su carb tune
- Thread starter garethp6
- Start date
Hello Gareth,
Provided the air being drawn into each carburettor is the same volume, proceed as follows.
Check to ensure that both floats are set to the same initial setting, as variations here will impact on mixture at all points in the rpm range.
Remove the dashpots and lift out the pistons then set the jet in each carburettor to exactly the same position in relation to the bridge. Flush is a good starting point. Refit pistons and dashpots.
Start the engine and adjust each mixture screw an identical amount. Winding in richens,...out leans it off. The mixture scres should only EVER be adjusted at idle. Using the lift pins or a flat bladed screw drvier to lift each piston in turn by say 1mm or so take note of the engine sound. Idle speed increases and remains so...too rich. Idle speed rises slightly then drops back to pretty much where it was initially...spot on. Idle speed drops and engine stalls or very nearly so...too lean.
Another alternative is to wind out each mixture screw until the idle speed starts to drop and then wind in one half turn.
Another option is to buy an exhaust gas analyser if you really want to get the CO levels close to the mark.
The beauty of SU carburettors is that they have a range of of A/F ratio settings that they can be set too. Slightly lean will produce best economy, slightly rich best power. You can tailor them to your drving style and what you would like to achive.
Ron.
Provided the air being drawn into each carburettor is the same volume, proceed as follows.
Check to ensure that both floats are set to the same initial setting, as variations here will impact on mixture at all points in the rpm range.
Remove the dashpots and lift out the pistons then set the jet in each carburettor to exactly the same position in relation to the bridge. Flush is a good starting point. Refit pistons and dashpots.
Start the engine and adjust each mixture screw an identical amount. Winding in richens,...out leans it off. The mixture scres should only EVER be adjusted at idle. Using the lift pins or a flat bladed screw drvier to lift each piston in turn by say 1mm or so take note of the engine sound. Idle speed increases and remains so...too rich. Idle speed rises slightly then drops back to pretty much where it was initially...spot on. Idle speed drops and engine stalls or very nearly so...too lean.
Another alternative is to wind out each mixture screw until the idle speed starts to drop and then wind in one half turn.
Another option is to buy an exhaust gas analyser if you really want to get the CO levels close to the mark.
The beauty of SU carburettors is that they have a range of of A/F ratio settings that they can be set too. Slightly lean will produce best economy, slightly rich best power. You can tailor them to your drving style and what you would like to achive.
Ron.
