drmini in aust wrote:
Forget yellow springs, they are too heavy for HS2 or HS4. Only any good in HS6.... maybe.
Blues are lighter than reds. Too light for a 1330 with HS2s.
I don't often disagree with the Doc, but...
Yellows are NOT too heavy for an HS2.
This goes against all conventional modern thinking, but is wholly in tune with old thinking. The determining factor for springs isn't the size of the carburettor, it's the size differential between the carb and engine. Vizard's suggestion was to use light springs with big carbs on small engines, and heavier springs on small carbs on big breathing engines. So, for a stock 850 running an HS4, he definitely wanted a blue needle. For a 998 with a flowed head, he wanted red.
I'll quote from page 8 of 'Tuning SU Carburetters':
It is best to use the Red spring but if the carburetters are suspected of being on the large size for an engine then the blue should be tried. Conversely if the instrument is only just large enough, a stronger yellow spring might be required.Italics are for emphasis.
I have a 1301 with a VERY flowed head, a warm road cam, oversized full race 3 into 1 headers (the join in the pipes is under the body), and twin HS2's. I'm running yellow springs. They work better than the Reds did. And it's most noticeable at part throttle, where I have more fine control than previously. The throttle is less of a switch between nothing and everything.
Everything I've read says you want the dashpots to be fully open 500rpm before peak torque. A weak spring - a blue spring - will allow the dashpots to rise faster than regular springs (red), and faster again than strong springs (yellow). If you have a shallow breathing engine - say, a stock 850 with a HS4 on it (yes, I know, a very unlikely combination), you just won't develop enough vacuum to fully compress the spring if you've got a yellow or even a red spring. You need the blue spring. That allows you to use the full depth of the needle. If, however, you have a deeply breathing engine with smaller carbs - say, a 1330 with a GR head and 'only' twin HS2's - then blue springs will provide far to little resistance. And red springs MAY also provide too little resistance - allowing the dashpots to be fully raised more than 500rpm before peak torque.
Your spring sets two things - one is the airspeed, the other is the timing of your fuel curve. Your fuel curve is set by the needle. If you have a heavier spring, then the dashpots will be lower - relative to weaker springs - which means you have a smaller aperture through which the engine can breathe. Which will therefore see an increase in airspeed to fill the engine. That increase in airspeed will also see an increase in the quantity of fuel dragged through the needle at a given station (higher airspeed = higher vacuum). It effectively richens the whole fueling curve. But the relative fuel curve of the needle remains the same, compared against itself. As in, the idle-end stations will still deliver proportionally less fuel than the wide open throttle stations, just the whole curve moves up the scale.
When I say the spring sets the timing of your fuel curve, I mean that it determines how much load it takes to get to full fueling. If you've got blue springs in with your engine, you'll have full fueling earlier than the engine wants it. Which means that you'll either run rich, or, more likely, you'll end up running lean at the top end, and fail to get the full potential of your engine. So, you could also describe the spring as setting the length of your fueling curve across your rev/load range - with a weak spring, all the fueling changes are in the first part of the rev/load chart, with too large a section left at max fuel. Too heavy a spring, the fuel curve extends beyond your engine's revs/load capacity (i.e. you might need to rev it to 9,000rpm at full load in order to drag max fueling - which is useless). So, having the right spring is pretty important!
Bear in mind, also, that ANY SU needle can be tuned at idle. And the standard carb tuning procedure for SU's doesn't determine correct needle selection, it only tunes the idle. It relies on you already having the correct needle selection. The same is true for a Colortune. Pretty well every SU needle for a given size carb is identical in station 1. I've only really looked in depth at 0.090" needles, but I've hardly seen something that doesn't have a first station size of anything other than 890
(looking at the best Needle comparison site, Mr Kerr's SU thingy). So, that means pretty well any needle - right or wrong at the top end, it's irrelevant - can be tuned at idle.
That tune doesn't indicate anything, you're better off reading the plugs (something I've had success doing - in spite of unleaded fuel).
The correct solution, with your engine and I believe also mine, would be not to find another spring, but to increase the carb size.
That's what WinSU recommends for me - twin HS4's, or a HS8. Can't imagine yours is any less - especially given it recommends the same needles for me! HS2's were correct for stock S's, but Police cars (which, from what I understand, didn't have worked heads) had HS4's. There are Minis on here running twin HS6's. With the quality of your engine, everything I see in it says the choke point in the engine is the carburetion. HS2's are on the small side for it, and a set of bigger carbs would certainly work. Definitely HS4's, maybe even HS6's (though they may then require blue springs!). Though I'm not in any hurry to spend the coin to swap up in carb size, and don't blame anyone else who feels the same way.
I wouldn't be afraid of trying the yellow springs WinSU suggests. That's why I went that way, and I've not regretted it. In fact, I'd go to the Yellow spring before I changed the needles...
AC Dodd recommends the order to set up is Damper, Spring, Needle, and Oil (last!). Most say twin HS2's are only good up to 75bhp, WinSU's estimates for power from our engines is over 80bhp. Whether that's right or not, it certainly shows that we're at the very top end - if not above the top end - of standard use for twin HS2's, which means we can use Yellow springs (going back to the comments from the SU Tuning guide above). My current needles - M's - are too rich according to WinSU, but with some tuning and the correct springs (because I used these M's with Red springs previously), I have no issues with fouling (which I had with the Red springs). It runs very nicely.
So we've got to stick with dark ages logic, rather than just getting the right carb size to run in the carb's sweet spot (with Red springs). 
Primarily because needles are generally $30/pair plus postage from most shops, and, without other options (other than M's) on hand myself, it was too much expense and effort at the time. 
Especially when my first move was to try the yellow springs, which did give a good result. 
My primary issue before going to Yellow springs was that I had fouling at part throttle. Before that, I was looking at needles. That said, if anyone has a set of ADN's they'd like to donate to a good cause... 
