Piston Drop Test SU HS6 carburetters
As you maybe know you may never swap the air chambers from your SU carbs. They (the air chamber and piston) are a factory matched pair. If you swap them or swap the pistons you'll probably get air leaks between the piston and the chamber which will result in an bad response to your throttle.
As I bought the Volvo 122s with twin SU HS6 carbies second hand I wasn't sure if the air chambers were on the matching carbies. But luckily there's a test which will determine if they're OK. This test is also known as the piston drop test.
- Loosen the 3 bolts from the suction chamber and take it off
- Remove the spring from the piston
- Remove the piston from of carburetter and get rid of the ATF in it
- leave the needle on the piston
- close the two holes in the bottom of the piston (I used a piece of a round rubber seal and made it thicker by wrapping it in aluminum tape)
- Remove the damper from the chamber
- Put the air chamber up side down on the bench and put the piston back in
- Hold the bottom of the piston flush with the edge of the air chamber (you can use the needle to hold the piston flush) and then let it sink in the air chamber
- If there are no abnormal air leaks this should take between 5-7 seconds
- If it's faster there's something wrong / wear (hopefully they're only swapped)
- If it's way slower clean the air chamber (you can use wax and grease remover and coarse polish to clean the suction chamber)
|Measuring the SU carburetter piston drop time|
|Holes in the piston plugged for the piston drop test|
|Part of Honda CB750 rubber oil filter seal used as plugs|
SU HS6 carbie Piston drop test
Synchronising SU carburettors
Another nice gadget tool is the Uni-Syn. With this tool is fairly easy to synchronise the SU carbies. I know that you can achieve almost the same results by listening to the "hiss" sound. But as I wasn't born as an experienced hiss listener this tool is helping me to become one..
|Uni-Syn synchronisation tool|
Tip: First check the airflow for both carbies without disconnecting the throttle, maybe they're already synchronised and it will save you the hassle. If there not synchronised loosen nut (1) and follow the instructions. If they're synchronised you need 0.3mm play between the lower tooth of the levers of the throttle spindles and rod on both sides. Because this is a hard to reach place for a feeler gauge I disconnected (2) and (4), let the rod rest on the lower tooth of the lever that's not loose en push the loose lower tooth of the lever softly against the rod and then fasten the nut (1) again. Now you can achieve the exact same play for both by turning (3) in or out. Another way to do this (when both levers of the throttle spindles are loose) you can find here: http://volvo-122s.blogspot.com.au/2013/07/tuning-volvo-b20b-engine.html
You can also use the Uni-Syn to synchronise the choke on your carburettors.
|Synchronising SU carburettors with Uni-Syn|
|Synchronising SU carburettors|
Creating the right air/fuel mixture
In order to create the right mixture I turned the mixture screws out a bit more then the specifications from Volvo/SU. This because the petrol nowadays has a lower octane (91-98) level than back in the sixties (100+). This will make the mixture a bit leaner. So turning out the mixture screws a couple of extra flats will compensate for the lower octane.
To check of the mixture is right, lift the piston a bit with a screwdriver or use the SU carburettors lift pin;
- If the idle falls or the engine stalls, it's too lean
- If the idle rises and stays high, it's too rich
- If the idle rises a little bit and then falls back to normal, it's the right mixture
|Checking the air/fuel mixture|
The right setting will give the explosion a nice blue color. To rich is orange and too lean is white/ very light blue.
I like to set the carbie a bit rich (orange) and then turn the mixture screw back in until the colour changes to Bunsen blue with here and there a orange spot, especially when revving the engine..See the diagnostic chart with all the colours (or must I say colors..) below.
|Gunson Colortune Diagnostic Chart|
Blue colour (sometimes whitish --> a bit lean)
Yellow / rich colour