Carb throttle butterfly spindle worn

Recently my SC has developed a habit of idling too fast, if you allow the throttle to slam shut by slipping your foot off the pedal and letting the pedal spring back to the rest position it returns to normal idle but normal decelerating (without slamming it shut) into junctions etc leaves it running fast.
I've had a poke around and have noticed that the throttle butterfly valve spindle has worn into the carb body and you can rock the spindle if you hold onto it from both ends. What I wondered was, how much play in this is acceptable and could this be causing an air leak past the butterfly in turn causing the erratic idling?
I'm fairly sure the mixture and timing are as spot as I can get, it's a bit difficult setting it as everytime you release the throttle something different happens, but I've eliminated any pinking under load and the exhaust note, lift pin and temperature would suggest that the mixture is within acceptable limits too.
Thanks for any advice. :D

Tom W

Active Member
Have you checked the throttle cable isn't sticking? I had to replace the throttle cable on my TC as part of getting it road worthy (old one was snapped and the outer was shredded :? ) It's not quite right yet, and seems to stick slightly, resulting in a fast idle. Physically lifting the throttle pedal returns things to normal. I'm going to have another go before I put the car in for the MOT next week. I used a universal throttle kit and I don't think I've set it up correctly yet.
On the SC model it's a rod and linkage affair. I know what you mean though I've just recently sold a TC and it had one sticky cable on that when I first got it.
Food for thought though, I'll get the oil can out and give everything a good splurge tomorrow in the light.
Thanks for your reply.
I'm afraid if you can visually detect the movement in the spindle then it is too worn and has the potential to give the symptoms you describe. Only solution I know of is a carb away full overhaul. They will either redrill and sleeve the body or give you a new one. Outside my scope, I'd need a decent modelmakers machine shop to achieve that.

Thanks Chris,
I thought that may be the case but wasn't absolutely sure. I think I've found a replacement carb so I'll fit that if suitable and I'll redrill the spindle in the old one and sleeve it. The linkage from the pedal upwards is showing signs of wear too so I'll do the lot I think.
Thanks for your advice. :D
Interesting stuff! I didn't realise the spindle itself wore like that, I'm used to carbs being made of pot metal (japanese ones) and the bodies wearing before the spindle does, food for thought that is. I've had a look at it today and for the moment I've given the spindle a little nip up and it's eliminated some play, obviously not the solution but as it's my daily something had to be done to keep it going. Happy to say though that it's running much better now, still sucking air on the overrun (makes that spluttering in the exhaust) but much better it even ticks over predictably. I've also made a sleeve for the throttle arm where it enters the bracket mount on the steering idler, and the pedal feels much better even though there is still wear elsewhere along the linkage.
Would you be able to tell me where you got that spindle/rebuild kit from?
Jon. :D :D
These guys have everything you need. If I remeber correctly the jet is not part of the refurb kit so you'll have to order that aswell. It made a big difference to the running of the car, well worth doing to the replacement carb if you choose to source one.
Thanks for the link, just had a quick look and now firmly bookmarked. Essential SU stuff there, just the sort of thing I'm after. I think they'll be getting some of my hard earned in the coming weeks then. :D


Active Member
I did an update on a mini 1-1/4" SU recently with a Burlen kit, and it came up very well. The kit came with a new spindle, and body bushes in case they were needed - they werent, the new spindles were an excellent fit. One of the reasons I liked the HD versions was the spindle seals they used that almost eliminated this problem. The kit included a new jet - check when you order.