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MG MGB Technical - Vacuum Advance Connection HIF 4

My 1972 MGB has an 18V582 series engine and during rebuild I have fitted HIF 4 carbs.

Can someone please advise where the vacuum advance connects at the carb end? There is no tube on the carb flange, nor on the heat block spacers. I have connected it to the manifold adaptor, but it means that advance is active at tickover meaning rich running and probably the cause of difficult hot starting.

Many thanks, Andrew
Andrew Walton

More info having looked at specs and read loads about 'ported' vs 'manifold' vacuum advance. Din't know it was so complex !

My engine is a 18V582 and distributor is 41288. Shown as requiring ported vacuum connection.

Can't find 18V582 listed, but specs say that an 18V584/5 should have a 41370 distrib with a manifold connection.

Is it the right distributor for a 582 engine is one question, second is, can someone confirm that the vacuum advance should be ported from the carb?

Many thanks, Andrew
Andrew Walton

Doesn't matter where the vacuum comes from when driving, the only difference is at idle. HS carb source gives zero advance at idle, manifold source gives a high vacuum. The difference is that a high vacuum at idle allows the idle screw to be turned down without stalling, which reduces fuel consumption and hence emissions. It was moved from carb to manifold for this reason. As soon as you start to open the throttle the two sources rapidly converge, then remain the same for the remainder of throttle opening.

By itself manifold advance shouldn't affect hot starting, many tens of thousands of cars were shipped from the factory like this. If your car has this problem then it's a fault due to other causes. Vacuum advance only affects timing, not mixture. If your mixture is rich that *will* affect hot starting, and should be corrected with the carb mixture screws.

UK cars from August 71 to November 73 had the 18V 581/582/583 engine with HS carbs with the ported source from the rear carb, and the 25D4 41228 distributor.

1974 to 80 cars used the 18V 846/847 engine with HIF carbs and the 45D4 41610 distributor. However my information is that the front HIF carb had the vacuum port until the 1977 model year, when it moved to the inlet manifold, i.e. same engine and distributor.

The 18V 584/585 engine was the North American engine, which was very different, being low compression and a series of different distributors.
Paul Hunt

I have a 74 1/2 English rubber nose 18V 847 engine, 45D4 41610 distributor, HIF carbs with manifold porting. All appears to be original with no where on the carbs to port from. Idle and starting work fine.
K Stuckey

Interesting, that's not what Clausager shows on pages 66 and 67. P66 shows a UK 75 with the vacuum pipe clearly going to the front carb, p67 is a 77 or later with it clearly going to the manifold. A carb part number change shows that also.
Paul Hunt

Thanks Paul and K Stuckey.

Thinking about it and looking at specs it seems that the vacuum advance mech is available with different part numbers. Is it this that dictates the advance curves or the moving plate in the distributor? so I need to feed my 41288 distrib from the carb or does it really make no difference?

When I say running rich I don't mean the mixture is rich ; the plugs are grey / cream after a run, but I wonder if the amount of advance at tickover is causing a strong smelling 'rich' exhaust?

What else could be causing a smelly exhaust at tick over? I am sure that the engine is 'effectively' rich at tickover but runs well when driven!

Cheers, Andrew
Andrew Walton

Hi, I had another look at the car there is no where to get vacuum from on the front or rear carby and on p66 of Clausager there is a red plug on the vacuum port on the manifold in front of the hose to the power brakes. This is where my dizzy is connected to. I cant see how vacuum advance could upset the mixture. Are your choke adjustments on the carby correct?
K Stuckey

There were definitely HIF's fitted with the ported vacuum - at least the chrome bumper models. The take-off was underneath the carb.
And most of the rubber bumpers seemed to have manifold vacuum, but I can't remember where the change point was.

Andrew - if the mixture is good at cruise and power but rich at idle, it indicates a worn needle. The needle wears most at the top, and this is also the point at which it makes the most difference as the gap between needle and jet is at its smallest. I agree with the above that the advance won't affect the mixture.

What you should do is get the engine warm and set the mixture to optimum at idle. Then take it for a test drive and see if there is any sign of hunting at a steady 30mph on the flat. If so, it indicates the needle is worn - and having to screw the jet up to compensate for the wear has made it lean everywhere else, hence the hunting.

The extra advance at tickover from manifold is just what the engine needs. To see what I mean, disconnect the vacuum, slacken the distributor and rotate it until max idle speed is reached - that's when the engine is at its most efficient. Lock the distributor and read the advance off with a light. And you'll find it not too far from what it is with the correct static advance and the vacuum connected. (But don't forget to reset the ignition back to its correct figure after this excersise!)
Some peopl make the mistake of trying to set ignition timing by the rotate-the-distributor method when the engine has ported vacuum or no vacuum. This always results in too much advance. It took me a while to work this out when I was a young fella, but I got there in the end :-)

Paul Walbran

The reference number on the distributor body defines both the centrifugal and vacuum advance characteristics. This includes the vacuum advance characteristics, which is what twists the points plate, although that capsule could have been swapped in the past. The centrifugal advance is controlled by weights and springs under the points plate.

The only way vacuum advance can upset mixture is if it is leaking air into the manifold or carb, but that will weaken it, and when and by how much will be determined by whether it is connected to the manifold or carb.

If you suspect it is rich do the lifting pin check, although this is more tricky with HIFs than HSs. Use the pin to lift each piston in turn by 1/32" i.e. a smidgen, and note what the revs do. If the revs die immediately it is weak. If they rise and stay risen while the piston is lifted it is rich. The revs should momentarily rise then fall back to what they were, even though the piston is still raised. As I say this is very difficult to detect on the HIF and requires careful listening and practice.

When adjusting the timing with a timing light manifold vacuum source must be disconnected and plugged, and if the strobe figures need the throttle to be opened as many do then even ported vacuum source has to be disconnected and plugged or you will end up with retarded timing.

Turning the distributor for maximum idle will almost certainly result in too much advance regardless of the vacuum source. You will start to get a very slight occasional misfire at a certain point, best observed with a vacuum gauge (connected to a manifold port, not the carb port) as an occasional downward tick. Retard until that just stops, then a further 3/4" Hg., and that should be the best setting for your engine and fuel. However that is with good springs, with the wrong or weak springs you could get too much advance i.e. pinking accelerating at higher rpms. So the best compromise is to set the timing just short or pinking at any combination of throttle opening, revs and load. This will take trial and error, so the 25D4 distributor with its vernier adjustment comes into its own here. You will also find that if you live in Norfolk and adjust for that, you could well find it pinks again if touring in the mountains or Wales or Scotland.
Paul Hunt

Many thanks for all the clarification and tips. I am now resetting the entire engine up from scratch.

Just to clarify, the dizzy characteristic shows strobe 13 BTDC at 600rpm. Centrifugal 3 deg at 600rpm and 9 deg at 900rpm. so, with vacuum disconnected do I time with strobe at 16 BTDC at 600rpm or is the 3 included in the 13?? Does it then follow that it should be 21 (13+9)deg at 900rpm?

I am going to leave the vacuum on the manifold, check and set dwell, timing, carbs and see how it runs !

Thanks, Andrew
Andrew Walton

Please could someone confirm the method for applying strobe timing figures for me - see above post. Many thanks, Andrew
Andrew Walton

The strobe setting specified in the workshop manual are the actual advance you should have at that speed (vacuum disconnected) - ie mechanical+static.

In your case, static 10 deg, mechanical advance 3 deg. total 13 deg.
Paul Walbran

Thanks Paul & Paul !

She is now running very smoothly indeed. New points, cap, set up dwell, timing, carbs.

If anything carbs were a bit weak - borne out by light cream plugs, so richened it and tested using lift pins.

Time will tell if restart when hot is now solved. Engine is still a little bit 'fumey' but that may be wear and tear?
Andrew Walton

As Paul says, the strobe figures given in the tuning instructions are the total advance observed at the crank pulley and timing marks. Manifold vacuum port must be disconnected and plugged for this, and whilst the very low strobe rpms in this case mean carb vacuum could be left disconnected with the higher rpms for some strobe figures that mean you have to open the throttle a little that vacuum source has to be removed and plugged as well.

The centrifugal advance i.e. curve figures given in some places do not usually include the static advance, they are just the additional advance, and they could be in distributor degrees or crankshaft degrees which are twice distributor degrees.

When you look at curve data for early distributors it is plain that the strobe figure is the sum of the static and the additional advance at the strobe rpm. But as you go through the distributors used over the years that gets fuzzy, with some it appears to be be adding only half the static figure, later still it doesn't seem to be adding any.

But at the end of the day the original figures are not that relevant with today's fuels. They are OK for a starting figure, but then 10 degrees static is all you need to use. After that the best approach is how much advance it can take without pinking at any combination of throttle opening, revs and load. Particularly at 95 octane many engines won't be able to get near the published timing without pinking horribly.
Paul Hunt

This thread was discussed between 19/03/2014 and 25/03/2014

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