Carburetors are one of our best selling products and as a result, they are one of our most returned products. We do not accept returns on carburetors once they have been installed, but people send them back thinking that they are defective. We will let you in on a little secret, most "defective" carburetors are not defective. The issue is that people think they have a bad carburetor, they buy a new one, and the problem still persists. The carburetor that they replaced is not the problem and the carburetor that they just bought, tried to return, and claim as defective is not the problem either. The source of the issue lies elsewhere. What happened or what changed when the “carburetor” suddenly stopped working correctly? Did you replace your fuel pump? Incorrect fuel pressure can make it seem like the carburetor is not working properly. And, it can happen with either too much (more common) or not enough fuel pressure.

Fuel Pressure is Too Low

When the fuel pressure is too low, the fuel is not flowing into the carburetor fast enough. This isn't the perfect analogy but it (sort of) works. Think of the carburetor as a swimming pool. When you have too low fuel pressure, the fuel pump is filling the pool (carburetor) with a garden hose while the swimming pool is draining itself with a firehose. The carburetor is sending fuel into the engine faster than it can pull fuel from the fuel pump.

Fuel Pressure is Too High

When the fuel pressure is too high, there is too much fuel going into the carburetor, more specifically into the float bowl in the carburetor. At this point any excess pressure that the float valve in the carburetor bowl can’t stop, will over fill the carburetor. Excess fuel can spill over through the vent, into the carburetor which can “flood” the engine. Other times, the float will stop the flow of fuel effectively but all the pressure on the lines, hoses and pump can cause the fuel in the lines to overheat and in some cases “boil” the fuel, causing vapor lock. In either case you might have one of the following problems:

  1. When too much fuel is overflowing (flooding) the carburetor you might experience, backfiring, hesitation, stalling along with black smoke from the exhaust.
  2. If the float is shutting properly, the engine may completely stall. Then it may not restart again until the engine, carburetor, fuel pump and hoses have cooled down.

If your engine is running too rich (blowing black smoke out of the exhaust) or you notice your tail pipes are covered in black soot, it may be fuel pressure or it could be carburetor jetting. Without checking the fuel pressure, it may be hard to identify where the issue lies.

If it is fuel pressure, the fuel pump is "flooding" the carburetor. When there is way too much fuel flowing into the carburetor, the air-to-fuel mixture is so far off that the engine won't run. The only thing you can do is wait for the fuel that is sitting in the intake manifold and the cylinders to evaporate. One way to speed this up is to actually “floor” the throttle while cranking the engine to push the excess fuel out of the cylinders.

Why is my Fuel Pressure So High?

It's the fuel pump. We and virtually everyone else have struggled to find fuel pumps that will deliver the right fuel pressure. Most fuel pumps put out way too much pressure, we’ve seen as high as 6 PSI and heard of higher.

What is the Correct Fuel Pressure?

The fuel pressure for air-cooled VW engines should be between 2-3 PSI for stock carburetors according to VW.

How do you Measure Fuel Pressure?

The best way to measure fuel pressure is to install a fuel pressure gauge in-line between the fuel pump and the carburetor.

How do you Adjust the Fuel Pressure if the Fuel Pressure is Too High?

  1. You can install a fuel pressure regulator inline between the fuel pump and the carburetor.
  2. You can change your fuel pump drive rod.
  3. You can add upper fuel pump gaskets to lower the pressure (We had to use 4 on the recent Super Beetle build).
  4. Replace your fuel pump (and adjust the pressure with gaskets as needed).

How do you Adjust the Fuel Pressure if the Fuel Pressure is Too Low?

  1. Remove your fuel pump and verify that you have the correct length fuel pump rod and replace it if necessary.
  2. Check to see if someone had tried to lower the fuel pressure in the past by adding upper gaskets and removing them if necessary
  3. Replace your fuel pump

We ran into this Problem Ourselves

When building our Super Beetle with a 1800cc engine and an EMPI 38 EGAS carburetor, we found the fuel pressure at 6 PSI. It was way too high. We tried different fuel pump rods, swapped out the fuel pump, and added 4 upper fuel pump gaskets. Eventually, Sam ended up using a Spectre fuel pressure regulator to fix the problem and and keep the fuel pressure constant at around 1.5 PSI at idle.