So, carburetors seem to be the story of my life at times when it comes to cars.
Last summer, the MG was parked for about 6 weeks when what I thought would be a simple head job ended up with a lot of complications. After throwing away a bunch of money on custom pushrods(if anyone wants a set of super light weight, normally $16 each pushrods for a BMC B series engine that are 3/4" shorter than stock, let me know), I ended up getting it running with some brass shims under the rocker pedestals. They were made from a $15 sheet of brass from McMaster Carr and I have enough to make about 4 more sets if the need arises.
In any case, in that time my gas went bad and I ended up with a nasty mess in the carbs. I cleaned them up, but I still had some issues. SU carburetors do not have a conventional "choke plate" as do American caburetors, but instead have a mechanism that drops the carburetor jets when the knob is pulled. This richens the mixture for easier starting. Whatever junk clogged the carburetors caused the jets to seize solid in the bodies.
These carburetors have known issues with wear in the throttle shafts that can cause vacuum leaks, uneven idle, and all sorts of problems. The only proper fix is to ream out the bushings in the body and replace them. Unfortunately, this really isn't practical as a DIY job as you not only need the correct size reamer but also a jig to correctly hold the bodies and ream the bushings parallel. The go-to guy for rebuilding in the US is Joe Curto in New York City, but he runs about 3 months.
Since I didn't want the car down for that long, I called a friend and he sold me a set of carbs off his shelf. They were on an MGB manifold and had MGB linkages, but I don't think that they were actually MGB carbs(for reasons I'll get into in a minute).
In any case, I cleaned them up, but when I stuck them on the car one of them leaked like a sieve at the bottom of the float bowl. Some diagnostics led me to a rubber seal that was missing, and unfortunately it's a weird enough part that I actually needed to get it from a supplier and not improvise(even though I tried). I called Joe Curto, and in addition to ordering the parts decided to go ahead with a "light rebuild." I bought a gasket kit, needles, seats, floats, and throttle disks. The last are a different story-basically the carbs had poppet valves in the throttles for emissions reasons, and the poppet valves cause problems. It's best just to replace them with solid disks.
In any case, I got the second set cleaned up, put together, installed, tuned, and the car ran more or less okay. The problem was that the my car(and distributor) were set up for ported vacuum advance. See the discussion a few pages back about ignition timing-the vacuum advance is used to adjust for load. Small engines like this really work best with ported advance(vacuum drawn from just in front of the throttle plate) but they switched to manifold advance for emissions reasons in '72 or so.
Without vacuum advance, both acceleration and fuel economy suffer. Finally, a couple of weeks ago, after talking to the guy who built the distributor, I connected the vac advance to a manifold vacuum source. It added an easy 5mpg on the interstate, but came at the cost of a poor idle.
In the mean time, I've still been sitting on the old carbs that do need to be sent off for a rebuild.
With that said, when I was searching for a manifold advance nipple, I remembered the box of carbs in my pantry(yes, I'm a bachelor). I found the nipple I needed on a spare manifold, but also found this
This carb is pretty well junk, but it has a nipple for ported advance. So, I decided to clean it and just transplant other parts onto the body. Of note, though, is the fact that this also has a solid throttle disk.
So, I pulled it apart. The body is pretty junky looking, but is functional and can be made to look better
I degreased everything, which I didn't show but basically I boiled it in methylene chloride and worked into the cracks and crevices with an acid brush.
Then, it was on to my "magic" derusting solution. This is basically hot acetic acid and EDTA, and does a good job of removing rust fairly quickly while leaving good metal intact.
After that, I went through a few washes of a little bit of bicarb in water to get rid of any remaining acid. Then, I boiled in IPA for a final degreasing and to "chase" water out of any cracks and crevices.
This left me with a clean but dull looking carb body
I oven dried everything overnight.
Then, it was time to wire brush everything to try and get it shiny. I'm still not happy with the body, so I'll probably tumble it in walnut and see what that does.
At this point, though, I had a bit more work to do. The base where they meet with the manifold gasket(or really the phenolic spacer and heat shield) is typically not flat, and it can introduce vacuum leaks.
So, it's time to start sanding. First of all, I took a sharpie and colored the base of the carb. This serves as a way to monitor the sanding progress and see when "low spots" remain.
When I'm trying to get a flat surface, I put the sandpaper on a sheet of glass. Also, I ALWAYS sand metal wet. This gets really important with aluminum as the paper will quickly gall if dry whereas the fluid(whether oil or water) will keep the mixture of metal and abrasive in suspension. I normally prefer oil for a nicer looking finish, but oil can clean the sharpie off so kind of defeats the purpose of it.
Continued in next message
