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MG Midget and Sprite Technical - SU vacuum port location question

I have a question about where the ignition advance vacuum point is located on twin HS2 carbs used on 1275 engines.

My car is a '74, US model, so it was delivered with a manifold source for the vacuum advance.

After researching the best vacuum source for these engines, I am thinking about drilling one of my carbs to use ported vacuum, as was used in the UK, and on earlier model year versions in the US. Reading up on "ported" vs. "manifold" vacuum sources for LBC engines, I have learned the vacuum pulses too much in the manifold to get a stable idle, when manifold vacuum is used, and fitting a hotter cam makes that get worse.

The hole can be drilled, and Keith Calver at Miniworld wrote these instructions:


"To drill a port to accept the vac pipe take-off it needs to be sited 9/16" back from the carb to manifold mounting face, offset slightly to one side. I.e. NOT at the dead centre, or 'peak' of the butterfly bore. And usually set off away from the crankcase breather port. Usually the vac pipe take-offs fit a 1/8" diameter hole - but measure whatever you're going to use before drilling. You're looking for an interference/air tight fit."


OK, now my question: In Terry Horler's book, the images show the vacuum port is located on the front carburetor.

=> Does anyone know why the FRONT carburetor would be the one drilled for the port?

Looking closely at the routing, it looks like the vacuum pipe then goes under the carbs, and all of the way around the rear of the engine to reach the distributor. Wouldn't it be a shorter path, and neater, to have drilled the rear carb instead?

So, I am wondering if anyone can think of why it was done that way so I don't do something stupid over here!

thanks so much,
Norm Kerr
Norm Kerr

Early engines had it underneath the rear carb - later ones on the side of the front carb - MGBs with HS4s had it on top of the rear carb, with early HIF4 setups it was underneath the rear carb.

Positioning is critical - it needs to be just in front of the butterfly when it is closed and the hole where it breaks through is only about 1/64th inch.

The easiest solution is to find a body with the takeoff in place!
Chris at Octarine Services

Norm
Picture is of the unmolested engine of a 72 RWA (UK spec). The pipe has an elbow fitting at the carb end and loops over the top of the engine.

Bill


W Dunsmore

Another picture showing the take off on the front carb at the rear of an oval boss.


W Dunsmore

Norm,
this subject was covered quite recently, sorry can't remember which thread/topic

photo below shows vacuum pipe and connection on carb, note I have a bigger hose on mine as I had problems with petrol being at dissy end of thin nylon pipe and this change seem to cure it (but I've not checked that for a long time!)

route and length of hose has changed since that photo as I could get it back to that exact route and tie-wrap fixings and have it sit so well, no idea why but I gave up and took another route and went for a drive instead of fussing any more


Nigel Atkins

Here's another photo of the pipe passing over the front rocker cover nut.

Another unmolested '72


Dave O'Neill2

And here's Bill's photo with an arrow added for clarity.


Dave O'Neill2

well done Dave, I intended to do a more close up cropped photo but by the time I'd added in another arrow it was "TEA" (or dinner if yer posh)

Norm,
parts 115, 116 and 117 are normally used as connectors and vacuum pipe which is very small, possibly something like 1/8" OD - http://www.mgocspares.co.uk/acatalog/MGOC_SPARES_ELECTRICS___DISTRIBUTOR__midget__164.html
Nigel Atkins

Norm
I have an actual blueprint supplied many moons ago to show where to drill and relevant measurements. I'll scan and e-mail it.
Alan
Alan Anstead

Thanks dave,

You beat me to it

Norm, im surprised yours dosnt have that little christmas tree port...did it break off or do you have old carbs

I doulbt manifold vs port vac will have much effect on this As the hole is so small, I doulbt pulsing will be an issue its just enough to advance and retard the vacume plate in the dissy...but I have been known to be wrong before...hahaha

Prop
Prop and the Blackhole Midget

Norm,
at the risk of feeling the wrath of the purist and to come back to your question "why the front carb". It make no difference which one you choose.
My transverse MG1300 has the take off on the top of the "rear" carb as standard (picture available at the weekend if required) which makes the pipe routing a lot easier.
It's your car, you choose... as long as it's in the right place relative to the butterfly ;)

Best of...
MGmike
M McAndrew

Awesome information everyone, thank you so much!

And especially Alan for sending me a scan of the drawing sent to him by BLMC for when he had to drill his carb back in '69.
Is it OK for me to post the scan here for others to use too?


Prop, the diameter makes no difference, if the vacuum is unstable, it creates "dither" and that causes trouble. The '73 and '74 US spec engines used manifold vacuum, and a unique vacuum advance unit that is not available new. My plan is to recurve my distributor for my modified engine and use a new vacuum advance unit, and convert my carbs to ported vacuum.


Hi Nigel, I remember the long thread you refer to, but this particular point (of whether it would make any darn difference which carb was drilled) was not covered in it. In that thread I had unearthed a whole ton of articles written for American V8 engines which all said to go the other way. But after discussion, it came out that small displacement 4 cylinder engines just don't have as stable a vacuum at idle, so BMC, apparently, preferred ported.



Norm
Norm Kerr

Norm,

I just remembered... minimania and I think moss sells a thin carb spacer in various thikinesses that has the vacume port just for what your discribing

It goes between the carb and the big block spacer...and its cheap as I recall

That might be the better way to go, that way your carbs would remain un molested

Prop
Prop and the Blackhole Midget

Here ya go norm... I know this is for a HS4, but they make them for HS2s also

They also come in various thicknesses as well

http://www.minimania.com/part/MFA338/3-Thick-Carb-Spacer-Hs4-H4

Prop
Prop and the Blackhole Midget

Norm
I have no problem with you uploading the blueprint for all to see and refer to.
Alan
www.masckent.org
Alan Anstead

Prop - that's no good - gives the same as manifold vacuum - not carb vacuum
Chris at Octarine Services

Prop, Chris is correct.

Thanks Alan,
Rest of the world, here is the drawing.


Oh brother. The file size is too big (3MB). Hold on, I'll try to upload it later.


Norm
Norm Kerr

I try again, upload of BLMC's "drill your SU" drawing...


Norm Kerr

This thread was discussed between 18/02/2014 and 19/02/2014

MG Midget and Sprite Technical index

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