Backfiring

If I rev the engine while the car is stationary, and as the engine speed drops to normal, there's a loud bang or poot from the muffler, and sometimes fire comes out of the exhaust. This also happens when I coast down a hill and I'm in gear but I'm not pressing the accelerator, I hear the same poots and bangs from the rear. Is this what they call backfire?

Sounds like a case of running lean to me -- the backfires on overrun are very common with a lean running VW engine.

The 31PICT/3 carb is usually jetted rather lean (a lot of VW carbs after 1970 were jetted lean by VW) and so you may find a simple rejetting will do the trick.

Like the 30PICT series, the 31 series carburetor is actually a little small for the 1600cc engine too (they have the same 24mm throat). That results in very high airflow speeds at higher rpm, and this can make it run a little lean too. Don't worry about the "little small for the 1600" comment though -- you lose only a few hp at the top end with this sized carby, and you actually get slightly greater torque at low/medium speeds.

The distributor/carburetor combination can make a difference, too. The 009 centrifugal-advance distributor has to have a carburetor that is jetted a little richer, or you get "flat" spots (hesitation) on acceleration. The 30 and 31 series carburetors are not as bad for that as the larger 34, but this hesitation problem can still happen when these carbs are teamed up with the 009 distributor. These carburetors, especially the 34PICT, are much better suited for use with a vacuum-advance distributor, and especially with the single-vacuum dual-advance (SVDA) distributor, which combines the best of both worlds.

With regard to jet sizes: for a vacuum distributor, try a 55 idle, 125 or 127.5 main jet and a 125 or 130 air correction jet (try the first number first.)

If you are running the 009 distributor, try a 55 idle, 127.5 or 130 main and a 115 or 110 air correction jet (once again, try the first number first).

I'll bet you have a main jet which is too small - a 122.5 or even a 120. Just changing that (without touching the air correction jet) may solve the backfiring problem.

These jet sizes should be very close to correct, but there is always some variation with set-up, engine age etc, so the alternate sizes mentioned might be needed.

The idle jet is a brass "bolt" on the right side, the main jet is in the bottom of the float bowl. The air correction jet is a brass screw with a hole in it -- vertical between the float chamber and main throat -- you have to take the top off the carby to get at it, and it's about an inch long. It has the emulsion tube attached underneath.

All of the jets have the number stamped on them in tiny numbers. In case you weren't aware, the numbers are the hole size in mm -- so 55 is a 0.55mm hole, and a 125 is a 1.25mm hole etc. With the main jet, each increase in jet size increases the fuel flow by about 2%.

In the case of the air correction jets, they feed AIR to the mixture (at higher rpm only) to stop the mixture richening to much at high airflows, so SMALLER means richer with these jets.

Another thing is that an aftermarket air cleaner and free flowing exhaust system also slightly lean the engine out, and so even if the carby is jetted "correctly" for the capacity, distributor and standard aircleaner/muffler, it may be running a tad lean for your set-up, resulting in backfiring.

And finally, the lean fuel-to-air mixture could be caused by leakage of air into the intake manifold. See our article on Air Inleakage. Dave had a very severe problem in this regard -- he finally found that the hole through the carburetor for the throttle shaft was worn to an oval shape, allowing air into the intake manifold AFTER the carburetor did its thing. This leakage of air into the system made the mixture way too lean, made it impossible to properly adjust the idle speed, and caused backfiring.