Carburetor Re-Rebuild

Well, it's been a while, hasn't it?

Lots has happened since we last spoke.  I finished Alice the MGB and both Alice and Gidget went to the 2022 BWOG.  They had a great time.  Juliette drove Alice and took third place in her class!  

Gidget took 6th in the Heritage class, which is a fair showing given I didn't really have time to prep her (because I worked up to the day before to get Alice ready).

So it was a great time.  Here are the two girls together after the show.  This is one of my favorite pictures.

Anyhow, this article isn't about that.  After the show, I noticed that Gidget had a ticking noise coming from her engine.  It had been getting worse, to the point where I stopped driving her.  So I finally broke down and pulled the cylinder head as I suspect the noise is a sticking valve.  I'll talk about that later.

Since I had the head off, of course the carbs had to come off too.  I figured I'd go ahead and go through them while I had the chance, to see how they've held up.  I have to say that they look good, but there was play around the throttle shafts that I didn't like.  So they came apart for inspection.

I took my handy bore gauge set and measured the throttle shaft bore in the carb body.  (You can get bore gauges for really, really small bores like this.)  And wouldn't you know it - they were actually out of round by .002".  The standard throttle shaft measures .248", and the bores measured a max of .254" on one carb and .252" on the other.  That's an air leak, folks.  It explains why it was so difficult to get Gidget to start without a lot of choke.

I had two options:

• I could bore out the carb body and install bushings to return the clearance to standard;
• I could bore out the carb body and install oversized shafts.

The HS2 carburetor does not have bushings for the throttle shaft.  Rather, the shaft rides directly on the aluminum body and is a sacrificial item.  The carb bodies don't really wear since brass is so much softer than aluminum - but almost 60 years meant there was a little, and it was enough.

I decided to go the oversize shaft route.  The shafts measure out at .258" (.010" oversize), and I confirmed that with my micrometer (which also confirmed that my mic was accurate).  That meant I needed a reamer.  Fortunately, I read a post that a 6.6mm reamer (.260") would work, and was available from Amazon for under $10.  Turns out they were right!

I also needed a vise that would hold the carb body.  Harbor Freight solved that for me, and I got a new tool.  I have a small bench top drill press that I was able to manipulate into being able to hold the vise and reach the other side of the carb body with the tip of the reamer.  Perfect.

I used the old throttle shaft as a guide to line up the carb body with the drill, and then swapped it for the reamer.

As Bob Ross would say, it was time for my bravery test.  I got some cutting fluid, turned on the press and slowly cut through the carb body.  And it worked.  I measured out the new bore at a nice round .261", which is a little more than I wanted but still okay.  And best of all, the new shaft fit like it was always there.  The alignment was perfect.

While I was busy patting myself on the back, I got another really good, but maybe bad idea.  I had seen a post on MGExp that talked about throttle shafts with O-rings installed.  I found those shafts, and I entertained buying them... but I already had two perfectly good new oversize shafts and that's what I committed to installing.  And yet, the idea beckoned... so I decided to really risk the biscuit and cut grooves for O-rings.

I practiced on the old throttle shafts, of course.  I have a kit of Viton O rings, and the A006 size is what I worked out I wanted.  The A006 is a .254" outer diameter ring and pretty thin.  Through trial and error, I found that cutting the throttle shaft to 0.148" would give me a completed outer diameter of .262", because the ring stretched a bit and thinned out once installed.

So more bravery tests for me... I marked out where the O rings needed to go.  I chucked the throttle shaft into the drill press, spun it up and with a thin file cut the grooves ever so carefully.  I first used a file with a sharp edge and slightly rounded back as it helped make a little clearance.  I followed that up with a flat, square edged file to smooth everything out.  This took quite a while and lots of measuring with my micrometer.  But I got there, fitted the O ring and it was precise.  I did the other side, and marveled at both my bravery and my work.

I lubed the O rings up with a bit of lithium grease and fitted the throttle shaft into the carb body.  It fit, first try!  I spun the shaft in the bore a number of times and it loosened up a bit, enough that I could spin it by hand with only slight resistance.  That's ideal.

After that, it was simply a matter of cleaning and reassembly.  And I was thrilled with the result.  There is no WAY that there can be an air leak now.  And all I need to do is replace the O ring for periodic maintenance.

I installed new throttle discs too, and everything's sealed up tight as a drum.  There's just the tiniest smidge of light coming through with the throttle fully closed. 

Of course, that was just one carburettor.  I had to do my bravery tests all over again for the second one.  But the second one turned out as pretty as the first.

All in all, it took me three hours including disassembly and reassembly.  I think Gidget's carbs will be good for a number of years.  Now I just need to get her cylinder head back, and installed, and everything put back together, and hope that the tick is no more.