Tuesday, March 20, 2012

Care and Feeding of SU H4 Carburetors (used on the MGA 1500 and 1600)

SU carburetors work very, very well.  Replacing your SU's with a Weber is normally not an "upgrade" - it's an admission of defeat.  A Weber carb is harder to dial in and will likely leave you with throttle lag and a lumpy idle.  SU's have a reputation for being leaky and requiring too much maintenance, but that is undeserved - set them up properly and then leave them alone.  They only have about three moving parts, and none require fiddling with - and if they are set up wrong, fiddling won't help!  Fix your SU's and you'll be happy.

I just recently rebuilt the third set of SU's since I learned how by rebuilding my own several times over the last couple of years - I kept doing it over as I learned more and I wanted to do it right.  I find that I really like this aspect of working on these cars.  Though I am not about to put Joe Curto out of business (he is the SU carb God and has rebuilt thousands of them - and I do buy parts from him), I think I will start working on carbs as a "side project".  I am already looking for any old sets that are available cheap - let me know if you have some non-working carbs you'd like to sell.  A "new" set from Moss motors costs about $1500, and a professional rebuild is about $300 or more per carb (MGA's have two).  I can generally rebuild a set for about $150/carb in parts - so those costs are not really out of line with the labor required, and the tools you have to buy.  I now have the reamers and taps and dies that I need.  I also figured out how to strip and zinc plate parts at home (to be a separate post) so that good parts don't have to be replaced just because they're rusty.  However, if you are interested in how your carbs work, and want to be able to service them yourself, there is no reason that you can't tackle this job!  This is by no means a definitive treatise, and I am only familiar with the type of SU's on our MGA's, but for my own benefit, and maybe - if you are reading this - for yours, I wanted to write down a bit of what I have learned.

Before you begin.

99% of all carburetor problems are electrical.  Before tackling your carbs, be sure to do or check the following:

* file or replace your points
* replace your distributor condenser
* check the polarity of your ignition coil
* put in new spark plugs (NGK BPR6ES standard, or slightly cooler NGK BPR7ES)
* replace your distributor cap and wires if you can't remember the last time you did it - don't forget the rotor
* set timing statically to between 8 and 10 degrees BTDC
* check all your valve adjustments - .015 cold
* Bonus hint - if the vacuum advance unit on your distributor is original or suspect, get a new one ($80) from British Vacuum Unit (http://www.britishvacuumunit.com)

By doing this basic tune up, you won't be chasing your tail later wondering why the car won't idle or run properly and start erroneously blaming your carburetors.


Remove the carbs from the car.  Generally it's easiest to remove the filters first, then disconnect the choke linkage and separate the carbs at the throttle shaft connection (pick one of the four 1/4" nuts on the wavy linkages to loosen).  Take the front carb off first.  That will give you better access to the vac line connection and the bottom manifold nut on the rear carb which is a bear to get a wrench on otherwise.  Installation is the reverse of dis-assembly.  When you take them apart there will be more than 200 pieces, and you think that you will never get them back together!  Don't Panic!  If you want, do them one at a time, so you have the other carb to refer back to - but it really isn't that hard.

I like to put a white or light colored terrycloth towel on the bench.  This keeps parts from rolling around, and helps me to see everything.  Here is a great exploded diagram showing all the parts and pieces:   

You will need a 1/4" small socket for the linkage bolts, a 15mm wrench, and a medium sized crescent wrench (spanner).  The fittings for the bowls and jets are Whitworth, so the odds of having those wrenches on hand is slim for most folks.  I think a 21mm wrench fits the bottom packing nut on the jet.  Don't use vise-grips!  You will also need a good small flat-bladed screw driver, and a pair of needle nosed pliers. 

Be careful taking the needles out of the pistons.  If you go after them with pliers, you will ruin them.  Normally, you should replace them anyway ("GS" for 1500, and "#6" for 1600's) so that is OK, but if you need to reuse them and they don't come out easily, just leave them alone.  Even trying to pull them out with your fingers could bend them.  If they come out, save them in case you accidentally bend the new one later on.  Check the number stamped on the base of the needle - can be very hard to read.  Make sure it's right for your engine and application - if not, that might 'splain a few things.

Don't mix up the dash pots and pistons - they are matched sets or at least you should treat them as such. 

When you have everything disassembled and laid out (no two pieces of metal should still be touching, you will need to clean up all the parts you plan to reuse.  If they are really grimy, clean them with a gas soaked rag, spray carb cleaner or brake cleaner, or dip them in "carb dip" before shining them up.  The best tool for this is a large soft wire wheel.  Mine is on an old-fashioned grinder that is belt-driven and has no guards so I have good access to the wheel with the part.  I also can put a wire wheel on my Shopsmith.  My bench mounted grinder doesn't work as well, since only a couple inches of the wheel is left unguarded.  A stiff wire wheel will just chew up the aluminum, so make sure it's a soft wheel. Wash the carb body, float chamber, float chamber lid and dash pots with your choice of de-greaser and a brush.  Then gently run them under the wire wheel which will clean and put a dull matte finish on the aluminum.  Some folks like the look of polished dash pots (the dome things on the top of the carb body), but I don't personally love the look - it requires a lot of sanding and polishing, and it has to then be maintained which is a pain.  I like wrenching and driving more than polishing.  If you do want to do it, be careful since over-zealous polishing on a cloth wheel can heat up and warp the dome.  Don't drop them either - hold them firmly while at the wire wheel or polisher!  All the linkage arms and brass parts are best cleaned using the wheel as well.  A small brass toothbrush helps to get into holes and corners.  You might have to use a dental pick and q-tip to get the last remaining bits of gunk and crud out of the crevices.  Feel the inside of the carb bodies with your finger - sometimes there is are burrs left from the original machining that you can scrape off with a knife or the tang of a small file works well also.

Carb kits and parts.

I have ordered carb kits from Moss Motors, Royze (through a distributor - check eBay), and Joe Curto.  Joe's are the best I think, and all are about $50/carb.  Another source for the UK is Burlen Fuel Systems - the original SU company (www.sucarbs.co.uk).  The kit should include all the gaskets, a new jet, (sometimes) a new needle, and a new float bowl valve.  Joe includes a new jet spring and needles as well, but some kits don't, so be sure to ask.  (Call him, don't email him.)  There are many needles available, so I get why they leave them out. IMHO, most carb running problems are not caused by needle selection but by basic set up issues (or electrical issues, see above).  Needle-swapping is for racers only.  You might also need other bits such as the clevis pins and retaining washers for the choke linkages, or even the brass choke linkage arms themselves if they are very worn.  The circular holes in them should have some slop - but they become ovoid with time.  Alternatively, you can braze them shut and re-drill them.

Teflon O-rings - The SU carb kits come with leather (as original) or sometimes rubber (modern alternative) o-ring seals for the top and bottom of the jet.  At Barney Gaylord's suggestion I replaced the leather "o-rings" for the jet bearing assembly with Teflon ones from McMaster-Carr.  Seems sacrilegious, but they don't wear out and don't leak - ever.  The part number is 9559K15, and you have to order 50 I think - but they're not expensive and so worth it.  You need 8 per set of carbs - I'll be ordering 50 more very soon. 

Checking and fixing the throttle shafts.

One of the biggest problems with SU's is that the throttle shafts and bores wear out allowing air to enter on the wrong side of the throttle plate.  This makes it hard to get a smooth idle, and can make the engine run too lean at low speeds.  When the carbs are on your car, if the car is running, you can check this by spraying carb cleaner around the throttle shaft bushings to see if the idle changes.  If it does, then you have a leak.  With the carbs in hand, you can wiggle the throttle shaft to see if it moves.  This is a subjective test, however.  There should be almost no play, though some wear is expected.  I have seen a spec that the play should be no more than .005" - but to measure that you have to mic the shaft and then the bore.  A simple check is to remove the thottle plate and slide the shaft over to where the unworn part of the shaft is in the bores.  If there is a lot less wiggle, then the shafts are worn and should be replaced.

 If "too much" wiggle, the fixes are these:  if the shafts are worn, replace the shafts.  The soft brass shafts get grooved out on one side and you can see it as well as feel it with your finger.  If the shafts aren't worn, but the carb bodies are, then you'll have to ream the bodies to accept the readily available over-sized shafts, or ream them out and insert bronze bushings which will allow you to reuse the original shafts, or original sized-shafts.  New carb throttle shafts (standard and oversize) are about $25 each.  They come longer than needed and have to be cut to length.  Bushings are $1 to $3 each.  The reaming tool from Moss is $200...ouch.  I got a 5/16" and a 3/8" reamer from McMaster-Carr for about $28 each.  

**Update:  I got some oversized shafts but don't now plan to use them - you have to also bore out the fittings which go on the shaft, as they won't fit otherwise, which I didn't want to do.  In another post I talk about an easy way to line drill the bodies to accept new bushings and allow the use of stock sized shafts.

To bush the bodies, you pass the reamer through both throttle shaft openings to keep it aligned.  If you are really ambitious, you could make up a jig for your drill press or lathe - it is very important that the holes line up.  But, this guided hand reaming method works fine and I see no need to improve on it as I am not doing this every day.  (I'll post pics of this as soon as I figure out how to do that.)

Once they are re-bushed or replaced, or just reinstalled, you need to replace the throttle plate.  They have a beveled edge all around, and it only goes in one way - so I mark the rear top with an "X" using a scratch awl or sharpie so that I know which way it goes back in later.  You also want to make sure you keep the same disc with the same carb body, unless you are replacing them.  They are probably fine, but they tend to wear right at the point where the throttle shaft enters the bushing, especially if the bushings or shafts were very worn.  Put it in place, then install the screws loosely.  (These screws come with the new shafts, but you should probably order a set if you are reusing your old shaft as they get munged up easily.)  Close the throttle and hold it tightly closed while looking up through the carb at a light.  You shouldn't see much light leaking past anywhere.  If not positioned correctly, gently reposition it with your finger and try again.  When it's seated tightly all the way around, continue holding it closed while you tighten down the screws using a good quality small flat bladed screwdriver.  The springs that hold the throttle shafts closed are handed - one for each carb - but you'll figure it out pretty quick if you reverse them.  When the spring is in place turn it one additional half-rotation and tighten.

Checking the dashpots and pistons.

The dashpots and pistons are carefully machined and should be kept as a matched set...so don't mix them up.  Don't just start sanding the insides of the dash pot or the outside of the piston if they are dirty - gently clean them as well as you can, and slide the piston back and forth.  If the piston is getting hung up anywhere, the dash pot might be a bit egg-shaped (if it was dropped).  Since they may have been swapped before, try switching them now and try again.  If one of the domes is out of round, you might be able to gently massage it back into shape with your hands, but I wouldn't start beating on it, or at least do it carefully.  I have had good success by lapping them.  Spread out a bunch of Old No. 7 rubbing and then polishing compound on the inside of the dash pot and lap it with the piston, turning it around as you push in and pull out.  To check them when you're done, you can do a drop test.  See John Twist's YouTube video for a description of this if you want to go that far, but if they move freely, you are probably all set.  Check the springs in the dashpots as well.  On my recent rebuild, one was stretched out and kinked, and the other wasn't even the right spring, so I ordered a new pair.

Float bowls - installing and setting the float height.
When you reassemble the float bowl, make sure the floats don't have liquid in them.  If they do, replace or fix.  You can fix it by drilling a small hole to let the gas out, then soldering the hole shut with a small piece of brass, and re-solder the perimeter seam and the seams around the top and bottom holes.  It might not look great when you are done, but it will work - even if you've added an ounce or two of solder.  

The height of the floats is very important to SU's - it determines the height of the gas in the jet tube as well - since liquid seeks its own level.  Too low and you'll run lean, too high and you'll run rich or flood.  The float bowl fork should be set to a height of 7/16" from the underside of the float bowl cover when it's resting on the pin - use a drill bit as a gauge, and bend it at the junction between the fork-y part and the straight part.  Remove and replace the needle and seat before adjusting the fork arm.  With the float bowl cap on the bench, there should be a small fiber washer below the float assy - in the kit is normally red, though there may be two sizes, a thin and a thick.  The thin ones are often white or grey.  I have always used the thick one.  (You are going to check the float height and set it anyway, right?)  I personally am running Grose jets - a ball bearing alternative to the needle type, and have been very happy with them.  You can still order these from Moss and other places, though since the original manufacturer's patent ran out, I have been told Moss is supplying inferior copies now (article in MGA! by Mike Ash).  If using the needle type, you will probably notice the newer ones have a Viton rubber tip.  No matter which kind you use, you should add an in-line fuel filter after the pump and before the carbs to make sure that sediment and rust and debris don't cause your carb float bowls to either overflow or starve.  Inside the float bowl cap under the banjo bolt should be a gas filter screen - often it's missing, or present but clogged.  If you have an upstream filter, you can omit these.

There are two kinds of float bowl "holding up bolts" - the banjo bolt (earlier) and the post and nut (later).  The grommets are the same for both - and are a source of leaks if they deteriorate or "perish".  There are rubber grommets out there that are incompatible with ethanol in modern gas, and they last about 3 or 4 months.  Moss and others now carry Viton versions with don't have this problem - but ask your supplier.  Some Viton units (from Moss) have a "V" cast into them, but others apparently do not have the mark, and yet are still sold as being Viton.

If you can't get the nut out, you can try heat - but you risk snapping bolt if it's really stuck (I just did one myself that broke off).  Then you have to drill and tap it out - and it's a BSF 7/16" tap - not readily available.  I ordered the tap and the die from a source in Canada off of eBay, as I have seen a few of these now with bad threads.  (the float bowl top fuel line banjos are 9/16" BSPP threads)

Assembling and Centering the Jets.

After leaky shaft bushings, this issue is the bane of SU carburetor owners.  It is imperative it be done well, however!  At the bottom of the main carb body is the jet bearing assembly.  It has the brass mixture nut at the bottom with its spring, the gland nut above, the lower jet bearing (brass tube thingy) and washer, upper jet bearing (stepped brass tube thingy) and washer, and the jet-packing washers and spring in the middle.  The jet size for MGA is .090".  The jet is normally stationary up in the carb body, but when you pull the choke it moves down to allow fuel unfettered access to the carb throat - which is why your car runs very rich if you leave the choke on, and your gas mileage suffers greatly!  Many (most?) MGA chokes do not work well, as this whole assembly needs some careful set up - but once done it is pretty bullet proof and you should not have to ever mess with it again.

Centering the jet is the process of lining up the hole down the center of the jet with the needle that is in the piston.  If not centered, they can rub, preventing the piston from rising and falling, and eventually wearing out the parts.  With the carb lying on the bench, disconnect and remove the choke linkage, then pull out the jet, and take the bottom brass mixture nut and spring off.  Leaving the spring off, replace the nut and the jet.  Back off on the large packing nut just a bit with your large wrench.  If you didn't just rebuild the carb, this can be tough to un-stick.  While holding the carb upright, raise the piston and let it fall, turning the brass jet bearing until the piston rises and falls freely.  When you have found the spot, make sure the flat side of the jet fitting is oriented correctly to accept the carb linkage.  I normally put the stamped face outboard.  Check again.  If still free, then lay the carb down on the bench on it's back, and mark the top of the tube with a black Sharpie so you don't lose the orientation.  Cinch up the gland nut and recheck that it's all still free (this can be a bit fiddly and takes a few tries sometimes, but sometimes you get lucky).  If still OK, pull the jet out, take the nut off and replace the spring, then reassemble, being careful to maintain the jet bearing orientation.  Hold up the carb and check again - voila!  Now do the other one.

Setting the mixture and choke.

The first thing is that, in the exercise above, the jet should slide out of the carb easily - with little or no resistance and still not leak.  If it drags, the chokes get hung up.  The Teflon washers help, but the jets that you get in the kit (any of them as far as I can tell) need a bit of polishing as well.  I have been polishing the outside of the jet with 600 grit (wet) sandpaper, then 1000 grit, then I use metal polish (MAAS - great stuff) with just my fingers.  Takes five minutes per jet and they slide much better afterwards.

The mixture nut should start by being backed off the fully-home position 12 flats or 2 full turns.  Later on with the car running, you can check the mixture by examining the spark plugs and adjust from there if needed - but don't start playing with the mixture until you are sure your carbs are well set up and all your other problems are sorted first!  To lean out the carb mixture, screw it in one or two flats, and to enrichen the mixture, back off one or two flats. (This should be done evenly on both carbs initially, but if you notice a difference between spark plugs one/two and three/four, you might have to adjust accordingly.  If there is a big difference in the appearance of the plugs, though - something else is wrong, so do a compression test and check the setting of your valve lash again, etc.)

UPDATE May 11, 2012:

New information:  When I recently installed my freshly rebuilt and reshafted carbs on my car, I backed them down each 12 flats and the car ran very poorly.  I did some research and tried this method:  Using my digital vernier caliper, I measured down from the bridge of the carb body at the bottom of the throat to the top of the fuel jet with the the jet all the way home (choke disconnected).  The measurement should be somewhere between 0.062 and 0.120" - big range!  I have been told that 0.75 is the ideal starting point, but I chose 0.10" and it ended up being spot on - plugs are just the right color, so I don't want to lean it out any more than it is.  Point is that they must be the SAME, and you can adjust from there by counting flats, but when I first measured them both - ostensibly the same at "12 flats" - they were very different measurements!  I then went back and took great care in adjusting the float bowl heights - again - the SAME at 7/16".  The car immediately lost it's lumpy idle and rans sweet and smooth with the first turn of the key!  Hooray! 

To adjust the choke linkage, connect the two brass linkage arms together with the L-shaped threaded connector.  As you lift up on the choke cable attachment point, both jets should be pulled down evenly.  Adjusting the position of the fitting on the L-shaped threaded rod by moving the two nuts allows you to synchronize their motion.  I actually mount the carbs to a piece of wood (a fake manifold - see photo at the beginning) so that I can check all the adjustments off the car instead of having to fiddle with little nuts where it is hard to reach them.

So with the carbs cleaned, reassembled, float height set, jets centered and everything working as it should, it's time to put the carbs back on the car!  

Installation on the car

Easiest way is rear carb first, then front carb, then air cleaners.  But leave the air cleaners off until you have finished all your adjusting and such for now.  

Eliminate vacuum leaks
Check or replace your carb to distributor vacuum line.  Mine had two breaks in it - a split by the olive/nut connector near the carb, and one under the clamp that fixes it to the back of the engine.  (While you are at it, throw away your old, dried out vacuum advance mechanism and order a new one from British Vacuum Unit - you won't believe what a difference it makes!)

Use new gaskets (they come in the kit) - the order is manifold-gasket-heatshield-gasket-spacer-gasket-carb.  The spacers don't come in the kit, and you should order new plastic ones from Moss if your old fiber ones are in rough shape.  You shouldn't need any sealer, but I like to use some Indian Head copper shellac on these gaskets - you don't want air leaks here.  The Volkes filter cans will normally have gaskets as well, but not a big deal so no sealer there.
Adjusting the idle and the mixture, and synchronizing the carbs

So, now the car is back together and we need to make it run.  There are lots of ways to do this, and lots of nifty different types of synchronizing tools you can use - but here is the method I like to use now.  There is more than one way to skin this cat, and my way is not gospel - but I offer it for what ever you think it's worth.

Don't connect the accelerator cable at this point, but do connect the choke cable.  Leave one of the inter-carb throttle connecting linkages loose and turn the idle set screw on each carb back out until it's off the carb body and the throat is fully closed.  (In this position the car should not be able to run - if it does, then air is getting in somewhere it shouldn't and you won't be able to control the idle.)  Turn the screw in until it jusssst touches down on the carb body.  This is the point at which any more turning of the screw starts to lift the arm.  The turn it one-half turn more.  Do the other as well, but don't tighten the linkage again (yet). 

Now - get yourself an analog dwell/rpm meter (Sears is a good choice).  Hook it up to the coil to measure engine RPM (1/2 of the 8-cylinder scale) and start the car (remember to pull the choke).  Get the car warmed up and release the choke, making sure that the jets are all the way home in the bottom of the carbs.  If you can't get the car to idle or run you may have to start with the screw one full turn in instead of one-half turn on both carbs, but they should be the same with the throttle shaft still disconnected in the middle.  Pick one carb and slowly turn the idle screw in until the rpm's rise - say from 1000 to 1200 rpm (not exactly - this is relative and an iterative process).  Then go to the other carb and turn that one in until the rpms get start to rise above that point, then back off.  Now, if either one is turned in at all, the rpm's will rise.  They are sync'd - so tighten the throttle shaft linkage back up with your 1/4" socket.  Now, turn the screws back out evenly so that the idle is around 800-900 rpm.  If you can't get to 800 rpm without a lumpy idle, then set the idle at 1000, and go back to your timing.  Loosen the dizzy nut, and slowly adjust the timing by hand so that you get the smoothest idle you can - not necessarily the fastest.  There will be a sweet spot where it just sounds "right".  Then go back and see if you can drop the idle to 800 without loping.  If your engine is a bit worn out, you might have to just live with the 1000 rpm idle.  (I had a devil of a time with this, and discovered I had the air leaks I mentioned above - and no amount of adjusting was going to help that.)

Please give me your feedback and let me know if I have made any errors or omissions.  I would love to hear from you, either here or by email.  Thanks for reading.


  1. Oh, no, Jim! Run away! Working on the car takes up enough time without stealing time to actually write about it. ;) Just kidding. I'm looking forward to following the details here.

  2. You have inspired me to attempt a carb restoration myself ... unless you want to do it for me, you have the tools ... give me a price.
    Thanks, Mike Greener Thomasville GA 229-224-5612

  3. I'm renewing my relationship with an MGA and the H4. What a great right up. Practical advice is what is needed and you've provided an abundance of it. Well done. If you have other info available such as photos or videos I'd love to grab copies.


    Brad Cronk

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