Mini Mad
Posted 21 January 2024 - 03:35 PM
So, we've just bought a 1990 RSP to which the previous owner has fitted brand new twin SU carbs.
It starts fine with minimal choke and once warm will tick over fine but when pulling away, there's a bit of flat spot and then once its *cleared it's throat* it seems to be fine. I've topped up the dashpots but that doesn't seem to have made much difference.
Could it be a weak mixture or some other adjustment that may be required.
Any help would be much appreciated.
Up Into Fourth
Posted 21 January 2024 - 05:15 PM
Learner Driver
Posted 21 January 2024 - 06:37 PM
It could be that your fuel mixture isn't optimal across the rev range or under different load conditions, and there are a number of possible contributing factors; you may find some improvement by experimenting with small tweaks if the base idle mixture is incorrect or by trying a different grade of dashpot oil (the oil requirement varies from carb to carb even with brand new units).
However if your carbs' needle profiles are unsuitable for the engine's fuelling requirements, or other issues are also present i.e. the carbs are out of balance or your ignition system isn't 100%, simply adjusting the base idle mixture won't work as a catch-all solution; you could end up solving one problem (the flat spot) but then cause another potentially worse problem, by introducing too rich a mixture at idle or part throttle.
AC Dodd has loads of useful and very informative videos on tuning the A-Series engine that may be worth a watch: https://www.youtube.com/@ACDodd , I'd recommend checking these out and using the info to check all the basics on your car to eliminate any obvious problems before fiddling too heavily with carb settings. AC also offers mobile tuning services which I highly recommend if you're struggling to get your car running perfectly.
Mini Mad
Posted 21 January 2024 - 08:56 PM
It could be that your fuel mixture isn't optimal across the rev range or under different load conditions, and there are a number of possible contributing factors; you may find some improvement by experimenting with small tweaks if the base idle mixture is incorrect or by trying a different grade of dashpot oil (the oil requirement varies from carb to carb even with brand new units).
However if your carbs' needle profiles are unsuitable for the engine's fuelling requirements, or other issues are also present i.e. the carbs are out of balance or your ignition system isn't 100%, simply adjusting the base idle mixture won't work as a catch-all solution; you could end up solving one problem (the flat spot) but then cause another potentially worse problem, by introducing too rich a mixture at idle or part throttle.
AC Dodd has loads of useful and very informative videos on tuning the A-Series engine that may be worth a watch: https://www.youtube.com/@ACDodd , I'd recommend checking these out and using the info to check all the basics on your car to eliminate any obvious problems before fiddling too heavily with carb settings. AC also offers mobile tuning services which I highly recommend if you're struggling to get your car running perfectly.
Thanks for the advice. I'll have a look at all the information you mentioned.
Mini Mad
Posted 21 January 2024 - 08:57 PM
Have you checked they are working in sync?
https://youtu.be/R1n...zJalbcqPlaMuGjD
Good source for information.. not exactly the same.. but principals are the same..
Cheers mate; I'll have a look at the video.
Mini Mad
Posted 21 January 2024 - 09:35 PM
Edited by ThermalEvent, 21 January 2024 - 09:38 PM.
Mini Mad
Posted 26 January 2024 - 05:29 PM
I have twin HS2's on a mildly tuned 1330cc and I've set my carbs as follows:
QA needles (I'm guessing you might have AH2's which are slightly leaner but fitted to the 1275 Cooper S's). AH2's should be fine if your engine is standard.
Blue springs
Jets 1.20mm down from the bridge (measured using digital calipers after dyno tuning)
ATF oil in dash pots (used SU oil to begin with but caused slight initial hesitation when pulling away)
It was dyno'd and was slightly rich in places but better than the AH2 Needes that were too lean. Short of custom needles it was the closest we could get.
The above settings should get you close I reckon.
Edit: I made the assumption they are HS2's, apologies if not.
How will I know what needles are in mine? I know my way around the spanners but I get a bit out of my depth when it gets a bit more technical. Also, when you say jets 1.2 mm down from the bridge; what am I looking for with this bridge? Sorry for all the questions.
One Carb Or Two?
Posted 26 January 2024 - 07:36 PM
The needle designation is stamped on the thick part of the needle that is retained in the bottom of the carb piston.
You haven't said what size the carbs are, so it isn't possible to make any sensible recommendations for needle and the needles can be swinging or fixed, depending on the carb type. If the needles are swing type take care removing the needle not to lose the bias sleeve and bias spring.
The bridge is the raised flat section across the base of the carb base which contains the movable jet. The 1.2mm refers to the depth the top of the jet should be below the raised flat section to give a base setting before trying to tune the idle mixture.
Mini Mad
Posted 26 January 2024 - 10:20 PM
The needle designation is stamped on the thick part of the needle that is retained in the bottom of the carb piston.
You haven't said what size the carbs are, so it isn't possible to make any sensible recommendations for needle and the needles can be swinging or fixed, depending on the carb type. If the needles are swing type take care removing the needle not to lose the bias sleeve and bias spring.
The bridge is the raised flat section across the base of the carb base which contains the movable jet. The 1.2mm refers to the depth the top of the jet should be below the raised flat section to give a base setting before trying to tune the idle mixture.
Hi, they are 1.25 carbs
Up Into Fourth
Posted 27 January 2024 - 09:38 AM
Mini Mad
Posted 27 January 2024 - 09:55 AM
Just checked the needles and they are AAC'S. These must be the needles fitted from new as the carbs are brand new and were fitted before I bought the car.
Are these the correct needles?
Mini Mad
Posted 27 January 2024 - 11:08 AM
Edited by ThermalEvent, 27 January 2024 - 11:16 AM.
One Carb Or Two?
Posted 27 January 2024 - 12:40 PM
Just checked the needles and they are AAC'S. These must be the needles fitted from new as the carbs are brand new and were fitted before I bought the car.
Are these the correct needles?
As the RSP is effectively an MG metro engine, which was originally fitted with a single HIF44 there is no standard needle for a twin 1.25" SU setup. The AAC is a bias needle so I was surprised to find that HS2 carbs can be had with bias needles but there we go. The M/AH2 needles used in the 1275 cooper S are fixed type and would need to be modified for use in a bias set up. Modifying them isn't hard but not for the absolute beginner. There are some 80 needles from AAA to AEC in the series of needles which will directly fit. The mintylamb needle comparison programme allows comparison with fixed and biased needles and trawling through them might let you find a needle which gives a bit more fuel in the low to middle throttle range, compaared to the AAC but it will possibly be an expensive saga to find the best. A rolling road test session might help.
Mini Mad
Posted 27 January 2024 - 03:53 PM
Just checked the needles and they are AAC'S. These must be the needles fitted from new as the carbs are brand new and were fitted before I bought the car.
Are these the correct needles?
As the RSP is effectively an MG metro engine, which was originally fitted with a single HIF44 there is no standard needle for a twin 1.25" SU setup. The AAC is a bias needle so I was surprised to find that HS2 carbs can be had with bias needles but there we go. The M/AH2 needles used in the 1275 cooper S are fixed type and would need to be modified for use in a bias set up. Modifying them isn't hard but not for the absolute beginner. There are some 80 needles from AAA to AEC in the series of needles which will directly fit. The mintylamb needle comparison programme allows comparison with fixed and biased needles and trawling through them might let you find a needle which gives a bit more fuel in the low to middle throttle range, compaared to the AAC but it will possibly be an expensive saga to find the best. A rolling road test session might help.
I have lost the will to live with it to be honest so I think I am going to try and contact AC Dodd; anyone know how I can do this? Does he have a website or what's the best way to get hold of him. The other alternative would be to revert back to the original carburettor that was originally fitted to the car. We are going to the Mini fair at Bingley tomorrow so I might see if I can do some more research there on the best way forward. As I said earlier, I am a bit out of my depth with this sort of thing so will have to call in an expert to sort it out.
Mini Mad
Posted 31 January 2024 - 05:35 PM
So, I had a local tuning guy look at the car today and he has set up the carburettors which are now running a lot smoother but there is an issue with the advance on the distributor so I am going to replace that and he will come and set it up for us so, after that, we should be sorted; fingers crossed.
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