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SU Problems
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Author:  Anto [ Mon Mar 22, 2004 7:58 am ]
Post subject:  SU Problems
G'day Everyone,

I'm hoping to get some opinions as to what is causing the flatspots that I am suffering from at the moment. I have just installed a HS4 carb in place of my old HS2, running a stub stack / sock arrangement and a straight through exhaust system. I am suffering from flatspots on acceleration low in the rev range, however full throttle acceleration is quite good (noticeably faster than my HS2). From what I can tell the car is leaning out momentarily, which leads me to think that either the oil in the dashpot is too thin or maybe a weak spring. I was initially running 3in1 in the dashpot, which I used in the HS2 quite successfully, but changing to 20w50 didn't fix it. And I have tried various different spring arrangements and that doesn't make much difference either.
Does anybody have any ideas?

Cheers,
Anto.
Author:  MiNi MuLiShA [ Mon Mar 22, 2004 9:02 am ]
Post subject: 
I'm not much help here anton... only thought is perhaps weights/vac advance in the dizzy... but if the carbs the only thing you've played with, it must be that.... I was always told to use 10 weight/grade oil in dashpots... eg: sewing machine oit... ATF (auto trans fluid or power steering fluid- same stuff).... I once had a set of twin SU's that I couldnt get right... found thay had diffrent colored springs (diff color, diff spring rate/tension) replaced them, and a few bits, and as they say.. problem solvered.


As for a single carb... no Idea.. Dr Mini Aus will know for sure.... :lol:

-Cam 8)
Author:  Mike [ Tue Mar 23, 2004 12:44 am ]
Post subject: 
The sewing oil thing comes out of colder places such as Europe where a thinner oil is required. In Australia where the temperatures are never really low (lets face it even if you live in the rain capital - Melbourne) you should just be using normal light motor oil (20/50).
Author:  MiNi MuLiShA [ Tue Mar 23, 2004 10:05 am ]
Post subject: 
Ahh ok, my bad...sorry lads.... and sorry ANTo... for calling u Anton.. lol didnt notice :roll:

-Cam 8)
Author:  drmini in aust [ Tue Mar 23, 2004 11:24 am ]
Post subject: 
Sounds like your needle is too lean in the midrange.. maybe..

I've got the same prob in the 1132 Clubbie, I just make it richer to disguise it..

When I fit the Dellorto 40DHLA I won't have the problem! :P
Author:  1380 yellow devil [ Tue Mar 23, 2004 10:01 pm ]
Post subject: 
Well I use a stright 30 grade oil in mine and yes it sound like it is to lean.
Author:  PhildoD [ Tue Mar 23, 2004 11:45 pm ]
Post subject: 
hey guys, I just had the 80 000 k service done on my clubby today. (27 years late, i'm lazy). I also had a flat spot in my carby (very similar condition). but not knowing a thing about em, wasnt overly worried.

The mechanic replaced the points and condensor for me, and tuned the carb. I was amazed at how much better it was after having a professional look at it. Now a 1.25 on a 1275 is too small, I know, but down low, it doesnt miss a beat anymore. He also had a look at the twins i have, and seems to think that they'll be fine to bolt on (BIG GRIN).

Anyway, this might not be much help, but I just thought i'd add it, since I had the same problem.
Author:  Anto [ Wed Mar 24, 2004 8:47 am ]
Post subject: 
Okay guys I believe I have found the problem, and you can laugh at me all you want for it, one of my less brainy moves :)
There was a slight vacuum leak where the carb attaches to the manifold. I didn't buy enough gaskets by accident so I thought maybe I could stoop low enough to use gasket goo, instead of making the trip down to buy another one. I must have been otherwise occupied when I put the goo on coz I used the wrong stuff (gasket assister not gasket maker) coz I couldn't find the gasket maker. Didn't think much of it at the time but of course it ain't gonna seal! When I was taking the carb off I realised immediately my stupid mistake and punched myself....
Also the throttle spindle is a bit worn so I'll replace that also (possibly explains crackling exhaust on deceleration).
So needles didn't fix the problem, although I could probably has concealed the problem with a rich midrange needle but that wouldn't have fixed it as such.

Cheers,
Anto.
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