How to sort out a poor running Type 1 motor (part 2 – fuel system issues and carburettor tuning)

Click here to check out Part 1 of our tuning guide, which covered valve clearances and electrical / ignition system etc

07  Fuelled up

There are a few different types of fuel pumps and different brands, but the main difference is that a fuel pump for a dynamo takes a 108mm pushrod and an offset pump, for an alternator there’s a 100mm pushrod. If you have the wrong pushrod the engine won’t run, so make sure you buy the correct pump and pushrod configuration. Some aftermarket pumps have a pin that holds in the foot of the pump; this can work out, causing the pump to malfunction. Worse still, some pumps have a plastic foot, which wears quickly. To test the operation of the pump, remove the fuel hose to the carb and have someone crank the engine over for only a second or so; you should see fuel pumping out. Caution: do not do this near a hot engine and have an extinguisher handy. Some pumps can put out too much fuel pressure, leading to over fuelling of the carburettor, causing running problems. To check this you’ll need a fuel pressure gauge. For a standard engine pressure should be around 3-3.5psi. If it is higher, you will need to regulate it by adding more base gaskets to reduce the relative throw of the pushrod.

08 Feeling choked

VWs from 1961-on came with an automatic choke. This is actuated by pressing the accelerator pedal once with ignition on, before starting the engine. The choke uses a bi-metallic spring to reduce the air intake into the carburettor, enriching the mixture. If the choke is poorly adjusted it can make the car hard to start and cause poor running. To adjust, the engine must be cold; you will need to remove the air filter. Remove the three screws that hold the choke onto the body of the carburettor. Be careful not to drop any of the small spacers as you remove the choke assembly. Now, you can re-install the choke back onto the carb, ensuring the hook on the bi-metallic spring hooks around the choke shaft lever. Once this is done, install the screws and retainer and snug the screws up to finger tight. Now, turn the ceramic element until the mark on the element lines up with the middle mark on the housing. With this done, turn on the ignition and press the accelerator pedal once. Now start the engine with the air filter still off and go and look at the top of the carburettor; the butterfly should now be closed; the stepped cam on the throttle lever side of the carb should be down fully, meaning the throttle is being held open by the choke. Now wait a couple of minutes for the engine to warm up; you should see the choke gradually start to come off, until it clicks off altogether. If this doesn’t happen correctly, you can loosen the screws and adjust the choke slightly, then wait for the engine to cool down and try again.

09 Unblock heat risers

The inlet manifold can suffer with a couple of problems. The heat riser tubes that carry heat up from the exhaust system, to prevent the inlet manifold from icing, can become blocked up with carbon, meaning the car starts to suffer with carb icing or incorrectly vaporised fuel. The only solution to this is to remove it from the car and attempt to unblock it; I’ve tried everything from heating and blowing with compressed air, to using an old piece of cable on a power drill over the years, with varying degrees of success. Sometimes, a manifold has to be set aside and a new one sourced, as, despite best efforts, the carbon refuses to budge.

Occasionally, if you have an aftermarket exhaust, the pre heat holes in the exhaust will not have been drilled out, so you will need to remove the manifold and drill the holes in the flanges, in order to get pre heat.

10 Coming up for air

If the inlet manifold is improperly sealed, or the ports on the manifold or carb ports aren’t blocked off correctly, then you may have an air leak. Air leaks will cause the engine to run lean; this will likely present itself as poor idling, flat spots and stalling, especially when releasing the throttle, or slowing down after a fast run. One very common place for an air leak is the throttle shaft on the carburettor. The carburettor body wears out of round here on high mileage/old carburettors and this causes an air leak.  If this is the case, rather than changing the carb, you can purchase kits to install bushes here, curing the leak and poor running problems.

The other places for air leaks is where the carb bolts onto the manifold, where the manifold bolts to the cylinder head and, on dual port engines, where the rubber boots connect the centre section of the manifold to the end pieces. Even if the gaskets have been installed correctly, the mounting flanges can be warped, due to excess heat. Sometimes you can rectify minor warping by filing the mating flanges with a large flat file.

If you want to identify where the air is leaking, get a can of easy start, then, with the engine running, periodically spray it around the common leak points. If there is an air leak, the engine should pick up/run faster for a few seconds.

11 High carb diet

Finally, the carburettor on an air-cooled VW is relatively simple compared to modern cars, but there’s still a lot that can go wrong, either causing poor running or, worse still, stopping the car running at all. As there are so many different types of carburettor, there isn’t enough space here to cover everything. Generally, the first thing to check is that the main jet is free. The main jet can be accessed by removing a brass threaded plug on the side of the carb, then using a flat screwdriver to remove it. Blow the jet through with compressed air, or just blow through it if you don’t have compressed air.  You should be able to see if it is obstructed. Next, if your carb is 1967 or newer, you should have an idle cut off solenoid mounted on the side of the carb. This is to cut off fuel to the carb when the ignition is switched off. If this is sticking, it can cause running issues. To test, turn the ignition on (do not start engine) and remove the wire from the end of the solenoid, then touch it on the end again. You should hear a click. If not, replace the solenoid. Inside the top of the carb (you will need to remove the top of the carb), there is a float needle valve. It directs fuel into the bowl of the carburettor and regulates the level of fuel in the bowl. If the float needle valve is sticking, it can cause erratic fuel delivery. New float needle valves come with all good carb rebuild kits. Lastly, you can check that the accelerator pump is squirting fuel into the throat of the carb correctly through the small injector tube. This can be done with the carb removed from the car and the bowl of the carb full of fuel; pull down the throttle arm and you should see fuel squirting out of the injector tube. If not, remove the tube with pliers and blow out with compressed air, or flush out with solvent. Okay, so that’s covered the most common issues and by following these steps, your engine’s bound to be running a little smoother!


TOOLS NEEDED Screwdriver; flat 10mm, 13mm spanners


TIME TAKEN 4-6 hours

COST DIY From £0 for parts and materials

COST PRO approx. £150 parts and labour

Words & photos: Mark Walker & Ian Cushway

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