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 SU Carburettor HIF4 dashpot/piston drop test

Drop Test
Once the piston and dashpot sections were free I cleaned them up so I could perform a drop test. This gives a general idea of the piston wear. As shown here the piston is put in the cover and a small nut and bolt fixed temporarily in one of the mounting holes. The trick is to put a large washer under this bolt head so that it overlaps the  edge of the dashpot cover and will stop the piston dropping onto the floor. If you use a metal washer you should be able to get an audible click when the piston drops and this helps with timing.

 Unless the  air bleed holes are blocked the piston will just drop without resistance. If you have some 6mm bungs then use them- I found that the end covers from filler gun cartridges fitted perfectly- but you have to find the sort that aren't ridged all the way down. Its then just a matter of inverting the dashpot and timing how long the piston takes to drop onto the washers- I had to use fibre washers to get them large enough which took away the nice "click" and made timing trickier but it was OK with a bit of practice. My piston dropped in 6.5 secs, just within the 5-7s window cited for carbs of this size. Slower dropping could mean oil or other obstruction on the sides (so clean it and repeat); faster probably means the piston is worn and needs servicing.

Cartridge gun caps used to block air holes

SU Hif4 carburettor removal and carburettor strip

There are a few signs that my carbs really need some attention:

Firstly, and most obviously, the idle is really rough- its hesitant and lumpy and sounds like a car that's always running  on the choke. The second problem (possibly of my own making) is that the idle speed will rise spontaneously from around 900 rpm or so to 2000, it will stick there and gradually come down. Blipping the throttle doesn't help and the throttle cam is right down against its stop. Idle will also rise spontaneously with the cables disconnected! Obviously this means that the car doesn't slow down as well as it should when you take your foot off the throttle. The lumpy running was evident from the start, I had expected a carb balance to sort that out (see my first post) and although they found a stuck piston, it didn't really improve matters. I tried to "help" by changing the air filters, using a Colortune to adjust the mixture and an air flow meter to balance the flows. I followed the various guides on the internet, Haynes and the MGB workshop manual.  I was able to improve both the idle smoothness  (although its still bad) and combustion gas colour, but it could be that after this the idle speed problem either appeared or at the least worsened.

Now researching this pretty soon showed me that this seems to be a very common problem on Hif4s- many of the reports eventually conclude that its a stuck cable or cam, or even dashpot oil level- but if the problem occurs even when the cable is disconnected and the dashpot refilled then these are not likely to be the explanation in my case. I suspect most would eliminate these possibilities before feeling the need to post and frankly I don't really believe these "resolutions"  and as the problem is intermittent, I suspect that sometimes dealing with these obvious things again does sometimes seem to have an effect but I doubt a permanent cure.  One problem I can eliminate is piston wear or poor fit. This would make the pressure chamber leaky, meaning that the piston would be held  lower for any given engine vacuum admitting less petrol and reducing engine speed; the opposite of my problem.

Anyway my considerations lead me to several possible explanations:
1. Air leak- I can't find one but many people report that warped and distorted manifolds are commonplace and they should be carefully re-flatted. WD40 spray- the usual test seems not to be particularly reliable. Anyway I can't find a leak that way.
2. Weak piston spring- permitting the  piston to rise more than it should in response to a given engine vacuum. These cars had "red" dotted springs and they can get weak- especially when hot.
3 Over-run (or poppet) valves sticking open. These allow extra air through when manifold vacuum is excessive- e.g. on overrun when decelerating from speed with the throttle closed- in other words just the conditions under which my idle tends to stick high. Again these springs can become weak and then the valves can  get gummed up or stick open and again heat can be a factor.
4. A faulty bimetallic strip inside the carb. This compensates for fuel temperature, admitting more as fuel temp rises. Its not clear to me that this can wear out, but if it goes wrong- or has been bent or installed badly maybe it could speed up idle. My suspicion is that such a problem could certainly increase idle speed; but it would stick there and wouldn't drift down again. Nonetheless this does seem to be a commonly suspected item- but I've never found a report where changing it had sorted the issue.
5. A faulty heat-shield could exacerbate all of these problems. My car has the original nasty composite fibreboard shield (asbestos???). I'd like to get rid of it although it means deviating from my goal of  "standard" condition.
6. Enrichment/choke device leaks. This device has a gasket, an O ring, a seal and a dustcap. All these older materials have been reported to fail in contact with modern ethanol fuels and leakage here would cause the rough "choke-y" idle I observed as well as  tending to enrich the mixture leading to the sooty plugs and exhaust I had also seen.

Problem 6 seems to be the most common fault - and also the cheapest to fix. You need part numbers seal AUD3577; O ring AUD3602 and gasket AUD3605. I have heard that changing the gasket for a rubber washer can also help. Perhaps if I had more time I would try fixing the choke on its own first, but as I don't wish to do this strip twice (and have to buy yet more new gaskets each time) I think I will change everything in one fell swoop as well as giving the carb and its components a very good clean. After all I may have a combination of several of these problems so a major carb rebuild does seem justified. I ordered the major rebuild kit from Burlen, this comes with new needles, jets and seals/gaskets (including the choke device seals above). It also includes new throttle plates (and spindle bearings plus seals) with or without new poppet valves (as you select) so it does address most of the possible problems. It doesn't however include the bimetallic strips or piston springs so I ordered these separately- springs cheap-ish but bimetallics seem a bit expensive! I will also check all manifold junctions for distortion and re-flat them where necessary.

As I think I have mentioned earlier, I can't leave a dead car on my drive (neighbour problems), and there is no way I can strip, rebuild, refit and retune the carbs in a day, so I'm going to get a scrap pair from AndyJennings MGB parts. He supplied the hubs and stub axles I used for my kingpin rebuild and it was a good service so I tried again and got two carbs, linkages and manifold for £100. These should allow me to strip check and rebuild the pair before doing a simple swap onto the car which should be do-able in a day. The only problem is that this used stuff isn't usually very clean...

Removing The Carbs

I received a pair of carbs plus linkages still attached to their manifold and old heat-shield. I took a few general views to see how things were attached before I started to dismantle.

Carbs front view. Steady plate (top) not apparently connected to anything- must find out where that is meant to go.

They were pretty dirty. Note vacuum takeoff for the distributor advance mechanism is on the manifold as it was with HS4 cars. I understand that this is sometimes moved to the carb itself although that's not the case with my car and these carbs match my own setup in that respect. 

Top view of dashpots and steady-plate. Vac takeof

They are still attached to the original heat-shield, made of some horrible looking composite board. This is clearly looking very sad, both in this case and in my car too. Although I do want to keep things original, I am concerned about poor heat shielding and it may be that I have to bite the bullet and fit one of the later replacement pattern metal heat shields.

Side views...

I took some general view of the linkages and springs to help with refitting

These linkages are all 2BA nuts and bolts. Throttle at rear and closest to the block, fast idle at front.

 Detail of fast idle actuation plate and the choke cable connection nipple (bottom centre) is a 5BA

The pistons aren't usually replaced in a rebuild; they come as a matched pair with their housing and they're very expensive. Wear here probably warrants a new carb. I checked that both pistons rose and fell easily with finger pressure and using the lifting lever. I will perform a timed drop check as one of the first things I do since I don't want to waste my time reconditioning carbs that are beyond help.

Pistons rose and fell easily.

Each carb is held on by 2 x 1/2" nuts, each with a spring washer, attaching to studs on the manifold. One of mine was missing its washer. The upper nuts can't be removed unless the carb is eased forwards so you need to slacken the bottom nuts as well, pull the pair of carbs forward and then remove the top nuts.

Upper carb nuts- cant be removed unless carb eased back

... and on the other carb

Nut and spring washer- 4 in total.

 The carbs then pull forwards off the heat shield/manifold and the two linkages separate as they do so. I had expected these to be fixed but in my case at least they just slid apart.

linkages- throttle top and cold start below

Choke linkage separated.

This leaves the heat insulating spacers on the heat-shield. The shield has clearly seen better days...

...and gives you two separate carbs ready for cleaning.

 I could now see the throttle flaps and both had poppet valves as expected for a carb similar to my own.

Old gasket stuck on flange- poppet valves in throttle plate.

Stripping the carbs
Unlike my experience with Dellortos, the SU carb isn't explained in a nice glossy up-to-date book. Where is Mr Hammill when you need him? Instead I found the Brooklands Guide "SU carburetters tuning tips and techniques" (Shouldn't that be "carburettors"?). This is an old book, its the best I could find but its turgid- with text that seems to have been written by Mr Cholmondley-Warner! There are no photographs (not even b/w!) only line drawings and they are very small. I will therefore try and put some more detail on the stripping and rebuild process as I experience it, but the usual provisos apply and it will be best to read in conjunction with a guide like this as I describe my experience. The text (well relevant bits anyway) from this book is available in the technical section of the Burlen fuels website. The first step is to mark dashpot cover and float chamber cover with punch marks for alignment when reassembling - I used single punch marks on carb 1 and 2 on carb 2 (duhhhh).

Marking dash pot cover and carb body no 1

The next step is to unscrew the three  screws that hold the dash pot cover (I found these really tight). I had also expected washers underneath them so I will check that (later found they are not needed here). Then...

 ...lift the cover vertically straight up revealing the spring...

 ...and any muck inside the cover.

 Remove the spring and lift out the piston.

 The needle is held in the piston by this retaining screw, before undoing it note the orientation of the needle holder in the piston as this governs in which direction the needle will be offset.

The etch mark is a small "V". In my case it points between the two air bleed holes.

Etch mark on needle holder- small "V"Note shoulder on needle is flush with piston surface.

 Remove the retaining screw, note it has a domed end and...

...pull the needle, its holder and the spring out of the piston. I will fit a new needle but reuse the spring and holder.

Next move to the bottom of the carb and unscrew the 4 screws that hold the float chamber base in place.

 Note that these screws do have washers.

 Lift off the float chamber base- might need a tap with a soft mallet. Note sealing ring in the base and this should always be changed if the base is detached. This reveals the float, jet and the jet adjusting bracket

 Some muck here but much less that I was expecting.

 To remove the jet first remove the jet bracket adjusting screw. This is the screw with the prominent cross recessed on the side of the carb.

 It unscrews and pulls out although its a bit fiddly to grab and pull. Note that it has  a shaped (reduced width) end and carries a sealing washer.

The jet adjusting bracket- bimetallic strip can then be removed by undoing the retaining screw from above.

... and remove the screw

The retaining screw has a reduced diameter end and carries a spring. It was screwed in tightly.

The jet then lifts out bringing the adjusting bracket with it and then...

the bracket is removed from the jet by swivelling the holder to align the lobes with the cut-outs in the bracket.

The two separate

Returning to the carb float bowl the float is held in  by a spindle screw

and the head of this screw is visible outside the carb

Undo the screw and remove it- it has an aluminium sealing washer just below the head

Loosen and remove the washer- it will be replaced.

The float then lifts out. I shook it to confirm that it wasn't leaky- no fluids inside! The needle valve is below and the needle simply lifts out. This was an all metal needle and will be replaced with a viton tipped modern equivalent from the rebuild kit.

The needle valve seat then unscrews using a 2BA socket

The seat lifts out and will also be replaced.

The main jet is retained by a central nut- 5/16 WW and easily undone with a socket

Poor focus here- sorry, but the jet bearing retaining nut then lifts out...

... and the jet bearing itself then pulls out - note the aluminium sealing washer on the jet bearing body. Later carbs have a jet bearing that doesn't have this washer so if its missing its not necessarily wrong! The test is to fit the jet bearing without the washer and check it doesnt protrude above the bridge under the throttle slide. If it does then it will need a washer to lower it. My newer carbs as fitted to my car are type FZ1229 (f and r) and these dont have the washer.

The next step involves releasing the two springs that drive return for the throttle and fast idle control levers/plates. I took some pics to show how the two were fitted. The fast idle spring is held between the operating cam plate and the fast idle spindle cover. Its top is hooked behind the cam plate and its lower end behind the cover plate. This spring is removed first but its bracket also retains one end of the throttle return spring. The latter is visible in the pic below at the top- the lower end of the fast idle spring is hooked into the fast idle cover plate. Note that the throttle spring is also held by the cover plate. 

Fast idle spring- note upper end (hooked) located behind cam plate centre right. Lower end (straight) located behind lug on cover plate. centre left. The throttle spring extends its straight end to locate behind the cover plate centre top

Upper end of fast idle spring (picture inverted)

The fast idle cam is held by a column nut, removed with a 3/16 WW socket.

The nut then lifts off revealing the tabbed lock washer beneath.

Remove the lock washer and ease the cam plate off the spindle, allowing the spring to expand as it comes off.

Remove the spring and the dished seal-covering washer beneath

The locations of the throttle return spring ends are clearer now that the fast idle control system has been removed. Note how its ends locate onto the fast idle cover plate and the throttle lever.

Side view... throttle return spring located in throttle pillar

The fast idle/choke system can be further removed- the small seal lifts off the spindle...

Remove the spring and...

... the two screws holding on the retaining plate/cover can be unscrewed.

 No washers here.

Cover removed

 and the fast idle system can be pulled out- note that the whole thing comes out not just the threaded section. I found it was pretty tight but it came out eventually.

 The inner shaft can then be pulled out of the outer- note the dust shield that was over the threaded end and the sealing ring on the outer shaft. There is also a gasket but this remained fixed to the carb body. These components seem a very common cause of the unstable idle speed I have been experiencing so should always be renewed.

Returning to the carb body the column nut holding the throttle levers undoes with a 3/16 WW socket.

There is a thick washer under the nut. I was expecting a tab washer so I'll check before reassembling. The  throttle levers can then be eased off the spindle. I found they were very tight.

Lever is in 2 parts... part 1 and....

... and part 2. Note sealing washer around spindle. In this case it was quite brittle and cracked.

The next step is to remove the throttle butterfly. There is a mysterious insistence on marking it and the "flange" although I don't know why. I'm not refitting this butterfly anyway so pictures should help more in reassembly. The butterfly screws are split and the ends bent to prevent them loosening in service.

Squeeze the ends to restore them and unscrew.

Once removed, slip the butterfly horizontal and slip it out avoiding the poppet valve

... and withdraw the butterfly

This leaves the spindle which...

... just pulls out of the body. Again pictures should help reassembly.

One seal came off with the spindle. The other stayed on the carb but both were in poor condition.

The spindle has obvious wear shoulders where it passes through the bushes.

The seal was brittle and broken.

Removing the seal from the other side (it was in similar condition- the edge of the bush is visible inside the throttle shaft boring (and also inside the Venturi).




That just about completes the strip. I now need to give everything a good clean and assemble my new parts. The spindle on this carb was clearly worn. Its possible I might have got away with simply replacing the shaft but the new shaft was actually a little narrower than the old and wear was still evident when it was slipped in. It seems therefore that I really need new bushes as well. The bushes are visible inside the spindle boring when the seal is removed. I will cover rebushing and reassembly in further posts.