Su Carb tuning

[1HRH]

Running freshly rebuilt 3.5 V8 in a land rover defender. Im using a pair of HS6 carbs with BAF needles but im not sure about which damper springs to use? Have tried 8oz springs but dosent seem to pick up very well, any ideas to which springs may suit best? Its got open K&N filters and tubular manifolds with performance exhaust. I have tried several sets of needles and BAF seem to give me average mpg with good power through RPM. The acceleration ok, but im told by changing the damper springs will increase the pick up even more. Any help/knowledge greatly appreciated.

[ChrisJC]

The oil viscosity in the dashpot controls the damping - have you tried a thicker oil?

Chris.

[RoverP6B]

A lighter piston spring,..ie 4.5 oz red spring will allow the piston to rise more rapidly whilst using the same grade of damper oil.

What type of oil are you using in the dashpots? Typically in a heavy vehicle like a defender, a thicker oil would be used so as to slow the piston from rising to rapidly which will allow a richer acceleration mixture.

Using a lighter spring will also mean you will need to change the needle to a richer one else the mixture will now be too lean.

Be aware that fuel consumption will increase as a result of using lighter piston springs.

Ron.

[kiwicar]

Hi the spring in the piston is a key part of correctly tuning SU's, you can change it at random with no referance to it's propper funtion but the chances of getting good results are slim. The spring is intended to control the velocity through the venturi over the entire rev range of the engine so it needs selecting correctly. You want the piston to get to the fully open point under full load about 500 revs before maximum revs on the engine (dash pots out), if it lifts to maximum before this then you have no controle over the top end mixture, it doesn't ever reach full lift then the carb is restricting flow too much and there will to high a velocity through the venturi for the carb to work properly. Once you have the correct springs then you can start selecting needles to sort out the mixture. The rate of rise of the piston is controled by the oil viscosity (as stated earlier) and controls mixture enrichment in transition so again this needs selecting after you have sorted the correct needle.
Best regards
Mike

[1HRH]

I have tried a number of oils in the dash pot. At the moment im using straight 20. Had a chat to Burlen fuels today, they are sending me some 4.5oz springs and also BBW needles. At least with some richer needles and diffrent springs i can play around with the set up. Thanks for your advice guys. I guess i havent helped my self by raising the overall gearing by 15%. At the moment it will do 70mph at just under 2000rpm in 5th. im always between 4th and fifth with any small hill.

[davemgb]

In my experience changing the springs is usually a mistake.

You say it does not pick up very well, what do you actually mean? Does it stutter when you apply throttle at low revs? does it pause once its accelerating? Get it diagnosed properly and find out if it pauses because of under or over fueling. Last time I had a car go soft and miss on pickup it was a burst vacuum capsule on the distributor. If the 8 oz springs are the original spec then stick with those until its been diagnosed properly.

If you have removed the standard filter housing what have you fitted the K&N filters to? there are breather holes on the outside face of the carb for the dashpot assembly and people often cover these up without realising what they do.

Dave

[RoverP6B]

The HS6 carburettor accepts a fixed needle where as the BAF and BBW are swinging needles which require a spring and seat for fitment into the piston.

Are those needles also available in a fixed format?

If you fit swinging needles into a piston designed for a fixed needle, then the engine will never run properly.

Ron.

[1HRH]

I have now fitted the new 4.5oz springs and for now using the BAK needles (fixed) with straight 20 oil. The pick up is so much better, dosent hesitate and really pulls well. After covering about 30 miles i have checked the colour of the plugs which are a dark grey in colour. It now pulls hard in every gear and also cruises nicely at 65/70 with my foot backed off. Filled her up with fuel and did about another 45 miles (short motorway/A roads,) returned 19 mpg! Its the best its ever returned alot better than 11 mpg when i first started with the carbs. Havent heard about the air holes on the side that the filters may cover, will have a look in daylight. Its a diffrent motor to drive, seems to have alot more power and dosent miss a beat.

[davemgb]

The HS6 carburettor accepts a fixed needle where as the BAF and BBW are swinging needles which require a spring and seat for fitment into the piston.

Are those needles also available in a fixed format?

If you fit swinging needles into a piston designed for a fixed needle, then the engine will never run properly.

Ron.

Helps to actually post something when you quote a message !?!

I don't have an HS carb to hand but the HIF has a large mounting plug that carries the needle and the needle has a retainer collar on the mounting part at the top, the collar could possibly be transfered to an earlier needle.

Surprised to hear that the weaker springs worked, would still be useful to know which style of filter you added, large cone type or shiny pancakes.

Dave

[1HRH]

The K&N filters im using are the cone shaped ones. The later HIF44 carbs use the swinging type needle, to use them in HS6 carbs all you have to do is remove the top part of the needle where the spring would usually sit.