Jul 26, 2014

Dyno - Machining the SU carb needles

Because the rebuild Volvo B20B engine has undergone some modifications the SU HS6  carbies probably need other needles. Although the engine was running OK I felt it wasn't always as powerful and smooth as it could be.

But what's the right needle to get the maximum out of your modified engine? There are over 260 different needles for the SU carbies.

SU needle dimensions and stations - page 1
SU needle dimensions and stations - page 1

SU needle dimensions and stations - page 2

SU needle dimensions and stations - page 3

SU needle dimensions and stations - page 4

SU needle dimensions and stations - page 5

SU needle dimensions and stations - page 6

SU needle dimensions and stations - page 7

And there's a real possibility that the right needle for your modified engine even doesn't exist. So in order to find the right one I'm gonna buy them all and try each and everyone of them...hahaha  No, there's an easier and quicker method to get your tailor made needles.


First some info about the optimum Air-Fuel-Ratio (AFR) and carbie needles.

Theoretical the optimal air to fuel ratio is 14.7:1 (determined for pure gasoline). But as most petrols have varying properties and additives it is commonly accepted that most engines respond to an AFR somewhere between 12.5 to 13.2 parts air to every one part of fuel.

Carburetor needles
If you take a closer look at a SU needle you'll see it is getting thinner at the end. When the engine is idling the needle is almost all the way in the jet. The jet opening is almost closed and only a small amount of fuel can pass the opening between the needle and jet.

SU needle and jet Volvo 122S
SU needle and jet (with attached fuel line)

Volvo 122S carburetor Needle in Idle position
Needle in idle position

If you rev the engine the carb will lift the needle which will allow more petrol passing the opening because the needle is thinner at that point.

Volvo Amazon carburetor Needle in Cruise position
Needle in Cruise position

SU Needle full trottle , see the bigger opening of the jet
Needle almost full throttle , see the bigger jet opening

SU needles are divided in stations. Normally around 16 stations. Sizes are taken and calibrated every 1/8 inch (ca 0.3175 cm) from top shoulder. The thinner the needle at a station is the richer the mixture will be at that specific rev point.

SU - KN needle with 16 stations - Volvo Amazon
SU - KN needle with 16 stations

Sooo... to create the perfect needle you have to measure the AFR at every station of the needle and a Dyno is the perfect tool to do this.

When you wanna do this make sure you have a near new needle and jet. Now it's a matter of fine tuning your needle.

With the first Dyno results you know the AFR at every Station and you'll see it isn't always within the 12.5 to 13.2 range. Every station that shows a leaner mixture (>13.2) will be machined/filed. This isn't a job you can do yourself, unless you have the right tools and really know what you're doing.

AFR with different speeds - SU HS6 twin carbies - Volvo Amazon
AFR at different speeds

The NGK BP7HS Spark plugs were also replaced with 2 points hotter spark plugs; BP5HS. In summer I'll probably replace them with 1 point colder ones; BP6HS. The problem with the BP7HS was they weren't getting hot enough (it's winter here at the moment) to burn all the carbon of resulting in a black insulator nose with a weaker spark.

After machining the lean stations and replacing the spark plugs the Amazon is now running super smooth over the whole rpm range of the B20B engine. Still running in so I have to be careful not to floor it, still a couple of hundred miles to go before I can redline the Amazon...

Test drive after dyno tuning

This time 3 bonus pics, made this afternoon when I was replacing the exhaust of a Saab 900 turbo.

Classic Saab 900 Turbo with Simons sport exhaust
Classic Saab 900 Turbo with Simons sport exhaust

Toolbox guard

So, where's my reward ?
Did you mention the word Schmacko????