the compression numbers don’t seem to indicate that. 118 is low but it doesn’t make me think it’s dead especially since the problem is localized to the driver’s side and the matching cylinder on the other side is the same and is running fine.

So what say you?

Sajeev answers:

Sadly I know more about fixing Ford’s awfully durable awful EEC-III induction than any carburetor. Weird vantage point in mind, procure yourself a grain of salt for this Piston Slap!

I drove a 1978 Continental Mark V with a Carter AFB Competition Series much like yours for a year, and I find it easy to understand. Truth is, all carbs have their pros and cons so I reckon yours just needs a ‘fixin (technical term).

While backfires are either an air-fuel ratio richness or timing/ignition issue, yours is fuel related from worn metering rods: running rich-ish at idle and going batshit rich with throttle (i.e. when vacuum decreases). If that Voodoo cam is the stereotypical dude bro high lift/duration unit with trivial vacuum at a normal idle, it could exacerbate the richness?

It could also include an issue with the accelerator pump and the aforementioned metering rods. No matter, I recommend a full rebuild…from someone who knows way more about carbs than yours truly.

Oh, and ditch the cam for something more streetable/torquey, if I’m reading between the lines correctly. What say you, Best and Brightest?

[Image: OP]

Send your queries to [email protected]m. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.

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34 Comments on “Piston Slap: The Land Ark’s Backfiring Belly?...”

  • avatar

    It’s a little hard to follow exactly what happened with your fuel system or the timeline. Your tale reads as if you’ve replaced everything in the fuel system except the carburetor and some (significant?) time elapsed between this work being done and being unable to drive the car.

    I can tell you from experience that with modern fuel if you are smelling strong vapors for any length of time your fuel is forming varnish/deposits as the light aromatics flash off. I don’t know how long that state persisted but the formation of varnishes and the addition of fresh E10 fuel causes those varnishes to flake off and flow through the fuel system.

    I agree a full carb rebuild is in order. Change the fuel filter. It might not hurt to replace the fuel pump about which I didn’t see mention.

    Your lower compression on cylinders 5 and 6 seems odd. Have you done a leakdown?

    You mentioned 5/7 had fouling plugs but have you checked 6/8? I suspect you’re running pretty pig-rich and the raw gas pooling on the intake floor is making the rear cylinders run over-rich due to it running downhill to the rear of the manifold (God I love MPFI). I mention a leakdown because if those cylinders are running too rich you may have carbon fouled intake valves causing a poor valve seat which would read as a low dynamic compression but cause much more rapid leakdown than a healthy cylinder.

    • 0 avatar

      I, too would start with pulling the carb off and going through it. Carter/Edelbrocks are pretty easy to work on. Use some PB Blaster to spray the internal passages clean. Afterwards, put a vacuum gauge on it and adjust your idle mixture and ignition timing.

      I recently put a Voodoo cam my ’99 Suburban, and although it cams hard it isn’t prone to vibration. I think you have another issue.

  • avatar

    I’ve had a few carbed cars but admit my knowledge of how they work was pretty low. I usually ended up buying a rebuilt or a new carb (often Edelbrock, which is essentially a Carter). Do the plumbing, set the idle, and then drive to see how it works out. Do further adjustments until I get the response I liked. Worked just dandy for my 355 powered Monte Carlo.

  • avatar

    This sounds suspiciously like a stuck float. The “old school” fix for this was to whack the carb body with a screwdriver handle while holding it by the shank. The stuck float leaves the fuel inlet needle valve open a bit, allowing the fuel level to rise too high. Result – very rich running. I agree about the fuel running downhill in the manifold and fouling the valves. Once this is solved, an “Italian Tune-Up” is in order. A carb rebuild will cure this, but the quick fix is easier and more rewarding.

    • 0 avatar

      The “stuck float” comment shook loose a situation I had in a carb on a UTV. It had been run on old gas with a high MTBE content and a switch to non-MTBE oxygenation (likely ethanol) caused an issue where junk built up on the needle seat. The float wasn’t stuck, but the needle didn’t seat so it acted like a stuck float. Some toothpaste on a Q-tip scrubbed the junk off the seat and some carb cleaner on the needle and float pin got it back in working order.

  • avatar
    Land Ark

    First, thank you Sajeev for posting this. I realize it is a TL;DR kind of thing and there are a lot of bits of info, but I wanted to make sure I mentioned everything that I would suggest to someone to try.

    I sent this in back in the fall, and nothing has changed. It has been sitting under a cover in the garage since October. Every once in a while I think I figure something out, try it, fire it up, pull out of the garage, it persists, and back into the garage it goes.

    For the record, the cam came with the motor. Since it has been in my car, all it has produced is motor-mount-tearing vibration and teeth rattling NVH. At no point has it ever sounded lopey or anything pleasant.

    I have considered loosening the clamps on the vent since when it stunk of gas at least it was running right. Just to see if that has anything to do with it. Am I grasping here?

    The time between installing the tank and fixing the fuel smell was about 2 weeks. Then, after a few quick trips where it ran fine, about a week later is when I tried to start it and it wouldn’t fire and flooded. So maybe a month passed in total.

    The fuel pump was replaced when I did the tank.

    Some of the confusion I have is that the time and mileage between the work being done and the issue starting means there wasn’t time for a lot of build up inside the engine or for the fuel to start to go bad. I’ve had gas in the old tank for years with no issues with the fuel.

    I’ve been mulling taking it to a shop, the unfortunate part is that the only shops I trust around me are not accustomed to working on non-FI engines. If I did, I will a request leak down test.

    • 0 avatar

      Quick thoughts:
      – Make really sure that the intake manifold gaskets are NOT leaking.
      – You said you swapped plug wires, and I took that to mean the wires are new. Did you try swapping the wires between #5 and #7 cylinders? Is it possible they are installed wrong, or as LTDWedge suggested below, that they are cross-firing?
      – Also, in regard to firing order and camshaft, are you sure that the Lunati camshaft is for the standard SBC firing order? I have seen some camshafts that are available in both the standard firing order, and a “4-7 swap” firing order. If you should happen to have one of the “4-7 swap” cams, you need to swap the plug wires to follow the cam.

      • 0 avatar

        Too late to edit my earlier post, but here’s an amendment:
        – If the engine really is a *Pontiac* 350, then please disregard my comment about the SBC firing order. However, I am not sure why a *Chevy* Impala would have a *Pontiac* 350, which is a very different engine, apart from the calculated displacement.

  • avatar

    I don’t recall, but if the carb has a power valve it is probably leaking raw fuel into the intake manifold. This will create all the mentioned symptoms.
    The power valve is in the bottom of the float bowl and has a spring loaded diaphragm. With normal manifold vacuum a plunger is pushed into a fuel jet. When the throttle is wide open and the vacuum is less, near atmospheric pressure, the jet is open allowing more fuel into the engine as the low vacuum would create a lean condition. There were power valves that were calibrated for different vacuum levels. A different one might be needed with a longer overlap cam.
    The diaphragm can deteriorate and leak. This could be because of the ethanol in the gasoline if it is an older carb. Hopefully replacement power valves will be compatible with the gasoline/alcohol mix.

    • 0 avatar

      This is a Carter AFB, so no power valve. The fuel enrichment under load is performed by the metering rods. This mechanism doesn’t normally fail. I have found these carbs to run quite rich with ethanol. Maybe you need to downsize the primary jets. Going back to your original problem, it does sound like stuck floats. Rebuild the carb and set the float height correctly. Look for degraded rubber tips on the ends of the needle valves on the float height mechanism.

      Lastly, for a car that sees little use, go out of your way to find ethanol free gas or always make it a practice to use Stabil fuel additive. As you never know when that last tank of gas may be the last tank of gas…for a good amount of time.

  • avatar

    Ok I can not help much but maybe asked around at the car shows you go to to get a shop they trust and send it there, AAA offers 100 mile towing if you bump up a level, or see if you can Borrow a Carb from a car show buddy for a quick change out and see if that fixes your problem, good luck I am sure your up to the roof in wanting this fixed or burning it to the ground depending on the day.

  • avatar

    Ballast resistor. Either that or the gas cap is loose.

  • avatar
    87 Morgan

    Land Ark, oh how I sympathize with your carb related issues.

    Tell me about your ignition system. A failing ignition system will have many carb like symptoms as spark will be erratic. I went through this last year with my SBC 327 which had the 64′ (Impala motor in a 57′ 210) set up of points. I removed that and went with a Pertronix HEI distributor.

    When all of this went bad, the car would run then stop and reek of gas, the symptoms were very similar to vapor lock.

    • 0 avatar
      87 Morgan

      Crap, I reviewed the photo. You are already running HEI. My bad. Any idea how old the distributor is?

      • 0 avatar
        Land Ark

        These are times when I realize I am no mechanic. Carburetors work on witchcraft as far as I can tell.

        The HEI was on the engine when I had it installed. I’m not sure what brand off the top of my head but it’s never had any issues up until this. I have looked it over and there didn’t seem to be any physical oddities with it.

        One day I should explain the nonsense I allowed to happen to get the new motor installed. Short version, I left my car with a dude from Craigslist who was selling a car with the motor in it and I had him swap the one from mine to the other car. No idea how I convinced myself to go with it.

        • 0 avatar

          You need to add more eye of newt. Rookie mistake.

          • 0 avatar

            How grey is your mechanic’s hair? Most of the ones who know what they’re doing look like either Wolfman Jack in his later years or Sam Elliott.

            Guy with a carb on his old Mustang

          • 0 avatar

            Just spray a can of Gumout in there. Either that or see if you have any ether laying around the house, probably under the sink someplace.

        • 0 avatar
          87 Morgan

          Yes, witch craft.

          I generally feel that carbs are a throw away item. Once they go bad, no amount of voodoo can bring them back. You might consider an Edelbrock replacement. I find them to be less temperamental than Holley. Keep in mind I said less….

          As soon as the motor in mine goes bad, I will be going with a FI throttle body set up. I have had enough of the carb life.

  • avatar

    aftermarket intake and carburetor. What size is the carb? Carter makes good carbs but like any carb if it sits too long things tend to get mucked up. Stuck float was mentioned by someone. also have to check the needle valve. On old cars junk from the gas tank might get passed the gas filter and mess up the needle valve. Check the fuel filter. A lot of gm cars had spring loaded fuel filters with a back flow shut off and when fuel pumps would get weak it could not push the back flow valve open. Gm made two kinds I always used the one without the valve. Better still to use a good inline filter. Do a rebuild on the carb and you should be good to go. Also better to time the motor with a vacuum gauge then the timing marks. shoot for 18-22 Hg unless you have a monster cam

  • avatar

    “Meanwhile my garage reeked of gas fumes.”

    Hope you don’t have a gas water heater in your garage (or any electric appliance, for that matter).

  • avatar

    My first thought is the neoprene tip of the inlet needle, it could be deteriorated by ethanol so that it doesn’t seat properly. Also, the needle could be hanging up in the seat due to the effects of the ethanol. I have actually incurred this where the fuel just runs right through the engine because it is constantly pressurized by the fuel pump but the flow never shuts off due to the compromised needle/seat.

    You are right about the cranking pressure, 118 lbs. is definitely low, probably valve seats but probably not enough of an issue (yet) other than to cause your motor to be down a bit on power as a result of those two cylinders.

    How’s the valve train? If your lifters are loose, you would know it by the clacking; I’m more concerned that the rocker arms may be down too tight. A Chevy V8 has adjustable lifters (a full turn down on the rocker arm from zero lash) but you mentioned a Pontiac engine and they adjust more simply (check a shop manual as I don’t remember how to do it on a Pontiac) and they usually don’t require additional adjustment later once the lifters are set up properly the first time. Probably more of a long shot but worth investigating as it is easy to do.

  • avatar

    That exact carburetor on my 1971 Ford LTD with a 429 is the reason that I splurged and installed a Holley 2V Projection throttle-body fuel injection system. I agree with the others – either stuck floats, float level set too high, or jetted too rich (which takes a chassis dyno or lots of road-testing and spark plug reading to get right).

    Honestly, I would try to find a correct Quadrajet carb that has tight throttle shaft bushings. People used to badmouth those carbs, but if you understand how to rebuild them properly and have the mixture set right, they really can’t be beat.

  • avatar
    velvet fog

    Running fine at idle and revving under light load while backfiring through the carb under load is a classic burnt valve symptom.

  • avatar

    Have you tried removing the air cleaner and looking down the barrels of the carburetor while it’s running? My dad’s 1967 Mustang had trouble staying running correctly – sometimes it would run perfect and literally minutes later it would loose power and just quit. The problem turned out to be with the Holley 4150 carburetor – fuel was dumping into the secondaries, but you would have had to look into the carburetor as it was running to see the problem.

  • avatar

    The problems started after the work, so something associated with the work has to be what caused the issue. I agree with excess fuel – how can that be attributed to the work? Is it possible you do not have proper tank venting and evaporated fuel is causing pressure to build up in the tank? This, coupled with a weak carb, might be forcing too much fuel into the engine…the float may not be fully closing off the fuel supply so the pressurized fuel might overpower the float’s ability to stop the fuel from overfilling the bowl…and an over-rich condition will result. Too much blow by and the engine will choke on excess fuel and stall…

  • avatar

    Is it an electric fuel pump? What is the pressure? Carbs should generally be no more than 3 psi. If the fuel pressure is too high it will overcome the float valves and flood the engine.

    Sticky or not properly adjusted regulators are not unknown. After sitting the regulator (If there is one – there should be for an electric pump) may not be reducing the pressure as it should.

  • avatar

    LandArk; My humbly submitted comments divied up
    into the Big Three categories:

    #1 Fuel – Vented fuel cap
    – Rebuild the carb to E85 standards
    – Fuel pump volume & pressure to spec
    – NO false (unmetered) air AKA, “vacuum”
    #2 Spark – HEI is very robust with a few caveats;
    – pickup coil resistance specification
    – pickup leads to the module can suffer
    from intermittent connection with a
    movable (vacuum advance) “breaker”
    plate (idk what version HEI you have)
    – the module itself may be junk
    – the very high voltages generated also
    can cause electrical breakdown in the
    cap itself, same thing can happen with
    the rotor, (about a 100% chance of a no-
    start) the screws attaching the coil to
    the cap sometimes can allow arcing
    – the coil may be breaking down under
    high temps or power demands.
    – 1 8 4 3 6 5 7 2 GM V8’s always had a
    weakness for inductive firing (misfiring)
    of cylinders #5 & #7 (driver side, rear)
    Extra care in routing the plug cables may
    all that is needed here
    #3 Compression – get a large faced vacuum guage
    This 1 tool will point you in the direction
    of mechanical righteousness as far as
    sticking valves, late valve timing, fuel

    I could write a book here and be banned from TTAC
    forever. Remember, you CAN eat the elephant,
    ONE BITE AT A TIME. Good Luck

    • 0 avatar

      Now that’s a good diagnostic procedure!
      Also if the motor is getting too much fuel, flooding, that could cause low compression due to poor ring seal. Get it running right and then retest compression.

  • avatar
    87 Morgan

    One last thing to check, that is fairly easy.

    Any chance you have a broken push rod? Might be worth it to pull the valve covers and have a look. I have had this happen and it produces the similar results in terms of back firing, running like crap.

  • avatar

    I am going to go out on a limb here and say the carb is ok. If it ran perfectly for 30 miles then something else is amiss. Metering rod wear would have to be constant and happen over time, not in a 30 mile drive. Have you tried running straight to the carb from a gas can? What size lines were used when the replacement was done? If the size is off it could cause a problem. I am ruling out debris as you have disassembled the carb and I assume, cleaned it as well. Are you using an electric fuel pump that is simply too much for this set up (or stock)?

    Outside of the standard checks – timing, idle speed and mixture, I would run it from a gallon gas can (if you can ie. no elec fuel pump) and see what it does. Also put a vac gauge on it. You can deduce a lot from vac gauge readings. The compression readings don’t scare me (unless we knew for sure what they were when you drove off from the shop).

    Lastly, do not start a parts-replacement-a-thon! That will muck up and complicate the troubleshooting.. Good luck and keep us posted!

    PS – if you already moved the timing around, be careful the balancer hasn’t shifted. This bit me hard once giving me a bad TDC ref.

  • avatar

    Vacuum gauge is a requirement when working on the carb. I would see if there is a race shop somewhere close as they should be able to go through your carb for you. There are parts in a carb that you cannot clean with just a spray can. The Carter is pretty easy to work on if you want to DIY.

    As a side note I had that carb on a small block Mopar with a big cam and it never ran properly. I ordered a new carb from Don Gould in Oregon (big Mopar engine guy) and it has run great ever since. But since yours ran prior to the work I have to think it probably only needs a GOOD cleaning. You can have them ultrasonically cleaned and it does a wonderful job.

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