Piston Slap: The Land Ark's Backfiring Belly?

Sajeev Mehta
by Sajeev Mehta
TTAC 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 sajeev@thetruthaboutcars.com. 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.


Sajeev Mehta
Sajeev Mehta

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  • Halftruth Halftruth on Jul 01, 2018

    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.

  • Flipper35 Flipper35 on Jul 02, 2018

    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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