How the SU Carburettor works - Part 3

[@Breezey1983]

The more you watch the more you understand that little SU is a much more complex and clever bit of kit than first thought……..and the hours you must have spent fettling and playing with them over the years is probably immeasurable lol. Looking forward to the next part, if not before the new year Merry Christmas AC 👍🏻

[@ACDodd]

That’s the point most overlook it’s apparent simplicity.

[@amidzlad]

Brilliant explanation A.C, Very informative as always.. Thank you 👍

[@somebloke5565]

Thanks AC. Love your stuff, no 'kin waffle - proper info. Guessing that explains why (0.100 needles anyway) all seem broadly the same at needle's thicker end, stations 1-4.

[@ACDodd]

Well they are different by the difference is in the tenths of thou range.

[@CheekyOllie]

Brilliant... Thanks AC!

[@ACDodd]

My pleasure!

[@anthonybezzina2638]

after watching this I realised I knew nothing about how the su carby really worked and how sensitive it was. One question I have why are some needles screwed in and flexible and some are hard fixed? I set up my triples with the jets set like you showed in a previous video on a motor like you said to do which will just get it started. However when I started the motor with the butterflies closed it strangely revved up high. I can't work it out any ideas?

[@ACDodd]

The butterfly’s have not been centred correctly is the usual problem, when you close the throttle the discs are not sealing the airflow off properly

[@robertf7015]

Great explanation, really enjoyed that thank you. Quick question, how accurate is the jet sizing with relation to production? Does this need to be considered with relation to needle adjustment?

[@ACDodd]

Yes I does vary but only in the tenths of thou range and yes it’s one of the reason that there are wide variations in fueling curves from carb to carb

[@keithoftheshire2805]

S U perb.......thanks for that 😎

[@ACDodd]

Glad you like it!

[@johnsharonwhitaker9242]

thanks AC. I am beginning to get a grasp on the complexities of SU tuning (HS4s on a '67 BGT here). How about a recording wide band gauge that could plot lambda against throttle position as you drove the car through a range of engine speeds and loads. Could analysis tell you whether you should change or modify the needle ? John in Manitoba, a long way from you.

[@ACDodd]

You could do that but a lot of people like to try and get way to much data, and that’s an example, you don’t need to do that in reality.

[@sjc75vs]

When you mention different positions of needle for different RPM, i assume you mean different airflows as for example a large throttle angle at 4k may be the same airflow as a smaller angle at 6k but both will raise the damper the same amount as its airflow lifting it versus the spring? And hence you can only pick one AFR for both of those situations even though in reality it might be better to be a little richer at WOT at 4k than at 2/3rd throttle at 6k as one is acceleration and the other is maintaining a constant speed on fast cruise?

[@ACDodd]

No the reality is there is no overlap on needle stations.

[@sjc75vs]

⁠@@ACDodd Surely if the airflow is the same at two different combinations of throttle/rpm then the damper would be lifted the same amount so how can there not be? If for example you are making 60bhp in both those situations then that would be the same CFM and hence the same lift? Or am I missing something?

[@ACDodd]

@@sjc75vsyou are indeed missing something, air flow is airflow, if the engine is producing 60hp it won’t do that at different throttle openings and different rpms. If it does that at a lower rpm its airflow will be the same at the lower rpm, and the engine will produce more torque because it’s doing it at a lower rpm. The throttle plate simply restricts the airflow so in your example at a higher rpm with a more restricted throttle setting the piston will be flowing the same air at the higher rpm, so the carb measures airflow, which is what makes it so good. 6000rpm on part throttle the piston is not in the same position as it would be if you were using half throttle for example. Don’t confuse rpm with needle positions. The air demand is what determines where the needle is.

[@sjc75vs]

@ thanks for the phone call earlier. Far easier than typing. To clear up for anyone else reading the comments. I meant for example a 80bhp peak bhp engine would make 60bhp at 4000rpm at full throttle and also make 60bhp at 6000rpm at 2/3rds throttle. And hence for those specific conditions both would have “60bhp of airflow” at that point.

[@stuwhite2337]

Brilliant explanation, thanks. How challenging is it with twin carbs to modify the needles by the same amount?

[@ACDodd]

It’s a challenge, and for that reason, in teach people to tune single carbs first, it they can then move onto twins once they have mastered the art of needle modification.

[@RussEdgar445y7tlfj]

...Do I now have a claim to fame as I was sat opposite Don at Burlen when he wrote the advice section 'Choosing a different needle' for the Profile Chart?. Unlike me , he's a very clever chap!. 🙂

[@ACDodd]

Hehehehe maybe!

[@ACDodd]

Although it now could do with updateing to include the need to use measuring equipment to track your progress and the dangers of engine damage due to over and under fueling

[@RussEdgar445y7tlfj]

@@ACDodd He wrote the guide back in about 1991/2. We'd been proof reading and using Burlen's first edition S.U catalogue before the profile booklet was printed.

[@RussEdgar445y7tlfj]

Nb. If Stan is still there, he'll remember as he was our manager.

[@RussEdgar445y7tlfj]

@@ACDodd I'm assuming no changes have been made to the original advice which was based as you've mentioned on mathematical area changes and not on flow data / rolling road. So purely a guide. You've gone to the next level with practical application. A lot has changed over the years, in particular the fuel. In 1986, the lead content in UK petrol was reduced by 30% and isobutane added. This certainly caused a few issues with pre-ignition, fuel vaporisation on many vehicles.