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MG MGB Technical - carb spindles

the butterfly spindles on my HS4's are moving due to wear in the housing, what the best fix for these? I thought i read somewhere that it was possible to buy over size spindles and then ream out the holes in the carb housing to the new spindle diameter, is what you guys do?

Gary Roberts

If you're in the US, try He can repair them for you, or sell you the parts and tools that he manufactures. His service is personal, professional and fast! He's been a great help to me and others on this board.
Jeff Schlemmer


You can buy a rebuild kit that includes all the parts, including new bushes. The new bushes are stainless steel with Teflon coating, rather than the original brass ones. I assume they are supposed to last longer. Now, I tried to re-bush them myself using a drill press and the correct size bit, the theory being that I could then push the new bush in. Unfortunately I made a bit of a hash of it and got the drilling just a fraction off centre! Result is a body which might not even be saveable! However, I reckon I could still have the original brass bushings drilled out properly and then a new brass bushing inserted.

I assume you are in Stafford UK. The people to talk to are Burlen fuels. ( They own the SU brand and manufacture all the rebuild kits, new carbs and genuine SU parts. They are very helpful on the phone and didn't even laugh when I told them what I had done! They charge 30 a carb (plus VAT and P&P) to re-bush each carb. So figure on 75-80 to get the bodies re-bushed. You may be able to get a local machine shop to do it for less, you may have the ability to do it yourself! I found Moss to be the cheapest for the rebuild kit, but if I was starting again I'd just strip the carbs and send the bodies to Burlen. They'd re-bush them and send them back with the rebuild kit (minus the parts they used) for you to put back together. Youd' be looking at around 130 all in I reckon. You can get refurbished carbs for that (MGB Hive I think) but I suspect they won't have been re-bushed, only had the spindle replaced. You need to talk to Burlen to establish the best way to proceed IMHO.

The one other thing you could try is simply buying the rebuild kit, trial fit the new spindle and see how much play is due to the worn bushes. Most of the wear in mine were in the spindles themselves, but I had those new bushes looking at me, and just a little play still there. I just couldn't help having a go!! Now I'll need to buy another worn pair off eBay or fork out to get the one I wrecked fixed!

Good luck.
I D Cameron

Oversize spindles (010 I think) are available from Moss and other places on their own or part of a rebuild kit, have a peek at their online catalog. I picked up an adjustable reamer of the correct size from an industrial supply place for a few bucks. I think there's a specialty reamer for a few more bucks. Either way, 60 careful minutes later the job is done, no more leaking.

Next time they wear out you need to get the full kit with the bushes.

It never ends does it? Every 30 years or so without fail you need to rebuild the d*mn carbs ;).

Mike Polan

Gary, there is a guy by the name of Ed Biddle (EB Engineering in Little Malvern) who does nothing but rebush and rebuild SU carbs, he also does excellent exchange windscreens for roadsters. He does about 1200 carbs anually for MGOC and the like. give him a call and I'm sure he will put you on the right track.
Iain MacKintosh

When we notice a play in the spindles, how much are we looking for? and in x or y direction? and could you get this Ed Biddle tel n?

Look for very little if any visible play of the spindle inside the bush. It's okay for the spindle to move in & out, though that can't happen when the throttle is closed, only as it opens.

Really the amount of play doesn't matter, the important thing is whether your idle fluctuates, and if it does, whether spraying oil on the spindles/bushes settles it, or if carb cleaner speeds it up. Either check suggests a rebuild.

HS4s are pretty simple, it's hard to mess them up if you've got any mechanical aptitude. I'd save my money by using the rebuild kit with the oversize spindles unless you know they won't fit. In mine both the spindles and the bushes were worn. An easy way to check is to take off the throttle plates and slide the spindle in the bushes to an unworn part - if you can still feel any wiggle you need oversize spindles or new bushes (and standard spindles).

If you have any doubt perhaps you can drop by the parts dealer and trial fit the oversized spindles. Or a micrometer can tell you whether you've already got oversized spindles already or enough wear to need new bushes. I agree with Iain I'd get someone with experience to press in new bushes.

Have fun :)!
Mike Polan

Alfredo, that's a very good question.

The spindles can be worn but it's only an issue if it won't idle properly. Sometimes it will keep running even with the idle screws right out due to the air leak. If this isn't a problem and you are happy with the idle then forget it.

The reason is that the disc valve and spindle are drawn forward towards the engine at small openings, closing any gap between spindle and body on the engine side of the disc. The wear/gap is only open on the atmosphere side so has little effect. As the throttle is then opened any wear is negligable compared with the bore.

That said, all this is true of old carbs with parts that have grown old together. If rebuilding do everything otherwise you can end up with new parts that don't fit so well with the old and throttle discs that close OK sometimes and not others. A case of if it's OK leave it or do the whole lot properly. Rich.


My question was because my 71b has a hs4, i had it for nearly 5 years, and i tuned it as per all manuals.
idling is ok, i run them rich all the times otherwise when traveling on a steady speed it coughs,thats why i am trying to figger it out weather is the spindles wear. They seems to be a bit loose but i have nothing to compare to.
Sending them to a proper guys to rebuild it would be wiser i think.

Alf, if it's doing it on steady light throttle about 2-3000 rpm it sounds like weak mixture but it's unlikely to be from the spindles at that speed. I've experienced it simply from trying to run too lean. It's called "hunting" over here, a sort of intermittent stuttering like it's hunting for fuel. Single or or twin carbs? Which needles are you running and are you on standard air cleaners or ram stacks? Rich.


i got twin carbs, with K&N air cleaners, needles are #5, but before i run it with standard paper filters same needles, i kwnow it runs reach because it failled on MOT, i had to lean to pass.

Alf, MGOC sell #6 needles for use with K&Ns. Looking at the profile they are richer from about 1/3 throttle to full throttle. Are the dashpot springs (red) OK? If they are tired it will run weaker. Recently changed mine which had shrunk 3/4". Also worth going through the ignition system and timing as this sort of cough can be tricky to pin down. I'm not sure where you are or how they test your emissions but most earlier Bs run rich at idle, quite lumpy, and if you lean them off at idle they then won't pick up so well. Rich.


This thread was discussed between 01/06/2005 and 03/06/2005

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