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Another SU question

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seminolecpa Bryan S
San Mateo, CA, USA   USA
I have SU carbs on both of my 67 MK3’s, however, one of them has some type fitting for a breather hose (I believe) that runs between the 2 carbs. Can’t seem to find any pictures or information on how or where they should be run. The attached photo is how they currently sit. The other car does not have said fittings.

Thoughts?

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jimgt6 Jim E
Seekonk, MA, USA   USA
looks like the carbs are from a later Mk4


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spitfire50 Avatar
spitfire50 Paul Mugford
Rochester, N.H., USA   USA
Bryan,
Years ago I spent the money to buy a brand new pair of HS2 SUs for my Mk2. They came with that same port. At first I plugged the port with a Wellnut, but they had a habit of disappearing. So I finally removed the tube, tapped the hole, and Loctited a set screw in there. I was easily able to do this because my car is already equipped with a Smith's valve to handle the crankcase ventilation. Your engine seems to have the Smith's valve too, so feel free to plug those ports.
All the best,
Paul

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seminolecpa Bryan S
San Mateo, CA, USA   USA
Do you see any issue with running a hose between the 2 as it currently sits? Was thinking the same thing with the Smith's valve and that they essentially serve no purpose.

Other odd thing between the 2 cars (both are early MK3's) is that the throttle linkage is different. One is the same as someone was asking about in an earlier thread (with a throttle stop as well as the throttle lever) and the other is more like a bellcrank lever and angled piece in the middle of the connecting rod with a hole to connect to the throttle. Having a hard time determining between the 2 cars which is the correct one and which one may have been upgraded/replaced.

Born Loser Avatar
Born Loser Silver Member Matthew Taylor
Land O Lake, FL, USA   USA
The carbs tend to run rich at idle with the hose not pulling a bit from the crankcase. If you tune them plugged and get the mixture right at idle, you will be lean under load at rpm.



Matthew
1960 Triumph TR3a
1970 Triumph Spitfire MK 3
2012 Mini Cooper SS Convertible
2018 Jaguar F-Pace

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spitfire50 Paul Mugford
Rochester, N.H., USA   USA
In reply to # 1526850 by Born Loser The carbs tend to run rich at idle with the hose not pulling a bit from the crankcase. If you tune them plugged and get the mixture right at idle, you will be lean under load at rpm.

Matthew,
That hasn't been my experience.
The original HS2s didn't have that problem with no air bleed from the crankcase.
I'm not sure connecting them together is a good idea, because they enter the carb's "constant depression" area and any imbalance between the carbs will mean airflow through the hose to the carb with the lower pressure in that area. Control of the pressure in that area also controls the pressure over the jet and affects the piston lift.
All the best,
Paul

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