Quote:
Originally Posted by Ron Kwas
AS; "the richer needles" implies there are a limited number available...in fact, I have no less than 3 8 1/2" X 11" pages full of dimensional data for metering needles for 0.100" Jets, the sizes of which are given in ten thousands of an inch accuracy (!)...determining which is "the one" for your engine configuration takes some engineering, and trial and error, accompanied by solid research and practical load tests, not just a quick look at the list...
Forum; Dashpot oil is indeed crucially important to be present, but viscosity 20W +/- is just not that critical for anything but where its necessary to get the absolute best out of the engine (and this applies only to transitions ie throttle response!), consequently a lot of people play with viscosity and swear they get a noticeable result...I have to question that...now if I had a lightened Flywheel and Intake and Exhaust flow which allowed a much faster throttle response, I might be tempted to try varying the viscosity (if I had a dyne or a lot of track time to check it).
Cheers
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Thank you for your informative post; the last time I played with SU needles was the summer of '68
Access to the data to which you refer was rarely available in those times, most BMC/Leyland dealers only carried standard needles, indeed even locating richer/weaker needles was a question of "ringing around" the county (Kent, Surrey, & Sussex at the time) to find alternatives. Those I found came from a now defunct small engineering/racing engine builder some 60 miles away.
Much of what was achieved was by trial and error and I assure you there were many errors; in stage 3 tune it was running a tuftrided racing cam (Piper's seemed less prone to wear than Janspeed's at the time), triple valve springs, coupled with a Lotus derived duplex timing chain with the associated gear mods, tuftrided fully balanced crank & lightened flywheel, plus a competition clutch - the engine was redlined @ 10,200rpm on 5* fuel meaning only one gear change was necessary to hit 60mph in a shade under 6.1 seconds - not bad on the 10inch wheels - it literally was a very hot little motor and would blow away most sports cars from a standing start, leaving many an Elan in its wake. Various heads were used over time, the best coming from a frogeye sprite which had a little more metal to play with when it came to skimming & smoothing again compression ratios of 10.6 to 13.1 were used over time - the best set up was achieved using 12.3:1, anything more than that just didn't work as road car + five star fuel aka the old leaded 101 octane became increasingly difficult to find at the roadside hence the sale of that wonderful engine in the summer of '75.
The twin 1.5" SUs on "normal" needles and Duckhams Q 20/50 made for a much more tame drive, but still enabled a very fast take off without the thirst of the Webers.
The point being that the SUs were designed for an SAE20 oil.