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Tuning SU's
https://www.ausmini.com/forums/viewtopic.php?f=2&t=71927		 Page 1 of 1
Author:  Tadhg [ Mon Jul 18, 2011 11:57 pm ]
Post subject:  Tuning SU's
G'day all. Over the weekend I changed the needles in my twin HS2's. I went to re-tune them, winding the mixture screw up to the housing (full lean for the needle), then winding it out 12 flats. But I discovered that my existing needles were at 18 flats, and I'd leaned them out compared to where they had previously been. Those needles were modified and were running too rich at low openings. I've put in unmodified needles (both modified and unmodified are 'M'), so they need to run richer than the old ones.

So, with this in mind, I've two questions. Firstly, how many flats richer can you turn an HS2's adjustment screw before you cause problems?

Secondly, I couldn't see anything on the thread size/pitch, so how far does each full turn (6 flats) move the Needle..? Just wondering, because I'm considering putting my car on the dyno for tuning, but I figure I'll probably need to come up with the needle size. But if I'm not running the known needle in the 'default' position, then it's hard to know which needle to get.

Any and all info would be appreciated! :)
Author:  TheMiniMan [ Tue Jul 19, 2011 8:04 am ]
Post subject: 
tuning SU carbs (or rather the starting point) is to have the engine at least 'running" enough to be able to get some warmth into the thing, it really does need to be at "Normal Running Temp" to achieve best results

however,,, that "Starting Point" that i mentioned is simply an "Idle" mixture thing

you can wind the nut up & down all you like , but while you`re doing it you may want to take note of the actual "Idle",,, the more rich (down) you go the thing will eventually (if you keep winding it down) drop in revs & then (if you keep winding it down even further) you`ll notice it surging & going Blup blup blup (like the choke is stuck on),,,, this point is "WAY" too rich

alternatively,,, if you wind it lean (UP) then you will also notice the revs drop at some stage also,,, but with winding it too far lean (up) then the revs will generally drop right off & the car will stall

so theres a fairly wide band of use going rich (Down) but there`s a smaller band going lean (up)

so,,, if you find the "Happy" spot in the middle , where the revs (ON IDLE) are the highest & smoothest,,, one would then assume that`s spot on right???

well,,, no actually that would still be a tad lean

the mixture should be set slightly richer that this "Happy spot (called "stoichiometric")

stoichiometric is a term given to the "Theoreticaly correct mixture,,, which in fact is a tad lean for "practical" use

so a slightly richer place than stoichiometric is where we want to be , on idle, at running temp

so,,, find the happy spot,,, & turn the nut down a bit further,,, just until you notice the slightest tiny weeny little (barely noticable) drop in the idles speed,,, that will do you for now

ok?

However,,, once you found that spot,,, then go for a drive,,, because now it`s time to see if the mixture needle taper is correct to suit your engine,,, remember that idle mixture with just about any needle is able to be achieved,,, but the rest of the taper on the needle is only able to be checked by going for a drive (or dyno-tuning it with a gas Analiser)

Be aware that the carby piston rise-rate can "Trick" a lot of people whilst attempting to tune,,, so there`s more to the carb than just the needle,,, the rise rate (& fall rate) of the carby piston needs to suit "Your" particular engine too

hope that helps
Author:  Tadhg [ Tue Jul 19, 2011 11:52 am ]
Post subject: 
Thanks Matt - much appreciated. :)
I've basically set up the needles to start tuning them richer than where the old ones came out. I haven't started the car since I put them in - been a bit crook. :(

My problem is that I know the previous needle's, being modified at the thick end of the needle, were rich at that end. :? But I didn't realize that they were set up rich as it started - they were at 18 flats, and I'd leaned them off maybe 8 flats from where they had been previously. :shock: The top end needs to be at least where it was before, as it was starting to run too lean up there, and the bottom end probably should be richer than standard for the needle, which will still be leaner than the modified needle (I got the digital verniers on it, they had up to 0.002" off the diameter in a notch). I just don't want to turn the nut so far that it messes things up - things like the centring of the needle. :?

As you say, the profile of the needle could be out, hence my thoughts about getting it dyno'd and using the gas analyser. :) I'd have just gone that way straight off, but I figure that you need to know what you're starting with in order to have a hope of knowing what you need to get. And, given the current needle's not near the 'default' tuning position of 12 flats... The more info I have to pick the next needle, the better. :)

I think my carbies are right for piston rise rate, though. I'm running red springs and light machine oil. I've changed the needles because I had a miss at constant light throttle (read: 60km/h in 4th, throttle down between 1mm and 5mm), I'd fouled plugs, the engine was full of coke (even on the back of the inlet valves!), but the top end was leaning out. :roll: That's what moving 2000' in elevation after getting a tune will do, I guess. :lol:
Author:  Tadhg [ Tue Jul 19, 2011 9:13 pm ]
Post subject: 
I guess I should resolve the thread. :) I had the needles turned out to 32 flats without any issues. :o Well, none in terms of centring. I did have issues with masses of black smoke and fouled plugs. :oops: I ended up at 22 turns from tight/full lean. After I cleaned the plugs, it ran really, really nicely. :) So nicely that I had to turn down the idle screw, and it didn't miss (or ping) up hill in 4th at 60km/h. :shock: :D The only disappointment is that the engine's internals now likely have some of the coke that was recently cleaned out back in there. :roll:

So I'm all set. :) Well, until such time as I can organize a visit to a dyno. :twisted:
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