Tech Tuesday: Carbs

The purpose of a carburettor is to mix the fuel with air which it does by metering fuel through adjustable “jets” and drawing the fuel into a venturi using a vacuum effect caused by air rushing through the venturi.


We race in X30 and so use the Tillotson HW-27A but these principles apply to other diaphragm carbs – please check your manufacturer’s instructions for specific jet settings, pop-off values and so on.

You will need:

  • A pop-off gauge – ours is from JHC – Hubchen and has a mounting point to keep the carb steady while you work on it
  • Three-in-one oil which is a useful alternative to petrol for checking the ‘wet’ performance of the pop-off needle
  • Carb cleaner
  • Something to hold the carb still if your pop-off gauge doesn’t have one
  • A 8mm socket/t-bar machined to fit in the narrow orifice where the needle seat is located. These are available from major kart shops as “Tillotson sockets”
  • Rebuild kit – for the HW-27A a DG “half” kit or RK “full” kit including inlet needle and seat

Carb 1

We rebuild carbs after every meeting as a matter of course – you will see the reason why from our photos.

To disassemble the carb, start by taking the jets out. Make a note of which jet is which, and which order everything came out. Until you’re familiar with the process you could put the parts down on a piece of plain paper and label each part.

Despite filtering the fuel there’s still some debris on the small gauze filter, we blow that off with brake cleaner until it’s clean.


Take the screws out and take the carb apart a bit at a time, shake the fuel out of it and clean it with brake cleaner.


Take the pump diaphragm off, you can see a circle shape in it and that it’s slightly out of shape on the parts that do the work of pumping, so we’ll change it.


This is the metering diaphragm – examine it and see what it looks like compared to a new one, whether it still has elasticity and integrity. This one is out of shape around the inner circumference so we’re going to change it.



The metering needle valve needs to be checked closely, this is the valve that controls the pop-off. We fill it up with two in one oil as it has to be wet, use the pop-off gauge and see where it pops off. In this case it popped at 0.75 bar and was holding pressure at 0.5 bar as recommended by Tillotson. That tells us that the valve is in good condition. It’s doing that consistently as it was a new valve and seat at the last rebuild.



The pop off pressure is controlled by the spring weight. There are various weight springs available however IAME UK recommends you stick with the standard spring as it gives the best performance across a wide range of conditions.

We’re going to take the needle valve assembly apart anyway to clean and check it. Remove the small screw and take the valve out. You can see the spring, valve and metering arm. Look at the end of the valve and see what kind of condition it’s in. When the pop-off pressure is tested, if the valve is holding pressure it’s safe to assume that the needle valve is in good condition.

Carb 2

Next remove the needle seat with the t-bar that has been machined down to fit in the hole.

Carb 3

Then get the small copper gasket out, this might be fiddly but often you can just tip them out.

At this point it’s completely stripped down. Spray brake cleaner into every orifice, you should be able to see fluid flowing through the high jet and the low jet. Then blow air from a compressor through the orifices.



Check that the butterfly valve doesn’t open past the middle at full-throttle, if it does, adjust it with the screws.

When reassembling, remember the gaskets always go beneath the diaphragms. Keep note of the order all the components must be in. If the carburettor goes back together incorrectly then the engine will most likely not start and if it does then it will not run well or in worst case scenario, it will seize!

Carb 4

Tillotson recommended jet settings are:

X30 Senior

Low: 1T

High: 1T 30-35 minutes

X30 Junior

Low: 1T

High: 1T 20-25 minutes


This article was originally published in Karting magazine in January 2016


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