Alex is correct. Don't do it unless you can measure it. Here are the hints I used for the needles.

Jerry //www.terryhunt.co.uk/tech/picsb/pics.htm

Unless you have a wideband lamda sensor installed or access to a rolling road and an AFR meter, don't start changing needle profiles - you will only make things worse.

a portion of this video shows a guy polishing the needle, is this the proper way to do it?

 

//youtu.be/_p23gFEeiYE

Jeg,

I only sanded the 5, 6, 7 positions as that was the area that was too lean. I then polished with 1500 sandpaper before trial.

I will take your advice on the whole flat change. Might as well. I know I had it starting better before.

Jerry

Cool, sounds like the springs have helped - I'd try leaning it out just a tad, maybe 1 flat and see if helps with starting.  It won't make too much of a difference to your running and it's most important to get the base of the needle sorted before worrying about the tip. 

Remember though, don't touch any area of the needle that affects the idling - only modify upwards of position 3 when necessary and work progressively higher on the needle when you do.  It's a bit like climbing a pine tree; start at the bottom, get it set, move up a bit, get it set, move up a bit etc...  Saves you time and money in extra needles.  If only I'd realized this when I first started modifying needles so many years ago.  (had twin HS2's on my old 850)

Update; I have made two runs with the car with new red springs. NO OTHER Changes were made. Just adding the red springs dropped the idle about 150 rpms. (or the wet weather is affecting the engine, but I doubt it). I left the 3 wt oil in the dampers. Butt Dyno: This is a run up my neighborhood that is slightly uphill and I try to run it consistently, speeds up to about 60mph. But I can usually tell prior to 60 if there was improvement. Low RPM- better, starts pulling right away and the air/fuel stays in the 12-13 range High Rpm- Can't tell much but feels better, the air/fuel still good. Noticeable improvement in acceleration in 4th gear.

Next changes to test: 1. turn the fuel down another 1/2 flat - starting is a little hard, and acts like there is too much fuel, starts better without the choke, but it is cold starting and likes a little choke till it warms up. Usually takes about 3-4 miles before the temp gauge starts to move. 2. Try some thicker oil in the dashpots 3. Drive it like I stole it!

some day, I am going to check the mileage again. Last check was 25-27 mpg.

Jerry

The way I learned it, allowing the piston to lift slower causes a low pressure betweem the piston and the bridge sucking more fuel from the jet all be it a smaller opening. And I thought Webers made you crazy. Steve (CTR)

Just got the new springs yesterday and will put them in today. But, It is actually raining in California! We have received over 4 inches in the last week, so I may not test the car till Friday when we should have a dry day. The last bit a polishing on the needles, really improved things. Still a little weak on accelleration due to too much fuel at the bottom of the rpm range. But it screams at the upper end.

More to come. We are doing a toy drive on Sunday and the mini will be going.

Jerry

How are you making out?

Not sure if this is what you mean..but , yes, the spring controls the height of the piston for any given air flow.  So the correct spring is that which provides full piston lift at WOT/max revs.  A spring that is too weak will mean that the piston reaches full lift before max revs which in turn means the mixture will lean out at the top end as air flow inceases with no further corresponding fuel increase...

All pistons can take fixed or sprung needles..  Fixed needles came first so when sprung needles were introduced they used an adaptor that allows the sprung needle to go into the same slot as the fixed needle.

While i've not done it myself, DV suggests that a simple flat filed on the needle is a simple and effective way to change the profile (as opposed to using a lathe to turn down the diameter....)  

Cheers, Ian

OK, are you saying that the spring is only effective as a way to decide when full piston lift happens? My thoughs were along the line of weaker spring allowing the piston to lift sooner to weaken mixture during transition. I would think float lever would be a factor. I enjoy talking with AC Dodd about SUs. He seems to think twin 1 1/4s are to small for all but std 998s. I have a couple of the SU needle chart booklets. I also have legal pad charts I have done so I can see the needles I have on hand side by side. I'm not sure when the floating needles came out but I have pistons for most size SUs for fixed and floating needles. While removing floating needles from crusty old carbs I have had the needles come out of the shouldered portion the spring attaches to. I have not tried reprofiling myself yet. Careful study of the chart and a call to Joe for the one I think and one either side of it gets close. My next project will be twin HIF38s for a big bore. Steve (CTR)

Placing an order for red springs now! good discussion. Smurf and I were have this part of the damper discussion off line and the writing you find out there is listed two ways: 1. lift the piston faster to get more air and lean out the mixture 2. Lift the piston slower to let less fuel and lean out the mixture.

I will approach it this way. the present system of #1 is not working perfectly, so I will try the red springs and see what happens. Fine tuning with different weight oils.

Thanks

Good discussion

Jerry

The spring used should allow the piston to be fully raised only at WOT under maximum load if ; too light a spring and the piston will rise too much, allowing for too much air through the carb and possible 'fluttering' instability of the piston at part throttle. 

Again, from Berlen's site:

"Piston springs

On 1 1/4 in., 1 1/2 in. and 1 3/4 in. diameter horizontal carburetters, the red (4t ounce) spring is normally used for initial testing and in most installations this load of spring will be effective, assuming that the size of carburetter has been correctly chosen. 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. If the absolute maximum of power is required, however, it is more usual to choose a somewhat larger carburetter, which will not obtain full piston lift until nearer maximum rev/min."

No way of really knowing what you've got at idle or low part throttle, but I'd suggest a deep breath, red springs to start, raising the jets to get it to run best at low loads and then work on the thinner needle points 6 through 9 - without planting your foot to the floor.  I use the handbrake and increase revs gradually to create more load at higher throttle.  After you've got the profile sorted, fill with 20W fork oil and then try a damper that gives the best pick-up.  If using AUC 8103, try the AUC 8114. 

Of course, a call to Berlen or Joe Curto could also help find a damper with lesser or greater restriction.

I would think a lighter spring would be in order. Steve (CTR)

Install heavier dashpot springs and try 3 in 1 oil

I have no "dyno" specs, nor anything more than butt-meter readings......but, I fit twin 1.5" to my near stock 1275 and the difference is very noticeable.  The HS2 pair I replaced were tuned well and mix was good.
I forget who I bought them from ( someone on here ).  I bolted them up to try them out of the box....never had to remove them.
The carbs have #5 needles.....they run so well, I'm scared to try anything else..lol

"I think the only thing I would like to change right now is the fuel dump on acceleration. I am getting too much right now. I put in a thinner oil into the dashpots (5wt) and that helped a little. Anyone know another way to stop the fuel dump?"

By 'fuel dump', do you mean excessive enrichment?

Which dampers are you using?  Changing to a different damper can increase or decrease the speed at which the piston rises, regardless of which oil your using.  Find the best damper, then fine-tune the oil.  A quote from the Berlen Fuel Systems website:

"Most S.U. carburetters incorporate a piston damper, the function of which is to restrict the speed of lift of the piston on snap throttle openings, and to allow the piston to fall at its normal speed on throttle closure. This one way damping is obtained by means of a non-return valve situated at the base of the damper.

When the speed of piston lift is retarded an additional air depression is put on the fuel in the jet resulting in an increase in the quantity of fuel discharged. A richer mixture is thus obtained until the piston resumes its position of equilibrium. This enrichment is necessary to provide satisfactory pick-up. The piston damper also improves cold starting and driveability from cold."

Another quote, also from Berlen:

"The primary purpose of the piston damper is to provide the enrichment necessary for a satisfactory 'pick-up' during rapid opening of the throttle.

This enrichment is achieved by the damper retarding the speed of piston lift, thereby creating an additional depression over the jet which increases the amount of fuel discharged. When the engine is cold, the viscosity of the oil in the damper is high and the enrichment obtained is therefore greater than when the oil is warm."

After I changed dampers, I went back to 20W (summer) and 15W (winter) in my HIF w/non-bi-metalic assembly.  When I had the HS4, I used AUC 8103, had tried the AUC 8110 and the AUC 8114 (the short version of the AUC 8103).  I've now got 4 different types for the HIF, and synthetic oils 5W, 10W, 15W and 20W. 

Edit: Quote from an MGA twin-cam service bulletin, 24 July 1959:  "The original dampers (AUC 8103)" could cause uneven running and a restriction in maximum speed but are interchangeable with the new ones (AUC 8114) giving an improved performance.  Alternatively, the old type can be converted to the later specification by machining .070" (1.78..) off the lower face of the piston."  (refering to the Damper Piston)

My 2 cents:  The piston damper is probably the most overlooked factor in tuning SU carbs.

You are probably right Norm, but who wants to brag about 1.25 carbs? I have a set, may try them. But since I was told the twin 1.5s would not fit onto a LHD car because of the master cylinder being in the way, I made them fit. I hate to undo all my work. It required tilting the engine toward the front, machining the intake manifold 1/8 inch, and building an airbox for the driver side air filter. A royal pain, but a fun project to fit. I keep going back to the cam being set up for more power above 3500rpm. I don't know the answer, but I keep reading stuff and a new idea will come up that is worth trying.

I believe the twin 1.5 's are right for your setup depending on the head you are using.

I would look at "Ask Doddy". AC Dodd can tell you what needles to start with and what profile he would recommend. He has a very extensive book on profiles used for tuning.

I know from doing this works effectively. Once profiled correctly the transition will be very smooth.

If you want further info contact me thru email.

Regards       

Joe    

Hi Jerry,

I wonder if twin 1 1/2 carbs is just too much carburetor for your engine, and if you changed them for twin HS2s the higher airflow velocity at partial throttle openings should improve things below 3000RPM.

Norm