Note- There is a lot of confusion out there when it comes to this job. This is understandable because the MGB used different carbs at different times and they dealt with the problem of throttle spindle mounting in different ways. If you just want to follow what I did with the Hif4s just skip down to the section "Rebushing". If you are interested here is the result of my digging on the subject:
The throttle spindle is a tricky compromise in carburettor design. Ideally, all air going into the carb should be regulated by movement of the throttle butterfly- but this has to be mounted on a spindle and thus there is always the possibility that air could be drawn in along the line of the spindle shaft, between spindle and its bore regardless of the butterfly position. Secondly the butterfly opens by twisting and allowing more air in above and below it as it opens. The centre of the butterfly should remain in smooth contact with the Venturi wall at all times and when closed should form a close seal against the Venturi wall. Consequently the spindle entry point should be both tight around the spindle shaft and completely and smoothly integrated into the throat inside wall to meet the butterfly edge. Any depression here would open a bypass path around the butterfly allowing uncontrolled inflow of air. Both these effects would be worst when the the butterfly is closed, i.e. at engine idle and as neither is controlled by the mixture screw, at the worst they could make idle speed uncontrollable.
For this reason most SU carburettors were made either with no bushes at all, or with bronze bushes that were cast into the body but actually stopped short of entering the carb Venturi. Both these designs give a smooth Venturi wall to meet with the butterfly and a tight fit round the shaft, the actual bore size being reamed to fit closely around the spindle... All of which is good when the carb is new. However, since the cast-in bushes don't penetrate all the way through, they can't be removed with a drift from either side. Consequently, if they should wear over time, then they have to be cut out. If the carb has no bushes at all (as in the HS), then the actual metal of the carb needs to be removed, allowing a bush to be inserted to mimic the cast-in type. Both these tasks are usually performed by plunging with a pilotted reamer (3/8" or 9.5mm) that takes its line from the bush opposite the one being cut. Secondly, as the bush will stop short of the Venturi, the reamer must be stopped at exactly the right depth to remove any existing bush entirely (or sufficient metal to insert one), but without breaking through into the carburettor body. This leaves the curvature of the aluminium inside the Venturi undisturbed. Once fitted the new bushes are usually then through- reamed to ensure a close fit with the new spindle.
However, the HIF4 carburettor was designed differently; its the only SU carburettor to be made from the start with removable bushes that do penetrate through the carb wall into the Venturi body! On the credit side this means that they can be drifted out easily for replacement and no reaming or drilling is required. However, it also means that the depth at which the new ones are fitted is critical since they must meet the inside surface of the Venturi exactly flush. They can neither project and jam the butterfly, nor be recessed allowing an air leak around it. As an extra safeguard against potential leaks, these carburettors also feature a moulded recess around the exterior throttle shaft entry points that houses a rubber seal. In theory then the Hif4 bush replacement is a much simpler process- drift out the old bushes, drift in and position the new. As these are internally teflon coated they should not require internal reaming either.
The spindle on this carb was clearly worn with obvious wear shoulders (see previous post). Its often possible to get away with simply replacing the shaft as it tends to wear more than the bushes, but my new shaft was actually a little narrower than the old (new shaft, 7.845 mm; old shaft, 7.887) measured in an unworn area and I thought it still seemed a little loose. I concluded that really need new bushes as well.
The old bush is visible recessed inside the spindle boring at the base of the seal housing when the seal is removed.
Its also visible on the inside of the carb and its worth checking this to make sure you know how to position the new one. I tried 3 ways to remove it:
Firstly drifting from the inside: This is a very thin bush (0.8mm wall) and this makes finding any form of drift that would actually work to tap it out without damaging the boring- quite tricky in practice. I found 5.5mm 1/4" drive socket was the right diameter but too short to drift it out completely. If you want to take this route then its probably best to machine a purpose made shouldered drift.
Secondly I tried a blind bush remover but this failed to grip the end of the bush and simply pulled out. I suspect the bush is just slightly too long for my puller mandrel.
Finally I used a 3/8 BSF tap to thread its way into the bush and then tapped this out using a drift inserted through the opposite bush. This worked really well except on teflon coated bushes where the cover tends to shear and strip off leaving the bush in position. However once the coating has gone the bush came out fine and provided the tap is inserted square there is no damage to the bore. I think this approach might also work on the blind cast-in bushes
Once out, the bore into which they were fitted did look a little rough. I cleaned it up a bit by inserting a 9.5mm drill bit and rotating it gently by hand. The drill wasn't a particularly precise bit at a measured 9.3mm, but it seemed to clean the odd spots of white corrosion well.
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| Drill did pass through bores, insert it blunt end first and rotate gently by hand once its across the carb. The shank then acts as a pilot while the bore is cleaned. In theory I could do the same thing with a 3/8" or 9.5mm reamer but I fear it would enlarge the hole too much. |
Cleaning with a reamer would be more efficient, but although I expected this bore to be 3/8", it was clear that it was actually a little smaller and wouldn't pass a reamer without some metal being removed. I really don't want to do that because the new bushes and spindle I have received from Burlen (unlike the spindle from Moss for instance) are not oversized; quite the opposite in fact. The spindle I have received is narrower that that I have removed! As received the bushes are a generous, loose fit on the new spindle. I assume that they will be compressed during installation closing them up to provide a good fit with the throttle shaft. The new bushes are 0.3777 inch external OD uncompressed- which means I would have only 2-3 thou of "squeeze" if I reamed the hole to 0.375 inches; is this sufficient? I worry that if I were to do anything to enlarge the bore then the bushes might not be compressed sufficiently to fit smoothly around the spindle so I decided to try fitting the first bush without enlarging the hole and reconsider my approach if that failed.
The next question then is how to fit the bushes. The advice is to tap the bush in to start it using a drift larger than the bush- a wide punch or the back of a socket, to prevent the top of the bush being belled out. Burlen sell this little bushing tool to position the bush inside the bore. You can use it upside down to start the bush and then flip it over to position it. The stepped end is a loose slip-fit inside the bush while the shoulder is so-positioned as to bottom out on the carb body when the bush is in the right position.
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| Burlen bushing tool and bush |
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| Bushing tool inserted into bush- Note good match of pilot section and bush length |
The problem is that this tool is designed for carbs where the bush boring does not go right through to the inside, and so is designed to position the bush short of the inside Venturi wall. How then can it be used in a Hif4? I did ask Burlen for their advice on how to do this. Although there is (as always) room for confusion I did specify the Hif4 and pointed out that its bush does penetrate the carb wall. The method I was given was exactly the same as that for HS2. In other words the bush should not be removed, but reamed out using a 9.5mm reamer- and that reaming should not go through to the inside of the carb. Obviously this would leave the stump of the old bush in position on the inside wall of the carb. Presumably the assumption is that this bush will be correctly positioned. The new bush is then pushed up against the old using the tool above, which presumably positions it against the stump of the old one without moving it. Any wear in the old bush remnant is not a problem since the spindle will ride in the new bush. As an approach it seems more complicated. Further it involves using an expensive piloted reamer and negates the design of the Hif4 which has done away with that requirement.
Anyway that advice came a little too late for me as the bushes were already out (as the actress said to the Bishop!). Consequently, using this tool in my carbs would leave the bush recessed app. 4mm from the inside Venturi wall, creating an obvious air leak around the butterfly. To solve this I used a section of 12mm round brass rod to make a modified piloted drift with 2 steps. The pilot guide is narrower than the intended spindle as anything larger would prevent the bush from closing up during installation- potentially damaging the bush and/or cracking the carb body. The pilot section is followed by a shoulder up to the OD of the bush and this is designed to fit inside the spindle bore of the carb body allowing the drift to push the bush deeper into the bore. The final step locates the drift on the outside of the carb and I tried to make it the correct length to place the inside of the bush flush with the inner Venturi wall. I made it by tailoring it to the carb by trial fitting and it may well be different for others, but for reference my sizes are below.
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| Brass bushing drift with 2 steps |
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| Bushing as located onto home-made drift, the shoulder below it will allow it to be driven deeper into the bore and the wider section will position it at the inner Venturi wall. |
Section Length Diameter
Pilot 12.5 7.36
Step 1 2.55 9.34
Step 2 (rod) 50 11.98
As a final insurance (plan B) I selected a Draper socket which was exactly the right size to make a smooth sliding fit inside the carb throat, fitting smoothly against the wall and covering the throttle spindle holes. Putting this as a plug inside the carb should allow me to tap in the new bush and prevent it from going too far and ensuring it fits flush with the inner wall. It may even impose a rounded profile on the end of the bush for a better fit with the butterfly.
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| Draper 28mm socket |
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| Socket fits pretty well inside the carb throat. |
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| I used Loctite fix'n seal on the bush |
... switched to the stepped brass drift to push the bush in deeper up to the the shoulder of the drift.
Any fine adjustments could be made from either end using the 5.5mm socket
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| Carb bush installed. |
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