MGB GT Loose gear knob fix

The gear knob in this car is loose, it flaps about loosely on the gear-stick but also buzzes annoyingly when driving. The knob I have is the black plastic type with overdrive switch in the top.

There is an indentation in the front of the knob, inserting a screwdriver here allows the cap (with switch) to pop off.

Indentation at the front of the gear knob to release cap.

 Inside the cap are the two wires that connect to the overdrive switch. These pass up the hollow gear lever. You can also see the ring nut that secures the knob from the inside.

Wires and ring-nut visible inside knob.

 Unscrewing the ring-nut carefully with a narrow screwdriver blade allows the knob to be pulled off.

Ring nut

 There is a slot up the side of the gear-lever and a corresponding ridge on the inside of the gear knob. This engages as a key to prevent the knob from turning and hold it steady.

Slot in gear lever and ridge/key inside gear-knob

 However in my case the wires inside the lever had split. They had been repaired with insulating tape which worked electrically. However it was a bulky solution and the wrapped wires wouldn't fit back inside the lever. Instead the wires filled the knob location slot (not a phrase you hear everyday) and prevented the ridge from locating securely into the gear-lever. As a consequence my gear lever could twist through + /-about 45 degrees. This I think is why the ring nuts wouldn't stay tight

I removed the tape and covered the split wires with shrink tubing

 For a much neater finish.

 I screwed the chrome bevelled nut down wards as far as it would go and then lined up the ridge/key in the knob with the slot in the gear-lever.

Ridge/key inside gear knob

I then cut a small square of heat-shrink tubing to bridge the slot in the gear lever as I pushed the knob down.This tubing is used just as a packer, not for its heat shrinking ability. I could then push the knob down engaging the ridge in the slot over the piece of tubing which  took up any wear and corresponding looseness in the ridge so that it sat securely on the knob.

Pushing the gear lever down over a patch of tubing covering the slot

Down as far as it will go.

I marked the contacts and the switch so that I could tell easily where the positive wires connected, and then refitted the cap

Finally The lower chrome bezel nut was secured tightly against the gear-knob. So far the knob has remained secure and the annoying buzz has been banished. If this doesn't hold I will repeat with Loctite to stick everything in place.

MGB GT Water pump swap

The water pump on this car was making a terrible noise and there was an obvious spray line across and inside the bonnet in line with the pump bearing. A clear sign that its been leaking. I drained the coolant by disconnecting the bottom hose but I didn't drain the block- probably a mistake (see on)

First step was to loosen the alternator mounting bolts

 ...and the clamp bolt on the slotted strap bracket

 ...so that the alternator can be swung inwards and the old belt removed. This will be replaced.

The pullet is held on by 4 7/16 bolts with spring washers

 Once the 4 bolts are out the pulley comes off. I cleaned it up and resprayed it where the powder coat was coming off.

Remove to hose

The pump is held on by 4x 1/2 inch studs, 3 are the same length and one is longer. 

Unscrewing pump mounting bolts.

Long bolt next to new pump. The long bolt passes through the thickest point of the pump and so penetrates only as far into the block as the shorter bolts.

The long one goes through the widest  part of the pump. All bolts needed cleaning and the lower ones de-rusting! In addition the forward alternator hinge bolt passes through the pump and so has to be removed.

Front alternator hinge bolt must be removed to free the pump.

The old pump was removed, which is when I discovered I should have drained the block as there was more coolant to be released once the pump was off! Its gasket could then be pulled free. It wasn't in good condition

Old gasket was decidedly dodgy

New pump compared with old.

Refitting is as they say the reverse of dismantling. You need to clean all mating surfaces carefully so that the gasket can seal. I didn't encounter any problems.

Refit top and bottom hoses

Pump on- refit pulley and alternator belt- check and adjust belt tension.

Refilling with coolant has to be done in two stages: Add coolant to the overflow tank to at least half full. Remove the block top-up plug and fill with coolant to the base of the plug threads. Note that you have to fill slowly as the liquid has to find its way down. Fill and wait for it to settle then top up again... repeat.

Start the motor and run it until the top hose becomes hot, then stop the motor and repeat top-up as described above.

SU Hif4 Carburettor removal, refit setup and balance

Removing hif4 Carburettors

Having rebuilt the two spare carbs it was then time to swap them for the originals. Before removing the old ones I took several pics of the hoses to ensure that I can refit them correctly.

Front cross hose, position of linkage clamps

Throttle cable and choke cable 

Fuel in to front carb

Vent hose from rear carb

Choke and throttle cable routing

cable connections

(The following is how I removed the carbs for a full strip down. However if you do not need to disturb the choke or throttle, its easier to tie the carbs together with string, compressing the spindle and cross carb fuel pipe. This holds the carbs together as a pair and saves disturbing the spindle and its settings. The carbs can be removed, worked on, and eventually refitted  as a pair.)

To remove the carbs I first compressed the clamps and then slipped them along the link hose before easing the hose off the carb cross connections.

 ... and disconnecting the vent hoses

 ... and fuel inlet hose- raise it to prevent fuel spillage.

 Remove the carbs by unscrewing the 4 nuts on the mounting studs. The lower stud on the front carb is both blind and blocked and a bit of a pig to get off. The tool of choice seems to be a 1/2" AF stubby spanner. Disconnect the choke and throttle return springs, slacken the throttle cable.

 Ease the carbs forward as a pair separating the linkages. The old gaskets are pretty tatty and were removed.

 ... and replaced with new ones. Before fitting the carbs, its necessary to set the jets so I screwed the jet adjustment screw fully anticlockwise and then screwed it back in until I could feel the jet start to move with an inserted finger (!). The adjuster should then be screwed a full further two turns from this point to provide enough fuel to start the motor.

New gaskets!

I slipped the carbs on, engaging the linkages as I did so. Devote at least 30 mins to doing up the hidden nut lower front carb! This is the work of the Devil to get at. I found that
1. Clean the stud and nut threads thoroughly before you try to fit the nut as doing as much up with finger pressure is ideal. If threads are clean then this should go almost all the way with finger pressure alone.
2. Use a magnet pick-up tool to feed washers and nut down to the studs one at a time and press the carb back or use a screwdriver lever to move them onto the stud. Fingers (as above) should then seat the nut.
Loosen the throttle actuating levers on their common shaft so that each carb can be adjusted independently. Screw up the throttle stops on each carb until they are free of the stop pads, then tighten then again until they just make contact. Finally screw each two full turns down to provide enough throttle (air) to start the motor.

 carbs installed.

Well there was good news and bad: The good was that motor started pretty easily... sadly the bad news was that the motor ran very lumpily and unevenly and  no amount of adjustment made any difference! I discovered that the front carb was simply not working. Opening the rear throttle led to a nice increase in revs and associated motor sound, opening the front throttle saw the piston rise and an obvious sound of increased to air intake- but no response from the motor- if anything it faltered and threatened to stall. Since fuel is clearly reaching this carb (it has to pass through it to reach the rear carb which is working) it appears that there must be some internal blockage in the fuel channels. Sadly I did not have enough time to remove and strip the carb again to sort this out since "Dads taxi" duties were rapidly approaching. I had no choice but to remove these carbs and refit the originals since I need to be mobile in fairly short order. However, before refitting them I now had the opportunity to examine their cold start systems which had been implicated in their problems.

Cold start mechanism, origin front carb

 I removed the column nut, tab washer, upper plate and spring. The cup washer beneath lifted off to reveal a brittle and broken shaft seal

 The broken part of the seal was found in the lower plate. I removed the two screws...

Broken seal fragment found, centre above lower plate.

... and lifted the lower plate off. In this carb the outer shaft on the cold start mechanism⁷s was integral with the lower plate and unlike the other carb, was not removeable. The O ring seal was also brittle and worn

I replaced the O ring, carb gasket and shaft seal before rebuilding the shaft. Examination of the second carb revealed a similar situation although the seal had not actually broken. I renewed these parts too, before refitting the  carbs as above.

I was then  able to start the motor- and immediately it was a much smoother tone. I equalised vacuum using an airflow meter and then installed Colortunes in cylinders 1 and 4. I adjusted the mixture until both burned Bunsen blue- although this is very difficult to see unless you have the car in a darkened garage! I found that the recommended method of raising the piston either with a screwdriver or the piston lifting pin simply did not work. The theory of seeking a temporary rise in rpm followed by a drop is clear, but it simply didn't happen! I found that raising the piston always caused the motor to falter regardless of the position of the jet adjustment screw*. This meant I was pretty much forced to rely on the Colortunes.
* Note added later- this problem with the traditional method was probably caused by poor timing adjustment. I later checked timing and found the car was set at 30deg BDTC at 1000 rpm. This large advance was probably needed to burn the very (very) rich mixture coming in from the carbs with leaky choke mechanisms. I'm guessing it needed longer to burn. I reset it to 10deg BTDC at 1000 rpm. This gave 32deg advance at 4000 rpm but revving the motor this high required a lot of nerve and I'm not guaranteeing that this is correct. However the cent advance does clearly work. In any event timing should be set better now and although it drives much more smoothly  and pulls better, its still not a smooth idle.
** note added later... Idle was eventually sorted by swapping for new carb butterflies without poppet valves see below

I then rechecked air flow and re-balanced with the individual throttles. When these seemed optimal ("even" motor rhythm and blue colouration) I was able to re-tighten the throttle levers back down onto their common shaft. This has to have a certain amount of free play between the lever actuating pin and the fork on the throttle plate into which they fit. The method is to insert a 12 thou feeler gauge between the throttle shaft stop lever and the cold start mechanism lever. Holding the  feeler in place, each throttle lever is then pressed gently onto its stop and held there whilst the bracket screw is tightened with a 2BA socket. When both were tightened the feeler can be removed. This will raise the throttle pins away from the lower fork of the throttle plate as it drops onto its own stop on the cold start lever. Finally the throttle cable was readjusted to take up slack but making sure that this free play was preserved. IE the idle speed is set by the carb throttle stops and not by the cable holding the throttle partly open.

I then removed the Colortunes in favour of the plugs and  refitted the air filters before road testing the car. Fortunately the motor ran much more smoothly and although it did seem to have a bit of a flat spot, it was running better then before, and most significantly the rough and choke-y idling had been cured completely! It still seemed that idle speed came down somewhat slowly after a throttle blip but whilst driving this wasn't a problem and. The spontaneous elevation in idle speed is still present but see on for fix!

I should add that examination of the plugs 24hrs after this setup showed that cylinders 3 and 4 were still running rich. I repeated the colortune setup  and discovered that the mixture on this carb could be adjusted considerably weaker with the beneficial effect of  improving the idle rythym.

I will examine the two rebuilt carbs and see if I can identify the blockage** in the forward carb; but if my original pair are now running in an acceptable manner then I may not refit them immediately.


**Stripped the offending rebuilt carb. The problem was quite simple in that the float had been pushed upwards during reassembly and had jammed in the closed position. All passages and the jet were clear. Consequently  fuel was simply not entering the float chamber and thus not exiting thru the jet. I freed it up and checked that it was opening and closing by blowing thru the inlet (finger over outlet to carb 2) whilst inverting and righting the carb. It seemed to be working fine now.

UPDATE- well I still haven't fitted these recon carbs. The reasons being that fitting new seals to the choke circuit of the old carbs was a cheap and easy fix for the horrible lumpy running and allowed me to set the timing back to normal- it was well out having been adjusted to burn Heaven knows what sort of mixture. However, this still left the spontaneous increase in idle speed that I experienced, idle set at 900 rpm would start to rise when the motor got hot and stick at 15- 2000 before falling back to 900. This seems to be cause by leaking poppet valves- I fitted new solid butterflies and carb behaviour is greatly improved, setup was more consistent and idle speed now steady!

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.