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Triumph TR6 - Mystery hole in top of SU carb

I've been reading all the threads, including the most recent one, on setting carbs with great interest. About ready to register car and want to try to smooth mine out also. Just to rule out obvious flaws I may have inherited, there is a threaded hole on the inboard side on the top of the back carb. The car came in pieces, so no idea what might have been there.

I have a '71 with a carbon cannister evap system. It has a hose on the forward side of each carb that goes to the can, with a T-fitting to the valve cover. That and the 1 small tube to the distributor for vacuum (advance/retard?) is all there is besides the fuel lines. The carbon can has only the one big line from the carbs, and a thinner one from the rear of the car.

The Bently book shows nothing coming out of the top, and matches the setup, showing nothing from this top hole. The Haynes book shows a setup that doesn't match at all so its useless.

Any of you with a similar setup know what this hole is for? Any advice appreciated.

Mark H

Mark--I have the same threaded ports on my 72. They are not used for anything except to make us wonder why they are there. Ignore them.

Rick O.
Rick Orthen

Mark are we talking about SU or Stromberg carbs?
With the description / setup it sounds like it is Strombergs which is OEM on the TR.

If it is the latter then the threaded hole is a vacuum line that is used for the EGR. HOWEVER this set up was only for carbs for cars 1974 onwards. Perhaps since your car was in pieces a alternate carb was included in the package. If this is true than there should be 2 connections to the canister from both sides of the carbs

Reading right off the top of the carb, they are Stromberg 175 CD-2. The Bently manual does list them as original for 1971 cars. Beginning in 1973-74 they list 175 SEV carbs, not these. So it seems they're original, but wouldn't match the 1974 setup.

I guess I'll see if there's any more feedback, but I can't help but like Rick's recommendation the best. Rick, did you even plug it up?

Maybe a good place for a cup holder if all else fails.


Mark H

The reason I asked about the originality is that you started the thread as "Mystery hole in top of SU carb" and first questioned is it SU or Stromberg?

1969-72 175CD-2SE
1973 175CD-2SEV
1974-76 175CD-2SEVX

However regardless of years, they are all stamped 175 CD - 2 and there is subtle differences between all of them, thus my question about originality is perhaps the hole in the top of the rear one just before the intake manifold is a later version fitted for the vacuum of the EGR valve that a previous owner exchanged over the previous years. Roadster Factory parts catalogue has a detailed explaination of each version. If you need a jpeg I can scan one for you.

You're right about the thread name, wrote it too late at night after reading several others...the SU stuck in my head. Charlie Ballard emailed me pix of his setup, and the hole clearly is located where he has a hose going to what I assume is the EGR valve I see you guys talk about, on the top of the head toward the rear. I don't have an EGR on my car.

So I guess I'll either plug it or leave it, and see how it idles.

Thanks again,
Mark H

Mark--Have a cold one. If you look closely, you'll note that the connection is blind; no need to plug it. Now, about those brittle throttle shaft seals . . . .

Rick O.
Rick Orthen

Talk about DE Ja Vu (spelling ..I am not french)re ZS/SU carbs. Mark, '71s do not have all that extra emmisions junk like later years (EGR valve).I agree with Rick O. My back one looks like it was drilled through and has a brass plug in it so do the same..plug it. Your setup is exactly 1971. Also make sure that where the hose connects to the canister that there is a small piece of plasic inserted that reduces the amount of air sucked from the canister. It acts like the "restrictor" piece of plastic that the hoses attach to on the carbs (TRF, Volumne 1, AX78). I am one of the lucky 1971 owners that has 2 vent holes on the top of the gas tank...interesting set up back there.
Rick C
Rick Crawford

This thread was discussed between 08/05/2002 and 10/05/2002

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