SU carb spring selection

[jerrytr5]

How does one select the dashpot spring when modifying SU's ?

If I want to find out about needle selection, there are a million documents online but almost nothing about the spring. Burlen suggest "When the correct spring is fitted it is usual to obtain full piston lift at full throttle and at approximately three-quarters of the maximum rev/min." So where do you start?

I've fitted red springs, but at 4000 rpm the piston is only about 10mm off the bridge which is nothing like full piston lift.

I might try removing the springs and see what 'maximum piston lift' really is but I'm not sure how that might help selection. It seems pointless to start selecting needles if the spring is wrong.

The engine is a Spitfire 1300 with HS4 carbs (from a Spit 1500), tubular manifold and foam filter but that's not really the point - it's the principle I'm trying to establish.

Incidentally, had fun playing with my new synchrometer - demonstrates just how useless it is to set the throttles at idle. I found that at 3000 rpm the airflow was half on the front carb that it was on the rear. So I synchronised them at 3500 which is rather more important than at idle. All these years listening at tubes at tickover and it's all been a waste of time.

Jerry

[Peter Cobbold]

Jerry,

It is almost impossible to get much lift with the car stationary. Burlen refer to an engine loaded on a rolling road - the "...full throttle..." bit is very important. You will not be able to hold full throttle on an engine in neutral let alone control the revs...bang! 4000rpm but with nearly closed throttles will not replicate Burlen's test. The reason is it takes very little horsepower, hence very little air, just to spin an engine. So I reckon getting 10mm lift is a lot of lift for very little air flow.

 

My SU book tells me the Spitfires use Red AUC 4387 4.5oz downforce at 2.6inches. You can measure it on kitchen scales - compress it inside the dome onto the scale pan.

 

The uneveness between carbs might be due to one sticking a bit. The piston should lift smoothly and easily with damper rod removed and fallback suddenly. If one gets stiff with the damper rod in then it could be slightly bent or the damper itself not passing oil.

 

Peter

 

PS I approve of removing the spring to see what happens - because there's nothing like an experiment !! But its not the answer.You then have just the weight of the piston acting against choke-tube dperession. You will get more lift but it will run like a bag of nails except at full throttle. Its a racing trick best never replicated on road cars!!

Edited December 20, 2014 by Peter Cobbold

[jerrytr5]

Hi Peter,

I did wonder if it was something that could only be ascertained under full load. It also means that checking the plugs for mixture after high revs with no load is also meaningless, I'll have to take her for a spin. There's a handy bit of dual carriageway with a parking slip road not far away so I can run her up then stop and check the plugs.

Both carbs are fine, the uneven flow was purely a result of setting the throttle connecting rod by eye and feel and then checking it with the synchrometer.

Cheers,

Jerry

[Motorsport Mickey]

Jerry,

 

If you have one of the small inspection cameras about 10mm head size with either an integral small LCD screen or recording SD card with it, position it under the bonnet with tank tape viewing the carb piston without airfilter, you can do full throttle runs and check the height of piston lift in action. For mixture checks cut the ignition under full throttle and cruise into the laybye under overrun and no power.

 

Mick Richards