Austin Healey 3000 SU problems

[Falcoray]

I recently bought an Austin Healey 3000 Bj8 I cannot get the tick over down. I’ve reset the carbs to the book about 3 times and it will not tick over slowly even with the slow idle screw completely closed, mixture is set ok any ideas please.

[2xChevrons]

It's likely to be an air leak from one (or both) of the carbs, probably around the throttle spindles or the gasket or spacer block (I can't remember exactly what the deal is on an A-H 3000). The genius of the SU (how it automatically maintains a constant pressure drop across the venturi) is also its weakness, because any vacuum leaks 'downstream' of the venturi will mean that the carb feeds more fuel to maintain the correct mixture, making it impossible to get the idle down. The idle screws can be all the way out but the engine will still draw surplus air through the leaks.

Spray all over the carburettor and inlet manifold with something safely unvolatile like carb cleaner while the engine is running. If the rpm drops (or, given that your idle screws are all the way out, dies completely) then the liquid has been drawn into a leak and temporarily sealed it. Keep squirting the fluid in more specific places to try and find the exact spot.

There are some simple things to check before doing that, however:

- Is the throttle linkage and both throttle levers actually at idle? Is the throttle cable tension set correctly and the cable isn't sticking or snagging on anything?

- Are the pinch bolts on the linkage between the two carbs secure? Are both throttles operating in synch across the entire throttle range? Try slackening the pinch bolts and using a finger to press each idle screw and throttle arm to make sure it is actually fully closed and that badly adjusted linkage isn't holding the throttle plate of one carb open slightly when the other is at idle.

- Are the fast idle screws fully clear of the fast idle cams when the choke is pushed fully in? As with the throttle, is the choke cable properly adjusted and operating fully?

- Check for perished, split or leaky vaccum hoses on the inlet side. A BJ8 will have a brake servo so include that in your checks, plus the vacuum advance line to the distributor. If I remember right a 3000 doesn't have any form of crankcase or rocker cover vent going back to the inlet, but if it does check that too.

[Falcoray]

Thanks for your help and advice I think it’s ok now... famous last words! The link bar between the carbs was sticking on one carb which held it open slightly, enough to make it idle really fast. I sorted that and set both up with figures from Bill Rawles and it’s much better.
Thanks again 2xChevrons.