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MG MGA - carb issue

ok, you guys were right. I write most recently on a rough idle that I thought was ignition. Well, replaced everything on that side only to migrate over to the fuel side again. It's getting fuel as the side bowls are full, but it seems as though running lean. I am on a budget and don't know if I am adventurous enough to rebuild them....any thoughts? I would love to plop a rebuilt set on there. I just want to drive her in this pretty fall weather!

For now...I am going to set the adjusting nuts all the way up and work down. We'll try adjusting them again.
Jim P 56MGA

Hi Jim. Rebuilding SU carbs is fairly straight forward, as long as you have a good manual with an exploded view of the carbs, and know a few essentials. Essentials are to first check the throttle shafts for wear. There should be very little wobble where the brass throttle shafts go through the aluminum carb bodies. If there is lots of wear, then the bodies need to be fitted with bushings. (Not a do it yourself job) If the bodies are not badly worn, then a rebuild is relatively easy. Remember to soak all the cork seals in oil for a few days prior to the rebuild. Soaking softens the cork seals, and make s them pliable. If they are not soaked until soft, they will cause many problems. The large cork seal at the bottom of the jet needs to be compressed A LOT. When the nut is tightened properly, the visible part of the large cork seal should only show about 1/32 inch of thickness. If the gasket is not compressed this much, you will not be able to adjust the mixture properly. The rest of the carb rebuild is fairly simple. Remove most of the dirt and grime with solvent. Dismantle the carbs. ( Keep the parts you remove laid out on a clean surface. Keep the parts in order, to assist with re assembly. Double check with the manual when re assembling to make sure that the parts go back in the correct order.) Thoroughly clean all parts and check for excessive wear. Re assemble the carbs, replacing any worn and usually renewable bits such as jets, metering needles, gaskets, etc. Additionally, on disassembly, be on the lookout for any EXTRA PARTS that should not be there! With my own carbs, I found several extra cupped brass washers where the two small cork Jet O rings were located! These extra parts were removed and discarded. Once the carbs are reassembled and bolted back on, they require adjustments and balancing. Again this isnt terribly difficult, and can be done with a manual, small jet wrench, slot screwdriver, and length of heater hose. (The heater hose is used as a stethescope to lisen to the air hiss at the air intakes. Hope this helps.
Glenn

Glenn,
thanks for the tips....I may make this my winter project!
Jim

Do check out the Air Piston/Dashpot assemblies for what is called Piston Drop. Below is a commentary of mine on doing this check. It is just as valid for H-4 carbs as it is for later SUs and after 50 yrs of use, can be revealing of why we sometimes chase our tails trying to get a set of carbs to function properly.

"While you are doing all the necessary things to ensure a good rebuild, also check that air pistons are working as they should. This is something that is NOT always mentioned in the leaflets that come with SU repair kits or even the SU shop manual, but it cannot hurt to do the test and it gives you the assurance that ALL is well with both your carbs before you start to do a tune up.

"Drop Test? It is covered in the Emissions section of the Bentley shop manual for MGBs, but a short description is that it is conducted by removing one air piston and (bell shaped) housing from its carb body, draining the oil from the dash pot, removing the air piston to remove the return spring inside, and then plugging the holes on the bottom of the air piston with modelling clay or plugs (even chewing gum works), putting the air piston back together with the housing (NO return spring!), and then measuring the amount of time it takes the piston to drop (hence, the name) while the assembly is inverted. As I recall it should come in at about 6-8 seconds.

"What you are measuring by doing this is the clearance between the air piston skirt and the inside of the housing. Air will escape at a certain rate in relation to the very fine (in the ten thousandths of an inch) gap, and too big a gap can indicate that a PO may have sanded, polished, or burnished the skirt and/or inner housing causing too big a gap and too slow a rise of the air piston as vacuum is applied to it from the engine. This is not always critical, but it does sometimes explain odd needle choices or the inability to get the carburettion sorted out. The test can sometimes also demonstrate that the housing or the skirt has been bunged up by being mishandled and catches at some point as it falls. This is not a good thing either. What it will do in free fall, it may also do when in use.

"But perhaps the most important thing checking both air piston assemblies shows you, is whether the two you have on your car even come close to responding the same at a given vacuum point. If they are not even close to each other because one falls significantly faster than the other or one gets hung up and the other doesn't, it is almost a sure bet that they will behave differently, one to the other, when you try to set them up and this will frustrate all the rest of your good work and new parts you put into your SUs to get your engine to run as it should.

"I don't know why SU did not cover this sometimes in their manuals or rebuild part literature, but in the era of emissions controls, it became important to make sure that both mechanics and owners had an awareness that such a test was important. If your carbs became out of kilter with each other, it was guaranteed that your car would have a tough time meeting emissions testing and so they had to teach us all something new about these carbs and how to ensure that a rebuild was both worth doing to a set of carbs, and also successful."
Bob Muenchausen

My version, with somewhat more detail on Bob's excellent advice, in the following thread:
T&thread=2006082423554020653
*****************
Posted 24 August 2006 at 23:55:40 UK time
don scott, Oregon, USA, donscott10@comcast.net
FRM
FR Millmore

Interesting learning exercise, FR.

Unfortunately, we can't always just buy a rebuild kit and put in new throttle shafts. There are other wearing parts, such as the Dashpot/air pistons which normally don't ever get replaced or checked, and if they have problems, their problems will continue to plague a carb long after the supposed rebuild.

Some of the problems I have seen relating to these parts are:

1) Carbs, which were installed OE as "matched sets" with similar air flow characteristics one to the other, are no longer matched. Someone replaced a (supposed) bad carb of a pair with one from another similar (or not so similar) car or out of the "good but used" bin and the matched air flow is lost.

2) The Dashpot/air piston of one or both carbs have been replaced. Carb might have been dropped, soft aluminum dashpot gets a dent and piston binds up, therefore it alone gets replaced. God knows from what source.

3) Well meaning owner (I did this very thing when I first got a twin carbed car in the 60s, how do you think I learned??) takes both carbs off of engine, dismantles both and throws both into carb cleaner can/tank. Air piston/dashpot parts get mixed up and whoopee! won't we have fun! Almost as much fun as finding extra parts in the tank.

4) After 50 yrs of heat cycling, overtightened bolts and nuts, and a few other possibilities, metals like aluminum can distort. Also, as mentioned above, they can have a little human help as well. Polishing or cleaning the inside of the dashpot with a scotchbrite pad or emery cloth can work wonders when you consider that most folks do it by hand and that the knuckles of your fingers create pressure points causing uneven wear while you are doing it. This is why body men use a rubber or other sanding block for a flat finish. And when you are dealing with clearances on the order of ten thousandths of an inch, it don't take much to leave a lasting impression.

Bob Muenchausen

Comment on Bob's comment.
1) I think all same series carbs are the same basic airflow, ie, they aren't matched as such. But the pistons and chambers are matched.
2) Therefore this doesn't matter, as long as each matched piston/chamber set is intact.
3) Yes, and as I noted, while this appears to be a 50/50 proposition, 75% of cars have the two sets mixed up. Switching almost always fixes it, then I mark piston and chamber as to where they go, and in the case of H*1 or 2, which side of the chamber goes outward.
4) The two evilest things in an SU's world are abrasives on the air chamber and piston surfaces, which can't be fixed, and dings caused by people hitting the chamber with a wrench, usually to stop flooding. (what? you say. Seen 'em do it!. This can sometimes be fixed by very careful scraping with a sharp blade. Either is a better reason for the, or at least, broken arm, penalty than most things.
FRM
FR Millmore

Comment on Bob's comment.
1) I think all same series carbs are the same basic airflow, ie, they aren't matched as such. But the pistons and chambers are matched.
2) Therefore this doesn't matter, as long as each matched piston/chamber set is intact.
3) Yes, and as I noted, while this appears to be a 50/50 proposition, 75% of cars have the two sets mixed up. Switching almost always fixes it, then I mark piston and chamber as to where they go, and in the case of H*1 or 2, which side of the chamber goes outward.
4) The two evilest things in an SU's world are abrasives on the air chamber and piston surfaces, which can't be fixed, and dings caused by people hitting the chamber with a wrench, usually to stop flooding. (what? you say. Seen 'em do it!. This can sometimes be fixed by very careful scraping with a sharp blade. Either is a better reason for the death, or at least broken arm, penalty than most things.
FRM
FR Millmore

This thread was discussed between 10/09/2006 and 13/09/2006

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