MG TD TF 1500 S.U. Carb Drop Test (Bell Chamber) Repair BBS discussion at MG-Cars.info

MG TD TF 1500 - S.U. Carb Drop Test (Bell Chamber) Repair

My carbs had been running rich, even with the jets twisted fully home, and I knew why - my bell chambers and air valve pistons had been aggressively 'cleaned' by previous owners one-time-too-many and they both failed the 'drop test' - took about 1.5 seconds to drop and they should have been 4-7 seconds. (Because the pistons wouldn't hold enough vacuum to lift the air valve piston properly, there was too much vacuum over the fuel jet orifice, and therefore the car ran rich no matter what I did with the jets.) So... how to fix this? Well, first, you need to know I'm basically really cheap. I am, after all, of Scots/Irish heritage. (Which also means I like to drink, but I hate to pay for it). So, buying new bell chambers and air valves seemed kind of an exce$$ive purchase. Putting the thinking cap on, I ruminated over a potential repair and I came up with two options. I happen to have brass air valve pistons in my carbs (with no springs), and there's really only a little bit of wear that needs fixing to make these puppies work like new. The two options I came up with were either (1) apply a thin paint layer to the sealing surface of the air valve pistons or (2) apply a thin solder layer (essentially just 'tinning') to the sealing surface of the air valve pistons and then, regardless of method, dress the repaired surface of the pistons down so that they are a snug fit in the bell and then use a fine abrasive to match-hone them into their respective bells. I chose to use paint because (1) it would be easy to remove safely if it failed and (2) it would be easiest to apply and to hone. I removed the air valve pistons and bell chambers and washed both in the parts washer, then wiped them down with lacquer thinner and gave them a final wash with dish soap and water... whistle clean, grease free. I then masked off the lower portion of each air valve piston (I didn't bother to remove the needles, and was *exceedingly* careful not to bend them), and I used a cardboard toilet-paper center to mask off the upper portion of the air valve piston (inserting it into the top of the piston). This left only the ribbed sealing surface exposed. I then applied two thin coats of 1200 degree (F) silver engine paint - it's quite resistant to gas and oil - and let it dry thoroughly. Once the paint was dry I test fit and found that the pistons would not go into the bells - too much paint was applied (which I expected). I chucked the air valves into the drill press (using the steel damper tube) and used a very fine flat file to lightly dress the paint down just a bit, repeating this process until the piston began to enter the bell. I then applied a little automotive polishing compound to the inside of the bell and to the piston's sealing surface and began to rotate the piston inside the bell, honing the painted surface until the piston was able to move freely all the way around and fully up and down the piston's travel. Once honed to precisely fit, I then cleaned the parts again with soap, warm water and a toothbrush, removing all traces of polishing compound, and dried them completely. Upon repeating the drop test the pistons are now at around 6-7 seconds - Perfect! I installed them and my mixture issues are *over*. The jets adjusted within spec and the car now idles smoothly. I'll be tracking this repair to see how long it lasts, and I'll report back at some point to let everyone know if it's successful. Given the paint I chose, I suspect it will last quite some time. After all, in service the outside sealing surface of the air valve piston should never actually touch the bell chamber so wear should not be an issue, and it's engine paint so it shouldn't be spoilt by gas or oil, either. Picture below is after painting and honing.

Kevin McLemore

This thread was discussed between 30/09/2014 and 02/10/2014

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