MGB Rebuilding the SU Hif4 Carburettor

Rebuilding Hif4 carburettor

Rebuilding- is as they always say the reverse of stripping. However this is how I went about it. Sorry about the focus on some of these pictures- camera AF tends to hunt.
The first thing I did was to sort out those components in the kit that can be fitted- O rings and crush washers, and put them on their respective parts.  Here is the cold start mechanism

Cold start system- this incorporates a gasket,  (fits to carb body), O ring (mechanism outer shaft) and a seal (threaded end of shaft) covered by a cup washer. Surprisingly there are no O rings fitted into the shaft itself- despite the presence of very obvious grooves. Later carbs can have 2 O rings and no gasket.

 ...and here the rest, float hinge-pin and jet adjusting screw- all with their new seals/washers fitted.

I started by refitting the cold start system. The gasket and the brass outer shaft collar that fits above it, both have a small cutout for orientation. This fits on the carb body with the cut-out lined up on the  upper screw hole, ie the same side as the piston lifting pin. This aligns the output hole in the outer shaft with the fuel passage in the carb body and the screw resting through the cutout ensures it won't twist out of alignment. It doesn't matter how the inner shaft is oriented because its symmetrical and the square end facet will put it in the right position when the operating plate is attached.
I took these pics during disassembly but they show the position.

Cold start tube aligned with cutout in brass disc against upper screw.

Close up.

This is the hole in the cold start shaft that has to align with...

...this hole in the carb body

Fitting a new gasket...

 The assembled unit was then pushed in (a little grease on the O ring) with the cut out in its rim positioned over the cutout in the gasket, i.e. towards the upper screw and the outer seal added and covered with the hollow cup washer.

 Next screw down the cover plate and position the spring.

 Add the top plate, engage the spring with the lower plate as shown above, then engage the upper terminal in the top plate and twist to tension the spring and align the rectangular section in the plate with the spindle.

 Watch out for loose turns of the spring though as they tend to get trapped under the top plate as here.

Free any trapped turns so that the plate can sit down flush. Fit the column nut with the tab washer and bend up the tabs to retain it when you are happy you wont be taking it apart again anytime soon!

 Next I inserted the throttle spindle- check the bushes are clean and then put a little grease on the shaft before inserting it- threaded end has to emerge on the same side as the cold start mechanism.

 Slip in the butterfly disc- being careful to check orientation- poppet valve goes in upper section and its bare face is exposed when screw heads are facing out of the carb. Check position carefully. Insert the screws, tighten and loosen them, operate the throttle a few times and retighten.

 Check that the butterfly sits evenly round in the carb throat- no daylight should be visible when its closed.

Spread the back of the screws when the butterfly is in properly.

 Add the seals to either end of the throttle shaft- here the threaded end.

 This end also takes the  throttle operating levers. Check orientation, butterfly closed when in this position.

 The plate is a really tight fit onto the rectangular section of the shaft- I believe this is intentional so I used the nut to force it down onto the spindle.

 Remove the nut again and fit the upper plate.

When I stripped this mechanism I found a single thick aluminium washer in this position. All the schematics I have seen are adamant that this should be a tabbed washer, so I have fitted that here- I may yet find a reason why the thicker washer was present- perhaps its to help seat the inter-carb linkages? *


* I checked the second carb and it too had a thick washer in this position, this time with the tabbed washer on top of it, here loosened to show both. I don't know why both are present but it does seem that I should have them so I had later to dismantle this carb and refit with the thicker washer in place.

 Add the throttle top plate engaging the upper tang on the bent lug. 
 Then twist the spring to engage the upper tang with the lug on the throttle actuating plate.

 View of both springs fitted.

Next I fitted the needle back in the piston- first drop in the spring

 followed by the needle.
Probably better to fit the spring to the needle and then fit the pair to make sure it seats properly.

needle fixed to spring before inserting into needle retainer.

Then slip in the needle retainer making sure that the etch mark points in the direction noted when it was removed. I used a straight edge to make sure that the retainer sat level with the piston groove before tightening the screw.

 This is the central jet- fitted with its new aluminum crush washer showing how it slips into the retaining sleeve nut

 Insert the jet/washer assembly. The washer is awkward to get to the bottom but it does go with firm persuasion. Make sure its well seated on the bottom of the boring and then tighten the nut. The nut does not go down flush onto the central column, it stands a little proud.

Jet seated in its bore seen from above. Jet is flush with the casting on this side.

Insert the float valve and tighten it home with a 2BA socket. There is no sealing washer under this valve seat which surprised me.

Pop in the new needle valve- this one is viton tipped.

Then refit the float using the hinge pin and its fresh washer. Check float height. Invert the float chamber so that the float rests on the needle of the valve. The float should sit about 1mm (+/- 0.5mm) below the level of the bowl rim at the lowest point in the centre. However its very tricky to see this gap so I placed a 1mm thick aluminium washer at the position of the specified clearance and used a straight edge to bridge across the chamber.

When clearance is right the edge will touch the washer but will not move the float. I found this easier anyway. You can adjust the clearance by bending the little brass tab on the float. Bend it down to decrease clearance and up to increase it but only small adjustments are needed. I found the new needle was sitting higher than the old and I had no clearance at all until I adjusted it. I will have to make a mental note to revisit this later as I do expect the needle to sit a little lower as it settles in in use.

 Next you can insert the jet- fit it first into the bimetallic bracket- note orient the jet to fit through the cut out then tip it forwards so that the lug fits into the cutout in the jet.  Finally clip the bracket into the cut out at the front of the jet. If you don't clip it home into this cutout then the bimetallic bracket will be strained when the retaining screw is fitted and jet movement will suffer.

 Fit the retaining screw and its spring.

Then fit the  jet adjusting screw and its washer- check that the pin like extension of this screw passes through the fork in the brackets lower arm.

adjusting screw needs to fit into the bifurcated bracket.

The pin is hard to see in position but its just visible in the bracket in the picture below.

Once its properly positioned check also that tightening the screw does indeed cause the jet to move- It will take a lot of turns before you see anything happening. Once satisfied screw the adjuster to align the jet with the bridge in the carb.

 Place the new seal in the float chamber base...

 and fit the base aligning the punch marks made before you disassembled.

 Sit the new needle into the jet and lower the piston into place.

Add the spring- I did order a new piston spring but as I ordered the wrong one I am still waiting for its replacement. I therefore re-fitted the old spring temporarily which is why this one looks so un-shiny!**

 Refit the dashpot chamber screws- and the jobs a good'un!

** When my new springs arrived (AUC4387) they were longer (15.5 cm) than the originals (13.0 cm) free length. I think the new springs are right but I'm unsure why different springs should have beeen present in the carb.

Here are a few views of the rebuilt carb- looks much better than my starting point I think!!

The second carb rebuild went pretty much like the first except that the throttle spindle was worn far less. There were obvious telltales where the shaft had passed through the bushes, but no shoulder to catch on a fingernail. The bushes in this carb looked like Teflon and I think have been attended to more recently. I checked the sizes and found both old and new spindles were very similar at 7.88 mm, which matched well to the unworn part of the other original spindle. However I will fit the new spindle and its butterfly because I want to match the butterfly over-run valves and I suspect the screw holes in butterfly and spindle are drilled as a pair. Once fitted the new butterfly was a very nice smooth fit and there was less play than in the first carb which I had re-bushed. I am starting to think that maybe QC at Burlen isn't as good as it should be and that my first shaft is actually undersized. I think if doing this in future I would question shafts that differ by a thou or more. However it is an improvement over what was fitted so I will try it and see.