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| SU Dashpot Spring Length & Type https://www.ausmini.com/forums/viewtopic.php?f=2&t=77217 |
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| Author: | Rocco [ Sun Jun 17, 2012 9:31 pm ] |
| Post subject: | SU Dashpot Spring Length & Type |
Hi, I seem to have flat spots when turning corners or just generally driving. I was told that the springs in the carbies could be the cause of the problem. Anyway, took out the springs, the LH spring measured 147mm on the bench and the RH measured 132mm. I have a 1275cc with twin 1 1/4 inch SU carbs. I assume that these springs need to be the same length for proper synchronicity of the carbs. What should be the standard length, and what spring colour should i be using? Thanks Roc |
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| Author: | PDJ [ Sun Jun 17, 2012 10:20 pm ] |
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Even though the springs are different lengths its how they compress that counts. Try compressing them with the same weight and measure the compression. Even then there are other factors to consider, dashpot condition, dashpot plunger, oil, etc. It’s possible for each carb to be set up differently to give the same result. The only way to know for sure is to get them set up properly. That or swap for a single 1 3/4". 
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| Author: | drmini in aust [ Sun Jun 17, 2012 10:58 pm ] |
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Running twins, I would buy 2 new springs.
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| Author: | 66S [ Mon Jun 18, 2012 8:44 am ] |
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Good advice Doctor, they are not expensive. Al |
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| Author: | Paddy [ Mon Jun 18, 2012 9:54 am ] |
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Where can u get the springs from? |
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| Author: | drmini in aust [ Mon Jun 18, 2012 9:57 am ] |
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SU Midel https://sumidel.site-trust.net/index.html Karcraft www.karcraft.com.au Carburettor Service Co http://www.carburettorservice.com.au/ Minisport SA http://minisport.com.au/category80_3.htm |
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| Author: | Tadhg [ Mon Jun 18, 2012 11:12 pm ] |
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I bought two yellow springs for my twin HS2's last week. Not yet sure if I gott he result I want - too early to call. Midel are not cheap (think $40-ish for two springs + post - that's what I paid  - post is a flat rate of $15, springs were around $13-$14 each), Karcraft and many of the other regular Mini suppliers will save you significant money.
Standard spring colour is red. Most cars - almost all cars - will do fine with that.  My red springs were about 115mm uncompressed, my yellows were about 125mm uncompressed. But as everyone else has said, that doesn't mean anything. If you can't see the paint on one end of the spring, then the odds are that they'd suit replacing.
That said, a 1275cc + twin HS2's can be almost anything. It could be anything from a torquey, economical little motor with 'Cooper' extractors and a standard head, or it could be a high revving screamer with balanced/lightened everything, race extractors and a race head (though the HS2's might not be the best choice with it - I know, my car's got race extractors, heavily worked head, and twin HS2's).  It might be worth putting up a more thorough description, or even downloading WinSU and looking at what it suggests. When you say flat spots through corners and generally driving, is it mostly at constant throttle, when you press the accelerator, under acceleration..? It could be springs, but they're not the only thing to check. Is there oil in the dashpots? If you can provide more info, that type of info, then people much more knowledgeable than myself should be able to give you more and better answers.
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| Author: | drmini in aust [ Tue Jun 19, 2012 8:58 am ] |
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I reckon yellow springs are way too strong for an HS2, or HS4 even. They are supposed to work in an HS6, but I fit reds.. works for me.
If you look at Vizard he suggests reds in twin HS2s, unless it's a small motor (850 or 998) then he says use blue. |
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| Author: | Tadhg [ Sat Sep 08, 2012 12:20 am ] |
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drmini in aust wrote: I reckon yellow springs are way too strong for an HS2, or HS4 even.
If you look at Vizard he suggests reds in twin HS2s, unless it's a small motor (850 or 998) then he says use blue. Thread revival! Well, after having the Mini off the road for about a month from when I last posted, then testing, I'm very happy with my new yellow springs. It's reduced (though not completely eliminated) my longstanding issues with missing at constant and very light throttle openings. I've also now got the best tune I've had since I movedl 2000' vertically and ruined the excellent tune I had done in Ipswich (plugs look beautiful - white electrodes, enough carbon on the top ring/landing, burnishing finishes 2.5 threads down the plug, transition in colour on the ground strap right on the corner - I'd been doing plug reading in the week leading up to that recent thread). It does like a lot of choke on a cold morning (needs it to start, and it gasps at lower-than-ideal (high idle) revs before it's warm), but get up to temp and it's beautiful. 
I did see that Vizard doesn't recommend Yellow springs in HS2's. However, he also doesn't recommend HS2's with heavily worked heads, hot cams and full race headers. On my engine, I'm sure he'd recommend HS6/twin HS4/twin HS6/DCOE45/etc. I know I could/should do that, but I don't rev my engine, and don't see the point of moving the powerband up. I prefer the torque.
So no yellow springs in HS2's for Vizard. But Vizard does show is a pattern.  Light springs for little engines that don't breathe hard, standard springs for bigger engines. So the progression would be that heavy-breathing engines (arguably like mine - even if mine's not the most powerful engine on here - that's not something I'd ever claim) either need bigger carbs, or heavier springs. Heavier springs to force higher airspeeds (and smaller venturi openings) for the the same rpm (compared to a lighter spring), in order to avoid a change in airspeed at higher rpms.
A change in airspeed is the enemy of all constant depression/vacuum/velocity carburettors. I've seen it written everywhere that you don't want the piston reaching the top of its travel until 500rpm before peak power. If you have a big engine breathing through a little opening with weak springs, it's going to compress those springs and reach full opening at lower-than-ideal revs. 
So, if Rocco's still round... Let us know how you've gone, and/or the spec of your complete engine. As I said before, it could be a torquey little stock engine, or it could be a fire-breather. And that will affect what you need to have in your HS2's. There's plenty of knowledgeable people on here (like DrMini), I'm sure they'd love to help you.
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