Having fixed my leaks, now it's time to tune.
I have read lots of stuff - everything indicates that jet height should be ~2.5 turns down on mixture screws.
Then some say 0.060" down (1.6mm). In my case that's only 1.5 turns down (although each carb seems different too).
Rather than relying on screws, am I better using the vernier calipers to set at the correct height from the bridge? And if so, at 0.060" - same blogs say twice that (3.2mm - 1/8").
My setup is weird - looks like e-type (???) and will have to adjust each carb flow at each linkage (cos the carbs cant be decoupled).

 

If your carbs can't be decoupled. Can you get all your butterflies to close completely so that you can set your idle properly on the idle bypass circuit screws?

 

Pardon my ignorance, but is it actually possible to have no way to sync the butterfly valves on a Jag linkage?

​​​​​​Doesn't make any sense...

 

multiple butterflies could be set on a single throttle shaft in the same orientation when manufactured or afterward if customized, i suppose, it's not impossible. ...not that i know that they are set that way on the threadowner's car. they are on typical non-SU carbs. although i would admit that i've never heard tell of an SU with that arrangement.

 

The way to set the throttle is to loosen the coupling drop arm at the throttle shaft. Each of those arms from the shaft you highlighted moves as one, but each is attached to the throttle shaft of the carb by a linkage. That coupling at the carb shaft is adjustable. You should only need to so that once to get the butterflies all moving together.

I usually set triple carb jet heights at about 0.075", but that setting depends on elevation. the type of air filter etc. Use it as a stating point and then fine tune with the wideband AFR meter. That setting should enable the car to start so you can tune from there.

 

This is a photo of a 4.2 E Type, the main difference is that the 3.8 has the main throttle shaft on top of the intake manifold ( like your car) while the 4.2 has it underneath. You can see the linkage attached to the throttle shafts of the carbs, with rods that go down to the main throttle shaft. It is these linkages at the carbs that you would need to loosen off to synchronize the butterflies of the carbs.

Note that all SU fittings are Whitworth/ British Association sizes, the bolts on the throttle linkages are 2BA size.

 

The only scres I have are: 1. Jets, 2. Fast idle on 2 carbs 3. Screws that only go the the throttle linkage. Thats all

 

Thanks! I will try 0.075"

 

Ah. You have solved my mystery. I did think SUs have some sort of idle butterfly bypass system. I thought I was going crazy!!

 

Thanks Glen, I can get them all to close at the same time - mine doesnt seem to have idle bypass screws.

 

By golly, I knew there had to be a way...

 

Yes it is.

 

So your SU's have no idle bypass (screw A)? Screw B is the idle mixture screw.

You set idle on the butterfly shaft like most conventional cars?

 

Not all SU's have that idle speed screw. E Type HD8's have it, but the HD8's used on Aston Martin DB5's do not for example.

 

Yes JB ~ I know Astons better than Jags & that is correct. Jeff says his Bentley's SUs don't have the bypass.

So do you set idle on the butterfly shaft once all butterflies are aligned as with a conventional car?

I tune Aston's once all butterflies are aligned & idle is smooth by plug wire pulling. If you get equal rev drop on each cylinder you are tuned & all readings will be correct at the exhaust (obviously with correct jetting). Exactly the same applies to multiple side draught Webers. The same applies to 12 plug Astons where you are synching twin distributors.

 

Refresh page for edits.

 

Yes, I sync the butterflies and the linkage between the carbs so the throttle shafts all move in unison, and then I can set idle speed by adjusting the speed screw that opens the throttle on one carb. If the slop is taken out of the inter-carb linkage, then it isn't necessary to use the speed screw on the second carb.

I'm going through this right now an a V12 E Type with the SU HIF44 conversion and get to do it on all 4 carbs. What joy!

 

to my knowledge, there are a number of different SU carbs that relied on cracking the throttle shaft with a dedicated idle screw BITD (among them type "H" in various throat sizes). many were standard equipment on MG's, Healy's and Triumphs. such carbs have one idle screw per carb, with one mixture adjusting nut that moves the jet up and down, and that's all. if the idle screw is backed completely off, the butterfly closes completely and the engine will die. the throttle shaft is retracted by spring tension such that the butterfly bottoms out in the throat of the carb before spring tension is exhausted.

i suspect the SUs with an idle circuit controlled with separate mixture and flow screws was developed to provide finer control at low speeds. as the previous method outlined above is fairly crude, subject to premature wear resulting in unsatisfactory performance. at lease that's been my experience as a previous owner of an MGA and a current owner of an MG TD. as i've mentioned, i own a '67 MK2 but haven't had the opportunity to inspect the carbs at all.

there's a fairly good synopsis of SU's on Wiki: https://en.wikipedia.org/wiki/SU_carburettor

 

Huey ~ the twin carb Triumph Spitfire was like that.

 

yes...

i have to laugh at myself, because up until i purchased my MK2, i've never had anything but MG SU carbs. and purely in defense, i've been forced to delve much deeper into their inner workings over the years than i cared to. consequently, when this thread brought up the point that some have what's commonly referred to as an "idle circuit" with its own mixture and flow control screws and a corresponding hole in the throat of the carb, i found it hard to believe!! smh.