Talk Morgan Forums Morgan Sports Cars Maintenance & Servicing tuning SU HS6 carbs in a '67 +4?

I am having a rough time tuning the carburators in my Morgan +4. I have idle going and am trying to adjust the mixture at each carburator. According to all the books you should be able to turn the jet nuts one way or the other and get an increase in speed or a falling off when you lift the needle. In the adjustment you are to take it right to the point where there is a 50 rpm increase and then a dropping off. I adjust the damn things, and there is never any increase in speed. It is very frustrating. Does anyone have any ideas what's going on?

It takes a little time to master the tuning of twin carbs. Screwing the jet upwards weekend the mixture, downwards to richen the mixture. Assuming your needles are correct I would wind both jets up to the weakest setting,then wind down 9 flats. Then balance the air intake and start from there. The speed increase comes when you raise the piston with the side pin. Best of luck.

Buy this Colourtune set.
http://www.carbtune.com/colortune.html

I agree with the previous screw up and back 9 flats. When I used to balance my Cooper SU's many years ago I found the best way was to put a tube to the intake and listen to the hiss and then compare with the other and then adjust the idle until they were the same. If you have not replaced the jets and needles it may be advisable to do so first as they will be quite wheel worn by now.

thanks for all the input. I have read about the unscrewing the nut to begin. In a TR4A book, and an SU Carburator manual I read two different numbers. One said to unscrew it two rotations (12 flats) and the other was two and half flats (15 flats). I have not tried the 9 flats, but probably got close. I will try again this weekend with the 9 flats.
That colourtune setup looks like it could assist in fine tuning.
Best regards

I used colourtune on my 68 Midget

Didn't he object???

Seriously though, a useful bit of kit, mine seemed to vanish during the last house move

Still got mine but the cars I own are all injected now.

Vitruvius (good book on civils by the way) I have the British Leyland w/shop manual from my Midget using HS2 SUs, happy to send you a pdf if you pm me your e mail address

The tuning algorithm above is correct but is predicated on the carbs being in good condition with no leaks around the throttle shafts or excessive wear on the main needle or jet. If the SU's are worn you wont be able to clock them in no matter what.

Ha ha Graham. A colortune on a midget?
I did find a very nicely concise description of tuning on a midget website..(MG). http://www.mgcarclubdc.com/techtips/sucarbs.html
I'm still failing to get the acceleration up when I adjust the mixture. It runs well until 4500 rpm and it stutters... Perhaps a distributor overhaul and carb tune up is due this winter for the Dutchess.