Tuning triple SUs.
Before you start you need to have the ignition and timing correct.
• Ideally, use new plugs and points.
• Plugs gapped to 25 thou.
• Points gapped to 15 thou.
• Ensure rotor button, cap and leads are serviceable.
• Timing set to 9 degrees BTDC at idle. Set vernier adjustment to the middle of its range to allow for later fine tuning.
• Ensure both the centrifugal and vacuum advance work.
In my view the only accurate way to set SU mixtures is by measurement of exhaust CO%. I use a Gunson G4125 analyser that costs around $200 from EBay.
You will also need to use a vernier caliper to measure jet heights.
Remove the air filter plenum.
Undo all the throttle links and ensure all 3 butterflies are fully closed.
Ensure that your throttle linkages are adjusted so that
• they will give full throttle
• do not catch on anything else
• return correctly to have all 3 butterflies completely closed
• there is the same amount of free play in all 3 actuator arms, so that all 3 butterflies are synchronised
Check that all 3 needles are correctly centred in the jets by lifting the piston with the damper removed. It should drop freely and stop with a clunk at the bottom. If it doesn’t you need to re-centre the jets. The procedure is described in the manual.
Remove the bells and pistons. Do not mix and match. Check that all 3 piston springs are the same rate.
Check that the needles are set to the correct height in the pistons. There is a groove cut in the base of the needle. Use a 2 thou feeler gauge in this groove to set all 3 needles the same.
Pour out any oil in the pistons.
Replace the bells springs and pistons. Oil and replace the dampers.
Set the fuel bowl float level to 7/16".
Bring the engine up to operating temperature.
Reset all carbs to the initial spec in the manual.
• Slow running screws. Close fully then open 2 full turns.
• Set the mixture screw by adjusting the jet on all 3 carbs so that it sits 60 thou below the bridge height. This is more accurate than using a certain number of turns as per the manual.
• Adjust the fast idle (choke) screws using a feeler gauge so that they clear their stop by 2 thou. Ensure the choke lever is fully off before you do this.
Restart the engine. Adjust the idle speed down to around 700 rpm. Check that all 3 carbs are synchronised using a flowmeter.
Using the CO meter and the mixture screws on the carbs, adjust first the left (front 3 cylinders), then the right (rear 3 cylinders) exhausts so that you have around 5% CO. You need to consider that adjusting the centre carb affects BOTH pipes and adjust accordingly. Once you have the right CO reading readjust the idle as above. This requires a little mental gymnastics at first but you quickly get the hang of it.
Then remove the bells and measure all 3 jet heights. Theoretically you should have all 3 at the same height, although this often doesn’t happen.
What I do then is reset all 3 at the average of the 3 heights, then readjust mixtures for CO as above, readjust idle using flowmeter until I have jet heights approximately equal, idle flow equal and idle speed at 500 to 800 rpm depending on the type of engine. This can take several run throughs. If you get confused and end up way off then just reset everything to the starting spec and repeat.