5 replies to this topic

[rolsevo]

Hi,
I seem to be having problems setting up a standard 1275 engine with twin 1.25" su's. The needles in at the moment are M needles but the car seems to keep sooting the plugs up. Am I correct in thinking that 12 flats of the nut down on each carb are where to start from? Bascially I'm screwing the nut up to weaken the mixture off until the engine revs increses slightly and then go back to normal idle when the pins on each carb are pushed up. The carbs are also balanced correctly.. If I'm doing everything correct to you thing I need a different needle such as the AN or Weaker GG ?

I have a Weber DCOE 45 to put on but as its too cold outside to start messing around cutting bulkhead etc I'd rather get the twin su's running first.

Any help appreciated... Also any thoughts on the 1.5 rockers of are they a waste of time and I should go for a cam and a ported head?

Cheers Roland.

[Sherlock]

You are correct with using 12 flats from the starting point, some people say more, but according to the SU manual in front of me, it's 12. If your plugs are sooty, there is no doubt the mixture is rich, I can see by what you have written you are doing this 'by the book' but you could weaken the mixtures a little more so when you lift the pins (only approx 1/32") it slows down immediately, to see if this makes any difference to your plugs. I think AN needles are probably what you need to solve the problem properly, M are probably a bit rich for a standard 1275.
Personal opinion, a cam & head are better than 1.5 rockers, but obviously a lot more work & expense to fit. The main thing I have against high ratio rockers is the extra loading they impose on the valve train, but as I say, it's a personal opinion.

[rolsevo]

thanks for the reply. I've already tried weakening it of but seem to loose top end power and it starts jerking when sitting around 70. i'll get some AN needles today. thanks roland.

[Dan]

Just to clarify, the initial position is 12 flats down from the jet being flush with the bridge. Not necessarily from being fully wound in. And the lift pin should cause the engine to hesitate rather than pick up. You seem to understand basic SU tuning, so I assume you know about the plateau method. Aim for two flats from the lean end of the plateau. If it really is rich and the range is wrong then try a new needle. Maybe AN, maybe EB but I'm not very experienced with needle selection personally so go with AN for now.

[Sherlock]

As a rough guide, AN were the standard needles fitted to the HS2 carbs on the 1275 Austin Healey Sprite / MG Midget. That engine is very similar to the standard 1275 so AN should be suitable, I used them when my Cooper 'S' ran for a short time with a 1275 GT engine with twin HS2's.

[bmcecosse]

Agree with all above - and yes - throw the daft 1.5 rockers away! Lots of extra loading - and 'maybe' about 1 bhp extra at the top end.