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MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG TD TF 1500 - Ported Vacuum for 45D4 Distributor (TF 1500)

The original distributor on my 1955 mgtf 1500 is in its death throws. I have a 45D4 requiring vacuum advance. It appears that I must use ported vacuum to drive the advance curve. My current H4 SU carbs are not drilled for a vacuum tapping. However there is a blank boss in the carb body at the throttle plate location on each carb. I am thinking that this boss can be drilled into the air flow and tapped to take a fitting for connecting a pipe to the 45D4 vacuum advance unit. Will this work for a '55 TF 1500? See the image of the boss position, it is offset approximately 30 Degrees from the bottom of the body.

Has anyone added a ported vacuum advance connection to a TF 1500 and/or installed a 45D4 distributor.

Ian Massey
TF 1500 9194



TF 1500 9194

The other way is to drill the insulation block (better still, both of them) between the carb and the manifold and bond in place a spigot using epoxy. Then use a Y piece and small bore tube to connect to the distributor.
Dave H
Dave Hill

Dave, the problem with your suggestion is that drilling into the epoxy spacer will result in manifold vacuum, which is not what I want. I want ported vacuum, so my tapping must be slightly upstream of the throttle plate. Manifold vacuum has the opposite effect to ported vacuum. I want high vacuum on acceleration and high load, manifold vacuum gives high vacuum on idle and light load cruising.

Ian M
1955 MGTF 1500 9194
TF 1500 9194

Can't see why that won't work Ian
At least being 30 deg. around will stop raw fuel running down the tube
It's a shame the butterfly doesn't open the other way then you could have it on top
If you go ahead I'd aim your hole as close as possible to the butterfly, maybe about.020" inside it max. so that you get the early advantage of the vac advance and keep the inside end of the hole small ,like 1/16" so that fuel can't well up in it

Actually , reading your post----
The vac. advance works differently to how you say--for ported vac advance
There should be 0 vac at idle, increasing as the throttle is partly opened for cruising etc but on full throttle the vac decreases and vac advance decreases with that loss of vac resulting in no vac advance with heavy throttle
William Revit

Interesting comment William, perhaps my understanding is incorrect. As I understand the situation, manifold vacuum gives max vacuum at idle, minimum vacuum at high load acceleration, and high vacuum at light load cruising. When I was a teenager I fitted a vacuum gage to the manifold of my MGYB. The above is what I experienced as I measured the vacuum in the inlet manifold. The objective was to maximize fuel consumption by driving in a manner to maximize vacuum in the manifold (no hard acceleration). I was a poor student in those days and gas prices were high in the UK fifty plus years ago. I had understood that ported vacuum, vacuum upstream of the throttle plate, works in the opposite manner, providing no vacuum at idle, as you indicate, but provided high vacuum during acceleration, making the engine more lively.

I guess I will purchase a vacuum gage and play. My TF already has a tapping in the inlet manifold. I will drill and tap one of the Carb bodies for the upstream vacuum port. Sounds like a fun investigation.

Ian M
1955 MGTF 1500 9194
TF 1500 9194

I measured the vacuum on my TD at idle when I installed a temporary vacuum gauge during the installation of the PCV system. I measured -0.7bar at idle tapped at both ends of the inlet manifold.
Regards
Declan


Declan Burns

It would be interesting to learn what you discover.
Dave H
Dave Hill

Ian,
If you connect a standard "el cheapo" vacuum gauge as in my photo above, you will need a damper or the pointer will go into orbit. I used a cigarette filter in the tubing.
Regards
Declan
Declan Burns

Ian
either connection should give roughly the same result from above idle onwards, the only differance should be that a manifold connection will give, as you say, full manifold vac. at idle whereas ported has nil at idle
The last thing you want is full vac advance on full throttle as the total combined advance will be too high causing detonation
The vac. advance unit works like a shock absorber adjusting the amount of advance compared to load (vac.)

hopefully your vac. gauge tests will show this
The main issue with having the vac. direct to the manifold is two fold--there is a tendancy to get a weak unstable idle speed because of the higher ign timing and because the idle is weaker the engine can sometimes stumble a little when moving off from standstill
IMHO- ported is the way to go
willy
William Revit

I spoke with Len Fanelli recently, he mentioned that you need to get the vacuum from outside the throttle plate.

Why not rebuild the existing distributor? Mine is at Advanced Distributor right now.

Peter
P G Gilvarry

Ian a well known Lucas distributor repair shop states the 45 D is "Junk".
Also Classic Sport Ignition "CSI" uses a 25D for T type housings.
The housing can be drilled and taped, BUT location of the ported vacuum hole is critical, the throttle plate uncovers it as soon as the throttle plate is barely opened, the hole is 1.5 MM diameter. The threads for the union is 6 MM.(For my TR2? front carb).
Peter, I gave up counting how many ways the original T type distributors can and do fail, Keep the OEM distributor for originality only, IMO.
I agree with Willy, Ported vacuum is the only way to go.
You can purchase a vacuum advance 25 D directly from CSI, asI did, as Moss does not offer them.
It comes with 16 different special advance curves to choose from.
A customer of mine found # 11 proved to be the best on the dynamometer.
Len Fanelli
Abingdon Performance Ltd.
Len Fanelli

Len, thanks for the recommendation on the 25D. I will purchase a 25D and see how it performs. I want to get my original TF distributor repaired, but it is always a good idea to have a reliable backup, just in case!

I will be removing the engine from the TF at the end of this month, a weep on the rear freeze plug! However, I hope to play with the Distributor substitution when I have the front fenders, and side skirts removed. I’ll report on my vacuum location test, and the 25D substitution when I have the results. It may be helpful to others. I will take Declan’s advice and purchase a high quality vacuum gage.

Ian
MGTF 1500 9194
The money Pit!
TF 1500 9194


There is a carburetor body on EBAY under "TF Carb" listing that has a metered vacuum port (front carb).
Richard Cameron

I spoke with Rob at British Vacuum Unit about a 25D distributor with vacuum advance for my 1955 MGTF. Rob indicated he had a NOS 23D with no vacuum advance which was supplied as an upgrade for a TF 1500 with the correct advance curve. It shipped out of BVU to my home this morning. I am going to try the 23D, it should be a drop-in replacement. My problems with vacuum advance are over (I think!). However, for interest, I will still carry out the experiment by applying a vacuum gage to my Manifold, and then drill and tap one carb body upstream of the throttle valve and retest to satisfy my curiosity. As I will be pulling the engine after an initial test of the 23D, it will likely be a couple of months before I reassemble and conduct the vacuum measurement experiment. I will report my findings.
TF 1500 9194

Ian
I hunted out an old SU carb and had a measure up---just to give you something to work off-
The ported hole , where it enters the throttle bore is .040" diameter
The interesting bit is the hole is closer up to the plate than I thought-
With the throttle plate closed right up (idle speed screw let right off) the edge of the port is still slightly covered (just) by the back side of the throttle plate- with it fully closed I couldn't poke a piece of 1mm mig wire through, it butted up on the edge of the butterfly
The hole becomes uncovered very early as the throttle is opened
I have taken a couple of pics to show how close up to the throttle plate the hole is
I'd say it's important to stick to the .040" size and position to get the vac. timing correct
Hope this helps or is of some interest
willy


Pic. 1
Butterfly fully closed and red tape up against butterfly



William Revit

Pic 2
Butterfly part open----notice the hole is only about the width of the hole away from the tape-
To be honest I thought they were a tiddle further back than that but there you go------


William Revit

This thread was discussed between 02/05/2019 and 08/05/2019

MG TD TF 1500 index

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