Classic Mini Tech: SU carb plug

SU Carb with nothing on itJohn Koo (of sracer.com) recently explained one of the quirks of classic Mini ownership, the emissions test port on the SU carburetor — well, most people don’t even give it a second thought, there it is on a small pipe hanging off the end of your carb. They use it for smog testing, as if the cars we drive ever get smogged (since the are so old). When the carb is not attached to a test apparatus, there’s usually a rubber plug stuck on it to block the port. If that happens to fall-off, get lost, or whatever, you have a potential problem. You see, this is where stuff gets into the carb, you get an improper mixture then way down in your engine you can experience detonation.

The Wikipedia states in it’s usual reference like verbiage “Detonation in reciprocating engines (sometimes called engine knock) is the uncontrolled supersonic explosion of the fuel-air charge, and may be caused by excessively high combustion chamber temperatures. It may occur prior to the firing of the spark plug in Otto-cycle engines. High compression ratios or high temperatures can cause the flame speed of combustion to exceed the sound barrier, resulting in an explosion or detonation and a pressure shock wave.”

Although that sounds relatively cool in a mythbusters sort of “I like to blow stuff up” scenario, it doesn’t bode well for a Mini’s little high-compression engine. Wiki goes on to describe the issue in better detail:

“Detonations are extremely destructive to common piston engines, and may result in mechanical deformation of the cylinder walls, piston rings, holes blown through the top of pistons, or cracks in cylinder heads. Detonation can also burn out a spark plug, reducing or eliminating its ability to produce a spark. Detonation is most destructive if it occurs during times in which the explosion forces the piston downwards while it is traveling upwards, which relates to engine timing.”

What does that mean to you? That means you probably want to do something about this hole in your carb so as to help avert tragedy from occurring, even if the likelihood isn’t in the extremely high probability, it is definitely there.

Enter the plug. This was designed and fabricated at SRACER PRODUCT for your Mini. It’s fairly small and straight forward to install, and can be done in less time than it takes to read this article.

Step 1:

Grab a good pair of vise-grips and pull the little pipe out of the carb.

Be sure you get a good hold on it, sometimes they are a bit stuck and need some elbow grease to get out, but they will come out.

If that fails, you can always tap in a screw or bit and extract it out that way, but for the most part it just comes right out with some force. If I had pics of someone messing it up and getting the pipe all pinched up in there or what-not horrors people can do to their Minis, I would include it here, but I don’t have such a picture, so you’ll just have to use your imagination on that..

This picture doesn’t seem to format correctly in the blog and I have no idea why, but there you go, lots of white space. Anyway, let’s move on, shall we?

this is where the plug goesStep 2:

Well now that you have that gaping hole there, you want to tap in the SU carb plug to patch it up.

This is a fairly permanent operation but it’s not like you are going to mess it up, since it’s so straight forward to push it in.

These pictures show someone pulling it out by itself, but generally you do this with the carb already installed on the vehicle, because all of that stuff bolted in helps hold the manifold and carb and all that, making it much easier to do.

If you want to do this to a SU carb before you put it in the engine, hmm, probably either want to wait until it’s on there, or if you really can’t wait, then get a buddy to hold the manifold or whatever stuff for you so that you can do it (unless you happen to have 4 arms, then, well you’re good to go on your own).

You’ll also notice in these photos there’s a stand-off and a heat shield on there where it connects to the manifold, yep that’s another SRACER PRODUCT invention.

Su carb mounted on 1968 mkII Mini with sracer bitsThe spacer helps give you more clearance for the carb in the engine bay (for mounting your filter) as well the heat shield, well I hope I don’t really have to explain what that does 🙂

One more pic before we close this bit of tech up for the moment and go grab a homebrew from the fridge.

Here’s the plug, installed on a car (that’s Henley, the Morris MkII from other videos and pics!) and I am so glad I won’t be worried about detonation at worst and suffering performance at least..

More Mini tech articles like this are in the works, so stay tuned to Miniology!

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