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MG MGB Technical - HIF4 Throttle shaft bushings

I've got my HIF4 rear carb apart. There seems to be a bit of play (maybe my imagination) in the throttle shaft.

Some bozo has cut the free extended end of the throttle shaft flush with the carb body. I don't think this has any negative effect but I'll probably replace it anyway. I saw in the Moss catalog that an oversized shaft is available but mine looks tight and clean even though I "think" there might be some play.

The previous "mechanic" had inserted the 2 rubber bushings into the body shaft holes and then filled the space with silacone....either to stop air leaking or to hold the rubber bushings in place. Since they popped right out with a 90psi air gun I suspect the latter.

With the rubber bushing removed from the free end of the shaft I can NOT blow past the shaft as far as I can tell when I kiss the end housing and "blow". This is probably a pretty funky test, but all I could think of!

So, the rubber bushings. They have a taper on one end and a grooved slot around the body at the other end. Since they are only about 1/4" long that's not much distance between the two ends.

I assume the tapered end goes out. Whether that is correct or not I would ask the purpose of the slotted end. It doesn't appear that the steel insert bushing has any sort of recessed ring that would capture the T'd end of the rubber bushing seal.

How do I handle this? I ask since it turns out the carbs were rebuilt by the previous owner and I may not even open the Moss rebuild kit, but rather just purchase individual parts for the float chamber area and the throttle shaft. The instructions with the kit might help, but I would prefer to return it unopened if I don't use it. You may recall this is all happening because of a fuel blow by problem.

Thanks,
BH
BH Davis

An SU HIF4 normally comes new with bushes in the body. Also the shaft has seals round it in the recess outboard of the bushes. What you describe sounds like the standard seals, which go hard with age and heat. The silicon you comment on is a bodge.

The shafts, bushes and seals are all available new from Burlen who took over the production when SU stopped making carbs.

The bushes are teflon? lined split tube bushes. The old ones can be removed using a junior hacksaw blade to make a second cut opposite the split. The bush can then be collapsed and removed. Burlen recommend running a 9.5mm reamer through before fitting the new bushes. I did this on mine I think it is agood idea because the original bushes had some thread lock on them and the reaming will get rid of that.

I fitted the new bushes using thread lock again. The old shaft with a washer and nut on the threaded end was used to drift the bush in. Burlen do sell a tool for this. The bush is a fraction shorter than the bore it fits in. I believe it is important for it to be flush with the inboard end of the bore so as to limit air leak around the closed butterfly.

Hope that helps and sorry ot is such a long post.
David Witham

David,

Ummmmmm....that's not long winded at all. You should read some of mine!

I'm a little deeper into the first carb now. I've removed the throttle shaft and can see the steel bushings.

When I insert the shaft into one of the bushing so it just protrudes into the inner chamber of the carb, the free end of the shaft can be wobbled back and forth 1/16" to 1/8". It would seem this should be a "tight" fit and there should be no wobble in the shaft as I described at all. Is this correct, or is the fit/wobble in the acceptable range?

Thanks,
BH
BH Davis

Because the box you type into is alot narrower than the screen it always looks as if I have typed alot.

I would think the wobble you are getting is on the high side. If I remmember correctly the additional cost for the bushes is not a great deal over and above the shafts and postage.

When I was considering doing this job some people whose posts I respect recommended just doing the shafts though. There were times when the junior hack blade was digging into my fingers that I wished I had done as they said.
David Witham

BH -

Two words come to mind: "Joe" and "Curto".

Allen
Allen Bachelder

David,

I have no problem buying the replacement bushings.....it's getting the old ones out as you stated. Your "junior" hacksaw must be really tiny!

Allen,

When I call Moss yesterday afternoon to discuss ordering the oversized shaft they gave me the number for Joe Curto Inc. I looked at their site and will be calling them tomorrow.

Thanks,
BH
BH Davis

BH, First, I agree that Joe Curto is a good place to get SU parts and service if you need outside help. I have reworked 3 or 4 sets of SU carburetors in the past few years and found the bushings didn't have excessive wear in any of them. The shafts were worn and replacement of the shafts helped. The seals come in the HIF kits. Some throttle shafts are a universal length, that may be the reason yours are cut, I didn't cut the ones I used because the external length wasn't a problem. This information may help you decide if you need new bushings. Standard shaft diameter is .310" the bushings are align reamed to .312". Oversize bushings are .320" and are align reamed to .323 ('P' reamer). I'm not a machinist and I don't know what a 'P' reamer means. I do know the reamer has to be aligned in both holes, otherwise the shafts may bind. Since your shafts may have recently been replaced you may not need new shafts. I would think .005" clearance would be satisfactory. In Hammill's boook "How to Build & Power Tune SU Carburetors" he states clearances of between .005" and .010" is one reason for idle speed problems.

Clifton
Clifton Gordon

Clifton,

Great stuff. I think my bushing have been replaced because I was able to easily drive them through with a drive pin. They pressed right out.

I'll check dimensions in the AM.

Regards,
BH
BH Davis

This thread was discussed on 04/07/2007

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