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.