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MG MGA - SU rebuild time

Ok, I admit it I a have a problem: if I haven't rebuilt it, I assume that something is wrong and feel compelled to rebuild it myself.

With that I've opened Pandora's box.

I've been having trouble with my rear carb not getting lean enough.

So in a fit of madness I've taken them out and intend to rebuild them in the next 3 evenings.

I've reached out to Joe Curto to discuss what parts I need to order.

This is what I've got so far.

1) Obviously a rebuild kit.

2) Tonight I will disassemble and check the shafts. If worn I will get new shafts. Question here, if the shafts and holes are worn can I drill them out myself on a standard drill press, or is this best done by a machinist?

3)Any other hints or tools I need. Is the centering too a must have?

4)Are the damper springs problematic? Do they need replaceing.

5)Any other signs of wear I need to look for?

Thanks mucho
T McCarthy

You can drill them on a drill press, the hard part is getting the carb body supported properly and lined up correctly so that you do bore straight thru the cenerline of the throttle shaft. You can get standard shafts and bushings to install or you can get 0.010 oversize shafts if the wear is minimal. The machinist is much cheaper then a pair of carb bodies. If you aren't too far away you could also take them to John Twist in Ada. MI or send them to Joe for machining, but any quality machine shop could do it for you.
J Heisenfeldt

Or you can just lap in the new shafts with some wet and dry. I've done that successfully a few times if the carbs are not too worn. If you keep trial fitting till everything is snug and not to tight, it's less risky than a drill press........
dominic clancy

What drilling are you asking about?

If the bodies or throttle shaft sleeves are worn too much then the old ones will have to be drilled out to just undersize of the new sleeves and then reamed to fit. After the sleeves are installed they are align reamed for the throttle shafts. In drilling out the old sleeves you have to be careful not to drill right through. There is a retaining lip in the casting. Drilling is best done on a lathe with the opposite shaft hole centered on the tailstock to keep both sides aligned. You will need two reams with extensions that pass into the opposite shaft hole to keep things aligned when reaming. As I recall the reams were 3/8" and 5/16". I believe some early carburator bodies did not have sleeves and the shaft runs in the aluminum body. These can be drilled and have sleeves fitted.

Or are you asking about drilling for the securing pin? This can be done with care in a drill press after everything has been assembled and the correct point carefully marked and centered punched.

Lapping in is only suitable if the wear is minimal and oversize shafts will be sufficient.
John DeWolf

John,

I was referring to the throttle shaft bushings. However I just got it apart and see your point.

How does one decide if they need to replace the throttle shaft bushings or go with a larger throttle shaft?

There was some play as I wiggled the shaft back and forth but I would call it modest. I even tried blowing air through the bushing with the shaft in there and I couldn't do it.

Is that a candidate for oversize shafts?
T McCarthy

It sounds to me like oversize shafts might work. There are some instructions for these operations in older Moss Motors catalogues. If you don't have one email me and I will send you a scan.
John DeWolf

SU's are the simplest and most reliable carburettors known to man but if they are not in excellent condition your car will never run properly and will be disappointing on the road. I used to fiddle them up myself (ie when the cars were "newer"), with varying results, but now get them properly overhauled by someone who knows and understands them (many "experts" do not!). Inability to make them lean enough is almost certainly due to worn jets and is an indication that a complete overhaul is warranted.
Barry Bahnisch

When I rebuilt mine, I used the Burlen Service kits (CSK40 x 2). These have all the gaskets, seals and jets in, but not the needles or dashpot springs - you should replace both of these. They also do Rebuild kits, and these have everything in if you need to replace the throttle shafts (Moss and B&G both do a reaming tool for the throttle shaft bushes). The Burlen kits come with a very comprehensive instruction sheet, which is easy to follow. An old T-Type tip is to soak the cork jet seals in light machine oil for a few days before fitting, and this works well for me. When reassembling, take care to ensure that the choke operating rod is in the correct hole in the fast idle cam - this makes a BIG difference to how the choke works in your climate - (and there is absolutely nothing about this in my version of the Works manual!!). The gunson carb balancer and Colourtune are, for me, essential for tuning, and both are easy to use. Good luck, you should notice a difference! AB
A Bennett

Thanks folks.

I spoke with Joe Curato. Very helpful. I'm going with the oversized shafts. He gave me the specific reamer to use and I'll chuck it up in a friends lathe turning the chuck by hand. Should be quite easy.
T McCarthy

The problem with not be able to get the carb lean enough could be caused by the cork seal (Moss #295-000)not being fully compressed. This prevents the jet from being raised high enough. The seal should only be about 1 mm thick when installed and it does help to soak in oil over night along with the smaller jet seals as mentioned. Also, check that there is only 1 washer under the upper jet bearing, as an extra washer can prevent the jet from being high enough.
Drilling the hole for the throttle stop can also be frustrating.
Berry Price
BTP Price

This thread was discussed between 02/06/2008 and 03/06/2008

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This thread is from the archive. The Live MG MGA BBS is active now.