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MG MGB Technical - Ignition/carb combination...help!

Hi,

Bit of advice………I’m in the middle of an mgb engine rebuild and I am getting ready to run it for the first time. Only problem is I have a bit of an odd combination of parts (all bought separately over a long rebuild period), I have the following;

• 25D distributor with vacuum advance,
• HIF4 carbs
• HS4 inlet manifold

Now, my understanding is that the early vacuum advance take-off (HS4) was from the carb body casting, whereas the HIF4 take-off was from the inlet manifold. Therefore I don’t actually have a take-off point in my setup.

Basically I guess my options are:
1) Take the advance off the manifold by fitting a connector to one of the holes that already exist in this (is the location on the manifold critical).
2) Drill and tap a hole in the HIF4 casting in the correct position (where? any critical measurements?)

Main question - I read somewhere that the earlier take-off position (i.e. in the carb body, just after the throttle butterfly) is the best location. I’m assuming this is because it responds more quickly/directly to throttle actuation due to proximity to the butterfly. However is this just an economy improvement, or does it make the car more responsive/improve acceleration?

Regards,

Ian
I E Mitchell

On '70 and onwards, the distributor advance vacuum for HS4
SU carbs was taken from the manifold balance tube - not the
carb body.

I didn't know that HS4 carbs will fit on a HIF manifold. Are not
the mounting bolt patterns different?
Daniel Wong

"I read somewhere that the earlier take-off position (i.e. in the carb body, just after the throttle butterfly) is the best location. "

Yes.
This from Peter Burgess' "How to Power Tune MGB 4-cylinder Engines" a book well worth having.
"In good condition the standard Chrome Bumber distributors are adequate for all tuning stages apart from full race. The rubber bumber distributor, however, has two problems, even for standard applications.
Firstly, the mechanicl advace curve is wrong. At idle the timing is a bit too retarded, while at high rpm there is too much advance. ...
Secondly, the vacuum advance is wrong. On the chrome bumber cars the vacuum advance take-off is positioned on the carburettor, upstream of the throttle butterfly, so there is no vacuum signal at idle., the distributor advance being purely mechanical, and as the throttle opens and the vacuum take-off hole is progressively uncovered, the depression across the hole increases which, in turn, pulls more and more vacuum advance."

My experience with the above is that at idle you need to adjust the throttle a bit more open than for a rubber bumber car. This means more air is going in at idle. When you crack open the throttle and get the added advance there is a small surge of power from the advance. Better response.
werner haussmann

Daniel HS4 and HIFs have the same flange fitting so can use the same manifold. The larger 1"3/4 carbs have a 4 bolt flange. Jim
jim soutar

UK and USA were very different, don't forget. UK didn't move from carb vacuum to manifold vacuum until September 76. HIFs were introduced for the UK market in November 73 (71 for export cars, which also changed to manifold vacuum), so these would have had vacuum ports as well as the HSs until September 76. The only difference between the two sources is at idle, manifold vacuum results in high vacuum nd advance at idle which results in lower idle emissions, carb vacuum results in zero vacuum and advance at idle. Off-idle i.e. all normal throttle positions for driving they are the same. Vacuum advance from either source is largely a cruising economy feature, although I have found it sharpens part-throttle accelleration as well.

Personally I wouldn't drill the carb body but get a port to replace one of the blanking plugs that you should have on your manifold. It isn't critical whereabouts on the manifold you take vacuum from, after all when it is carb vacuum it is only from the rear carb. V8 HIF carbs have a vacuum advance tapping, but because the butterflies operate the other way it is on the bottom which means fuel runs down the pipe and rots the vbacuum capsule diaphragm unless you fit a separation chamber higher than the carb port. You would have to determine which way the butterflies operate on your HIFs before you could drill for a port, it must be positioned such that as soon as the butterfly starts to open it it is on the manifold side of the butterfly, not the carb side.

The most important factor is does the mechanical (and vacuum) advance characteristics of your distributor match the requirements of your engine and the fuel you use, which is a hornets nest. Any 25d or 45D distributor that physically fits the engine will run and can be adjusted for no pinking, but the further the distributor curve is away from the engine curve the more performance and economy you will be losing. You can get very expensive electronic distributors that are said to match the original curves very closely (something which an old mechanical distributor and even a rebuilt unit is unlikely to achieve), but the original curves are irrelevant with todays fuels, so much so that one supplier of these distributors recommends the MGA version for the MGB!
Paul Hunt

Thanks everyone for your help.

Paul, your extensive web pages are very interesting and useful, its a real help to those of us just starting to get to grips with MG's!!

I think that the way forward is to get the engine running with the vac take-off in the manifold as Paul suggested. I am in the middle of an MGA rebuild (with B engine, hence the posting here) so I'm just eager to get the thing running, I can play with the various combinations later. I might even fit HS4s when funds permit.

Thanks again.

Regards,

Ian

Ian Mitchell

This thread was discussed between 23/04/2009 and 24/04/2009

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