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Unclogging the Carburetor

I bought the beetle in the middle of July 2006. I drove it 400 miles to Middlesboro, and then drive it around town and to flying lessons for weeks. For over 1000 miles the engine ran like a sewing machine. Then one day, coming back from Knoxville, it wouldn't run right. Once it was totally warmed up, it would almost run Ok, but it wouldn't idle quite right. I actually died a couple of times getting warmed up, which hadn't happened during the time I'd owned it.

I don't have a lot diagnostic tools, so I had to do a lot of guessing. I decided it had to be the carburation system. The engine would start (cold) and then run perfectly for 30 or 45 seconds, and then sputter and start to die. I could keep it running if I fought it with the accelerator.

It just didn't seem like a vacuum leak or an ignition problem (it just wouldn't run right at all in those cases). However, if one of the jets in the carb was plugged, then the engine would start and I could keep it running with the accelerator pump, but it wouldn't run without that (which was roughly the way it was).

Before cleaning the carburetor, I wanted to eliminate crud coming in from the fuel lines. After disconnecting the incoming fuel line from the tank, I opened up the fuel pump to check the screen and found this:

The bottom part of the pump is under the screen, and is on the downstream part of the pump. So that stuff was going into the carburetor. So I bought a carburetor rebuild kit (that contains gaskets and replacement screws and valves), a new fuel pump, and a set of in-line fuel filters, and a bunch of fuel hose. The plan was to clean out the carburetor, and then put it back into the engine having cleaned out the upstream components.

So here's the Solex 34PICT-3 carburetor out of the car and on the work bench. The thermal-electric choke is on the upper right, and the electromagnetic cut-off valve sticks out to the bottom left on this view.

The carburetor with the top half and float assembly removed. The float bowl is the part on the left. You can see the crud that has collected in the corners. I don't know if that was harmless or a symptom of the same stuff that eventually clogged the carburetor. The basic operation of my "repair" was to use compressed air to blow out all the orifices and "jets" in the carburetor to remove contaminents and such.

The bowl of the carburetor acts as a "sediment bowl"; that is, chunky stuff floats to the bottom of it. Unfortunately, that's where the main jet of the carburetor is:

so whatever stuff gets into the bowl gets sucked in there.

One thing that threw me for a loop was this brass piece:

It's a brass colored insert threaded into the body of the carburetor, just like all the other jets. However, it doesn't have a hole in it. I blew at it with high pressure compressed air, thinking it was plugged. Then I looked a second time at one of my diagrams of the carburetor. This is actually the plug over the check valve ball for the accelerator pump (the pump itself is on the outside of the carb at the top of the photo). So it's not supposed to have a hole in it. Good.

Here's the famous automatic choke that Muir hates so much:

One thing that threw me for a loop was the design of the replacement fuel pump is different than that of the old one. Here's the new pump next to the top parts of the old one:

The old pump had a screen under the top cover, and the cover was held up by a screw in its center. The new pump is entirely one piece, with no way to take it apart at all. I assume that means that the mechanical parts of the new pump are the same as the old, but that it doesn't have a fuel screen at all (because there would be no way to clean it)? I had already ordered in-line filters as a supplement to the screen in the pump. It's good that I did; I would have definitely wanted one with the possiblity of the pump not doing any filtering at all.

Here's the engine all assembled and ready to test:

The carburetor is all installed and set up. Note the fuel pump just to the left of the generator, without a screw in the top. Note also the in-line fuel filter that I've installed downstream of fuel pump:

It Is Alive!

After all that, the engine actually runs. Whoo-hoo! This is three or four minutes into idling, which means that the problem definitely has been fixed. A sucessful conclusion to my first real car repair.

One last tweak before I considered the car commissioned again. In the previous photos, the fuel filter is sitting on the floor of the engine on the cooling tin. Well, the underside of the engine is where the "hot" air goes, and so in my first test drive, the fuel filter got pretty hot down there. So here I've re-secured it up away from the floor, but still keeping the fuel hoses away from the generator belt.

The left fuel hose (to the carb) is secured to the air tube to the heater box, the right fuel hose (from the fuel pump) is secured to the vacuum can on the distributor.


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