Tuning twin SU Carbs

[Bullmansboy]

My 948 engine is fitted with twin SU carbs, any advice on tuning these would be appreciated.

Alternatively, would I be better reverting to single carb?

[picky]

you would be much better off with a single carb, much easier to setup. I would recommend a HIF38 if you can get one, or a HS4.

[eastona]

try this link, contains stuff about twins. echo the advice about ripping them off though

http://www.mmoc.org.uk/index.php?name=P ... ic&t=16656

andrew

[Alec]

Hello Bullmansboy,

S.U.'s are pretty simple as long as you have the specifications. And once set up require little maintenance and do not require regular tuning.
If the car is running well, I would leave them alone, but if you suspect that they are not set correctly, I would get a manual to guide you.
There are a few basics, needles should be set in the piston so that the shoulder is flush with the piston base. The dashpots internally and the piston major diameter should be clean, use only chemicals to clean these, by the way (carburettor cleaner or thinners).
Basic setting of the jet is two full turns down from the jet being flush with the jet bridge, this should be close to the correct mixture setting.
Set up the idle, balance and mixture with the throttle interconnecting link disconnected.

Alec

[bmcecosse]

Really an overkill on a 948 engine - does it have any other modifications ? A single 1.5" SU (later HIF 38 if possible) on a decent alloy inlet manifold will be a better bet. The only advantage your twins will have at the moment is that the ingoing charge will not be heated by the exhaust manifold - and this is worth 3 or 4 HP! So a single on a combined manifold (with that awful heating) would likely be worse - big single on unheated alloy is better.
To 'tune' what you have - take off any inlet filters - and rev up the engine while watching the two carbs - do the two pistons rise at the same time ? If they do - then just check the mixture on each one in turn - by lifting the little 'lift' pin and if the engine flutters and slows then too weak, if revs up then too rich, if nothing much then just right! Adjust as necessary - and try try again.
If the pistons lift unevenly - then first listen to the idle air on each carb with a piece of 1/4" bore tubing - if one sounds more than the other then adjust the idle screw on that one down and vice versa on the carb that sounds less. As Alec says above - you may need to slacken the joining link to do this - although there is often enough slack in the 'fork' connections for small adjustments. Then when the idle is same - you will need to again check the piston lift - and if still not right - adjust the link connections so both throttles open exactly together. The twin carbs will look great - and of course extra Pub Points - but to be honest - a big single will work better and is more easily set up.