Tuning twin SU carburettors on 1500

[Hexaflexagon]

Hi,

I'm on the last lap of putting my Spitfire 1500 back on the road after a layoff of circa 3 years and need some help re tuning the carbs. At least I think they need tuning since the engine was quite jerky when pootling around our road, which fortunately is private so no need yet for MOT/VED and Insurance.

I've been reading that the first thing is to get the tickover to circa 800 rpm and turn the adjusting screws until the noise of the inrushing air is consistent between the two. (I'm doing this with a plastic tube and listening with the Mk1 ear so  no technical gubbins)

The next step apparently is to use the piston lifting pins to lift the piston a little tio check the mixture richness. Apparently if on lifting the piston the revs drop, that carb has a weak mixture and the adjustment screw needs turning clockwise to richen it. And vice versa.

The difficulty I have is that the revs do drop but after adjusting the screw clockwise to richen the mixture the rpm idle then goes well past the 1000 rpm, and I'm in a chicken/egg situation.

Can anyone offer any advice? Is this the best way to tune it or are there other variables. I did wonder about the choke but I'm tuning with the engine hot and no choke, or at least the choke is pushed fully home. The air filter box is also removed whilst I'm doing this so not sure if that might affect things.

Usual TIA

[esxefi]

it's a balance between mixture and throttle opening,only one setting will be right.

if you richen and speed increases,close the throttles slightly.
keep doing this until you are happy,the ear is as good as any equipment to set the balance and you only need to lift the piston a tiny amount 2mm or so.

forget the choke for now,that can be set when engine is cold :)

[Andy_w83]

http://www.mintylamb.co.uk/?page=sutune.htm

Have a look at the above link, it seems helpful.

Andy

[Pete Lee]

Also search the MGB EXPERIENCE website for tuning twin su carburettors.