Carb Refurb & Rebuild

Happy New Year!

You might recall that I had started to dismantle the twin SU’s for the Spitfire earlier in the year.  Predictably, life & work got in the way and it’s only now with a short time off over the festive period that my mind has once again wandered towards the garage and the job(s) in hand.

I had a quick browse on the ‘interweb’ and came across an article on the ‘North American Triumphs’ Facebook page covering the rebushing and refurb of the carbs from his Mk1 Spitfire which he presented in an easy to follow photo story.

I didn’t write the article and the credit is not mine but should go to the North American Triumphs page!

I liked the way it was presented, the content and the photography – clearly a talented chap who also runs the Vintage Triumph Register of North America.
I’ve reproduced the blog below for your info but it’s worth your while following the links to see what else is available.  Isn’t the ‘interweb’ a great source of info for our hobby?

Refurbishing & Rebushing Twin HS2 SU Carbs

Today is a relaxing day…

HS2 SU carb rebuild day. I’m at the garage of Mr Triumph himself Chuck McGuire. I’m using his fine media blaster to clean these twins up and then he’s going to show me how to rebush the throttle shafts. He’ll show me how in a way in which anyone can do it. This is a step by step on rebushing HS2 SU carbs. all you need to do this is a simple vice, flat table, and a good drill press. His knowledge is invaluable and in an effort to share some of that knowledge, we wanted to show you it is possible for anyone to do some of the stuff that seems a little scary.

Each photo has a caption. Follow along…..

Here is the start. These HS2 SU carburetors were in serious need of a complete rebuild. The throttle shafts were very loose and clicked when wiggled in all directions. If the throttle discs don’t make a good seal and the air escapes from the loose throttle shafts on either side of the body, then these will not operate correctly and will be very difficult to get a set an even idle.

We have a couple dirty carbs and a bunch of new parts. All these parts seem to be great quality, boasting the “Original SU” parts company. Thanks Moss Motors!

Before cleaning. We want a length of 1/4″ brass rod. We’ll show you why in the next photo. Ours is about 4″-5″ long.

This is what we are going for with the brass rod. What we want to do is hold the carburetor straight in the vice to allow an exact alignment of the bushings we are about to bore into the carb bodies. The length of rod is important at this point because we will clamp it in the vice at the exact height where we can drop the carb down onto and using the drill press to bore out the hole with the exact outer diameter of the bushings we bought. Once the bit hits the brass rod, we are deep enough. More on that in a bit. First things first, let’s clean them up.

About a half hour in the media blaster cabinet, it is looking great. I made sure to put each piece in the cabinet individually to allow all angles to be blasted. Can you tell which one is blasted? ha ha

One thing to note; this is not sand blasting, this is media blasting. Much finer and much less abrasive with less damage to the piece. Meant for polishing.

A closer look. These are looking better than new. All new seals, jets, floats, springs, bolts, etc and this will look really nice and better yet; it will operate as it should.

With the brass rod clamped in the vise to the exact height and straight to hold the carb true, the 5/16″ bit needed for the bushings to bore into the body until the bit touches the brass rod. That way it allows a shoulder still and we can go too far.

So using the brass rod method, we slid the carb onto it and rested it on the vise, brought the rod down to where it still had a little bit in the hole to hold it straight. Then we tightened the vise to set the rod. Using the drill press, we got a sharp 5/16″ drill bit and bored the hole out until we hit the rod, leaving a lip. The depth and bore of this hole should be measured and calculated by the bushing you buy. Some come as brass bushings, some are stainless steel. Measure twice, drill once!

Using a bolt that fits into the bushing perfectly, light taps until the bushing fully seats to the shoulder we left in the body.

The other side is a little more difficult as it is not as simple as just flipping it over to bore the other side. Due to the vacuum advance outlet tube, a nut makes for a perfect spacer. The rod height needed to be reset, but the same drill press process applied.

Another angle of the spacer nut resting under the carb and the rod reset to the correct height.

After boring the second side and both the bushings were tapped in, the shaft needed to be inserted to test for any burs or rough spots. It should rotate freely with no roughness.

The front carb’s throttle shaft was perfect. No hang ups or rough spots. The rear shaft had a very slight rough spot so an old shaft with some valve grinding and lapping compound made light work of the rough spot.

Here is the grinding and lapping compound used to smooth out the rear carb’s throttle shaft. We started with the blue and then moved to the red. Worked like a charm. Smooth rotation on both throttle shafts means the next step is reassembly of the pretty, new carbs.

Discs installed and adjusted so it makes a good seal. Looking up at the light should show that. This may take a time or two. When you have it right, don’t forget to bend the screws on the back side of the discs to prevent them rotating back out.

Second carb’s throttle disc done. This turned into a couple of nicely operating carbs. There’s a lot to do to the rest of the car but we are starting with the fuel system since the fuel tank is new.

I hope you found the article interesting and informative.  We have a wealth of tips, knowledge and experience within our club so if anyone fancies taking some pictures and writing a similar article it would be welcomed with open arms! (and we can supply any help required).

All the best!




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