13 replies to this topic

[nath]

do i need to change the jets on these carbs,they are 1.1/4 su carbs that were fitted to a 1275 and the engine there on is a 998,does any one know how to set them up the car runs ok but there is a flat spot and the exhaust fumes smell a bit rich...thanks for a ny help

[Dan]

You don't change the jets with SU carbs, you change the metering needles. Yes it's almost certain you will need new needles and a proper setup. If the carbs are currently built for a 1275 they most likely have M needles which will be too rich for most 998s. You should try GY needles as a starting point.

[nath]

You don't change the jets with SU carbs, you change the metering needles. Yes it's almost certain you will need new needles and a proper setup. If the carbs are currently built for a 1275 they most likely have M needles which will be too rich for most 998s. You should try GY needles as a starting point.

thanks very much,can i use the needles that in 998 carbs or are they different,and also how do i know if they are m/gy needles are they marked with them letters sorry if i sound thick but im only 16 and learning

[Dan]

The needles used in a single carb car won't be the same spec so even in a single carb 998 fitted with an HS2 (if you can find one) you'd be unlikely to find a GY, it would use a GX or an AAV. The needles are marked at the top.

[jaydee]

The M was used in the 1275 coopers, it is too rich, especially at the top end revs.
You have to match the carbs with a good breathing head (like a 295) and a good manifold (like an lcb or a cooper freeflow) to notice any performace improvement, otherwise they will only drink fuel.

Edited by jaydee, 13 October 2009 - 11:55 AM.

[Dan]

That simply isn't true. You are right that new carbs aren't going to make the engine perform any better, not noticably anyway but they won't have a serious impact on fuel consumption either. Twin carbs are not more thirsty than a single and if setup right will actually make the engine more fuel efficient.

[nath]

That simply isn't true. You are right that new carbs aren't going to make the engine perform any better, not noticably anyway but they won't have a serious impact on fuel consumption either. Twin carbs are not more thirsty than a single and if setup right will actually make the engine more fuel efficient.

ok thanks can you tell me were i can some needles from,thanks

[Dan]

Mini specialists, carburettor specialists, Burlen Fuel Systems, tuning specialists. They aren't hard to find really.

[nath]

Mini specialists, carburettor specialists, Burlen Fuel Systems, tuning specialists. They aren't hard to find really.

thanks very much dan ..ill some phone calls in the morrning..i take it once ive done the needles it should run a bit better??

[Dan]

Should do but it will still need some setting up.

[bmcecosse]

If you are fitting twin 1.25" carbs in place of a single 1.5" carb - you may actually LOSE power!

[nath]

right ive taken the needles out of the carbs and they are marked up AN not GX or AAV any differents

[Dan]

I wouldn't have thought they would have been very nice on the 1275 they came from with AN needles. Those needles are used with twins on a 970 Cooper S or in single carb 850s, I can see them being too rich for twins on a 998 higher up the rev range. I still think you should start with GY needles as a starting point and take it to a rolling road.

[Whatty]

I wouldn't have thought they would have been very nice on the 1275 they came from with AN needles. Those needles are used with twins on a 970 Cooper S or in single carb 850s, I can see them being too rich for twins on a 998 higher up the rev range. I still think you should start with GY needles as a starting point and take it to a rolling road.

I just switched to twin HS2's on my 998 and AC Dodd recommended GY needles - suffice to say they work well.