mk1-forum.net

Hi Folks, before rolling road tuning what would be a good starting point needles for twin 1.25 SU on the following:
Standard 998 Cooper with SW5 cam
Standard 1275 Cooper S (with plus 40 rebore and SW5 cam)
Thanks a lot
Mark

good idea is to take a few needle with you when you go. where are you going?

ian will be able to help you more

I'd go with stock rich needles and do a couple of base line runs and go from there. Your not going to luck into the perfect needle from a consensus of forum members........

I would start with M needles, they will work well in both engines.

hahahaha, I have just found a tube full of new old stock needles. First 2 out of the hat were M needles.
Ho ho ho. Thanks a lot guys
Would you suggest blue or red type dashpot springs?
Do they generally wear out or are they pretty fail safe??
Cheer

Blue with the 8114 dampers

Dean wrote:ian will be able to help you more

... but:

minimans wrote:Your not going to luck into the perfect needle
from a consensus of forum members........

Random Carb Needles:

997cc Cooper - Standard Mixture - GZ - Red

998cc Cooper - Standard Mixture - GY - Blue
998cc Cooper - Rich Mixture - M - Blue

1275cc Cooper - Standard Mixture - M - Red
1275cc Cooper - Rich Mixture - AH2 - Red

The images below have been truncated at index 9 as this is
"Working Range" for the 1&1/4" carb.

The areas are in square thou...

Click on the images a couple of times to make them bigger,
press "Back" on your browser to return.
(Mark is so clever making it work like this...).

... Now BIGGER... If you want the spreadsheet that made these charts, go here:
http://mk1-performance-conversions.co.u ... l_carb.htm
The actual download file is dated 16th October 2015,
not as shown on the site...

Ian

For the twin HS2s these all used 8103 dampers as standard. I've not tried 8114's, do they make much difference?

With the dashpot springs I don't think they weaken much with age, but I have found that some of the recently supplied red springs are a lot heavier gauge wire than the originals, and also give higher loads under same compressed length.

So don't assume two red (or blue) springs will give same loads. They need to be matched, otherwise you will have real bother getting the carbs balanced and working to their best.

carbon wrote:For the twin HS2s these all used 8103 dampers as standard. I've not tried 8114's, do they make much difference?

They did on mine. I was running a 998 bored +40, block decked 20thou with Powermax flat tops machined to be flush with the block deck and a 295 head. The head was machined to give 11:1 compression (chambers machined so they were all the same) and all the ports were cleaned up. It had a 731 cam and an Aldon yellow distributor. Twin HS2 with E3 needles, 8114 dampers, blue springs with the inlet manifold cleaned up. I ran it with Triumph Spitfire air cleaners because:
a: they bolted straight on
b: I didn't have the Cooper air cleaner box and adaptors
c: they were cheaper than the Mini ones at my local parts shop and came with a gasket...

I set the piston fall time to be the same (this took ages) and made sure the fitted load and spring rate were the same.

At first it had 8103 dampers but the tip in pick up was not great so I swapped to the 8114's and that made it much better.

It was quite quick for a 998, it would go right off the end of the Cooper 105MPH speedo and into the 1/2 tank on the fuel gauge at 7000rpm on a 3.44 diff.

Given the inaccuracy of Smiths rev. counters and speedos I'd guess at around 100MPH 'real' speed on a flat road.

Sold the car and engine to a mate who wrote it off 2 weeks later by parking it in a farmers field on its roof....

Good info. Interesting to hear you matched the piston fall. I am trying to achieve the same. What did you do? Cheers. Mark

Unfortunately I think it was by a method that would expensive and impractical these days.
This was back in the early eighties, armed with a screwdriver and a cloth I went to my local scrap yard (run by a bloke who lived two doors away from me) and from his pile of 'to be weighed in' carburetors I removed the tops of all the HS2's I could find. The cloth was to wipe the piston rod to inspect for damage to the chrome. It was quite a task but having stripped out about 50 units I ended up with about twenty that were visually ok.

I made an MDF jig to hold the chamber inverted and after trying to do it with a stopwatch and getting results for one assembly that varied by about 20% I got my brother to build me an electronic timer. It used two micro switches one to open as the piston left the bottom of the damper and one close as the rod arrived. With this I was getting times varying less than 1% so I was happy.

Then I measured all the pistons in all the chambers, doing 5 timed runs for each, discarding the highest and the lowest reading and taking the average of the rest. Then I selected the two closest combinations. They were pretty close if I recall correctly so I used these.

I did try messing with the vent hole on the bottom of the piston on two of the assemblies that were the furthest apart, opening the hole on the slowest one by the use of a small tapered reamer. That got them exactly the same. I tried that pair on the car and could not tell any difference as far as I remember but having spent all the time finding a 'pair' I put those back on and that was how it ran until its untimely demise. Don't be tempted to change the time by modifying the piston outer or the 'bell' chamber, this makes it undrivable as one carburetor piston will rise much faster than the other (due to leakage around the piston) and this makes the fueling to one pair of cylinders very rich and the other very weak.

I did try increasing the hole size at the bottom to see what happened (did two 'pairs', one with two holes diametrically opposite same as original size and one with a single hole 5/16 diameter in the original place).

From memory I think it didn't make a huge difference to the feel on an acceleration from rest, however after changing gear the next acceleration was very poor. I put this down to the piston falling much more quickly and thus the fueling went weak during the time the throttle was closed and it then took a little while to get it back to where it needed to be. I can't for the life of me remember which was worse though....

One thing that makes a big difference in the feel is the setting of the needle and jet. I made a jig to push the needle into the piston so that it pushed on the step and seated against the bottom of the piston. This made sure the needle was always in the same relative position to the bottom of the piston. I also made (out of an old piston) a DTI clamp so I could measure the actual jet drop from the bridge. All this setting it flush and then counting the flats down seemed a bit hit and miss to me.

The other big influence is the damper and oil. Make sure you have two dampers the same and always use the same oil. I would never use 20/50 10/40 or other engine oil, it is too thick but some swear by multigrades. Quite why I have no idea because the temperature in a carburetor is not that high and the variation in temperature is small so the viscosity change due to temperature is very small. I used motorcycle fork oil 2.5 weight. It is very close to the SU oil (which is an SAE 20 grade). You can use ATF but it is thinner still. It is most important to use the same oil that you do your set up on.

The next unit I built had twin HS4's so I had to do it all again for those....