Misfire at low revs

[66DC]

or

How a Pair of Kitchen Gloves, a Swiss Army Knife, and Bicycle Degreaser Fixed My Misfire


Hi everyone,

Long-time reader, first-time thread starter -- I thought I would document the process to hunt down and cure a misfire that happened over the weekend in my 2.0 66 normale with distributorless ignition by Electromotive. This is my first 911 and first carbureted car.

After accidentally filling up with 10% ethanol (as noticed by the Mrs.), I began to feel a misfire and stumbling at around 1900 rpm. This was noticed most when starting from a dead stop. Over the next 4 hours and 75 miles or so between DC and Annapolis, it started to get worse and was significantly apparent by the end of the day.

After about two hours of reading here and on Pelican, and recalling hours of puttering around downtown DC, I thought for sure I had fouled a plug. After spending $2.75 on a replacement NGK at the FLAPS, I donned a pair of rubber gloves and went about unplugging the spark plug wires at idle to find the bad cylinder. Why use rubber gloves? Try it without and ask me again. Turns out unplugging #1 didn't change what idle I had, but unplugging the rest did. Pulling the plug and noticing the flawless condition with a dusting of greyish-brown, I swapped it with cylinder #2. No change. After checking the wires, I surmised it was not an electrical problem and I would have to get all up in the Weber.

Luckily, the first recommended step is to look at the idle jet on #1. Not having a reasonably short flathead, I used my swiss army knife to pull out the jet and LO AND BEHOLD...a tiny piece of black something was blocking the top hole. Again using the swiss army knife toothpick, i flicked it out, shot some Finish Line Instant Degreaser (awesome stuff) into it to clear it out and re-installed to a perfect idle. I wish I knew this when I was on the road. I literally could have fixed this misfire on the dime, with a dime.

So, my question is: How did that black piece get there? It looked way too big to get through the fuel filter (about 1mm long). Could it somehow have gotten dislodged from my K&N? But that wouldn't explain how it got to the idle jet.

[304065]

It is a piece of rust from inside your air cleaner. Either that or a particle of rubber hose from the breather.

[66DC]

It is a piece of rust from inside your air cleaner. Either that or a particle of rubber hose from the breather.

Nice. Now that I think about it, I bet it was a tiny piece of rubber that broke away from the rubber washers on my K&N rainshields.

[1QuickS]

Most efforts to stop debris from entering the fuel bowls revolve around good fuel filters and fresh fuel lines along with a tank cleaning. All these are good but there are other sources to check:

• tank filter screen cleaned
• vent pipes in top cover of Webers rust internally and drop flakes of rust directly into the fuel well; wire brush the inside of the tube and the inside of the top cover where the tube is riveted and apply "Extend" or equal; or remove, replate and reinstall
• spark arrester in OEM air cleaner housing (25mm diameter, right angled tube) is a good source of rust
• also the trough where the seals are installed will collect water, rust and pinhole after time which allows flakes to be drawn into the fuel well (I suffered this one for awhile before I found and fixed it)
• poor sealing of air cleaner housings (OEM and after market also) with sealing gaskets allows other debris to enter your fuel well; apply a thin strip of oil resistant, closed cell, self-adhesive foam to the existing gaskets to provide a compliant air seal
• spark arrester hose may harbor debris which may be drawn into the system

The smallest orifices in the Weber carb are the idle jets and the accelerator squirter nozzle tips. The idle jets fuel supply is through the main jet which is rather close to the bottom of the fuel bowl while the accelerator pump inlet is much higher. Anything smaller than these two orifices will be consumed by the engine or will stay in the bottom of your bowl.

Note that the air cleaner housing operates at a low vacuum so it is natural for atmospheric air to be sucked past poor seals and to bring crud with it.