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MG MGB Technical - 1980 weak performance

Hi All,

I bought my MG a few years ago in the following config:
1980 US spec, ZS, C.E.I. distributor, airpump removed, gulp valve removed, cat conv removed.

Falcon stainless falcon sport exhaust installed.
I have rebuild the ZS already.

I would need some help on the following problems...

The performance of my car is very weak, especially uphills (compared to my previous MG's 1969/1972).
The engine idle is fine, but after prolonged driving the idle is very rough for 1-2 minutes, then stabilizes again.

I am confused about to what stage the emission control stuff can be removed. Sorry to say, but I live in the Philippines with no emission limits...

With great interest I was digging in the archive, but I still can not figure out the following:

1. The vacuum advance is (ZS standard) connected to the manifold. Is it better to connect it to the carb port with a T junction below the ERG valve? I learned that manifold connection does not give advance during acceleration. Is that true?

2. Should the TCSA solenoid (4th gear) better be by-passed?

3. Is the ERG valve needed in my setup?

3. How do I properly time the engine and what needs to be disconnected (and plugged) during dynamic timing (I do own a strob light)?

4. What needs to be disconnected (and plugged) during carb tuning?

I do apologize for the number of questions. Any, as well partial answers are highly appreciated.

Thank you for your help.

Reto, Philippines
Reto Schlumpf

Where are you in the Philippines ? I am spending the winter here and am in Pampanga.I have had a fair amount of experience with MGs over the years and currently own 3 in the USA.
Gil Price

1. The only difference between manifold and carb vacuum is at idle, when manifold gives maximum vacuum hence maximum advance which is an emissions reduction measure, and carb gives zero vacuum and advance at idle. There is no difference between the two just off idle and with wider throttle openings.

2. TCSA is best bypassed as it limits vacuum advance to 4th gear only. While the better mileage aspects of vacuum advance are usually only applicable to 4th gear, vacuum advance in all gears does results in crisper acceleration at part throttle. However vacuum advance makes no difference to full throttle performance as there is no vacuum and hence no additional advance.

1st 3. The EGR is an emissions measure and can be dispensed with along with the other stuff like air pump, gulp valve etc.

2nd 3. Unless you have access to the original fuels when the car was manufacturerd and the timing specified, or can spend a lot of time on a rolling-road, it is all guess-work. The original values for your car were emissions related anyway, you can do better. But without changing components the best you can do is use maximum advance without getting pinking/pinging at any combination of throttle, revs and load. But with a low compression engine like your you may not *get* pinking/pinging depending on what fuel you use, so you will have to use comparative 0-60 or distance timings to get the best performance.

4. When timing an engine with manifold vacuum you must disconnect the vacuum and plug the manifold end. This may well result in a drop in idle revs, so turn that up while setting the timing, and back down again when you have finished and reconnected the vacuum.
Paul Hunt 2

Hi Gil,
Thank you for your reply. I am staying at the Taal lake, Batangas. Perhaps you could send me an e-mail to the above shown address and we could get in touch.
Reto Schlumpf

Hi Paul,

Thank you for the excellent explanations.

I am very glad to receive advise as I am battling with this engine now for more than a year..

I hope I am not asking too many questions again, but I really would like to understand how this ZS piece of art works..


Would this imply, I could disconnect the vacuum line for the ERG valve (but leave the ERG valve attached) and use the free carburetter ERG vacuum port for the vacuum advance of the distributor?

What would I gain by doing so?

Would the timing procedure in this new configuration (carburetter vacuum) require to disconnect and plug the vacuum line too (during the timing procedure)?

For tuning the ZS properly, which lines (hoses) have to be disconnected and does the air filter need to be attached during the tuning procedure. I have read at several places that for the tuning procedure the air piston shall be lifted slightly to check the mixture. How can the air piston be lifted with the air filter still attached?

Why do I not get a response so far of the engine by turning the coarse idle nut or fine idle screw. Only if I use the main mixture adjust tool the mixture really changes (carb is rebuild and no leaks are present).

Many, many thanks for clarifications!

Reto Schlumpf

The fine idle adjust is worthless for all practical purposes on the MGB. Idling will be less than perfect no matter what you do. Also, hot weather does not agree with the ZS carb in general, and even a new one will have trouble when hot. Hot starting and low-speed performance in high temperatures are particularly execrable on this carb, even when tuned perfectly. Performance in cold weather, however, can be pretty good.

If you can't even get it idling reasonably well, suspect an air leak. The manifold commonly warps, so check that you haven't got a leak at the manifold.

EGR is harmless when functional. If it's working, leave it working. The valve closes at full throttle or anything close, so there is no power to be gained by disconnecting it. Likewise, the gulp valve. TCSA depends on your particular setup. If the car pings in 3rd without it, put it back in.

The poor design of the exhaust manifold is the single biggest impediment to the car's performance. Yes, more so than the catalytic converter, more than the air pump, more than even the carb. You've surely noticed that the car can't breathe at over about 4000 rpm - replace your manifold with headers or an earlier manifold, and you lift that ceiling.
Sam Good

Hi Sam,

Thank you for your inputs. It makes a lot of sense to me that the exhaust manifold is a mayor problem. I have learned a lot in just 24 hours...

The only uncertainty I have is regarding the tuning:
Shall I change to carburetter vacuum and would the timing procedure in this new configuration (carburetter vacuum) require to disconnect and plug the vacuum line too (during the timing procedure)?

For tuning the ZS properly, which lines (hoses) have to be disconnected and does the air filter need to be attached during the tuning procedure. I have read at several places that for the tuning procedure the air piston shall be lifted slightly to check the mixture. How can the air piston be lifted with the air filter still attached?

Any one? Thank you!
Reto Schlumpf

From what I've seen the vacuum port on the ZS can be used for vacuum advance as it seems to operate in exactly the same way, but I've not tried it. It will only change total advance at idle though, and not affect performance.

In theory for setting timing you should be able to leave vacuum advance connected with carb vacuum as it should be zero at idle. But if you need to open the throttle for higher revs to set dynamic timing you may get some vacuum advance, so it would be better to disconnect and plug the carb port just the same as for manifold vacuum.

At idle the air cleaner should make no difference at all to mixture, if they do then they will be pretty restrictive at higher throttle openings and airflow. SUs have external lifting-pins for the pistons. I don't know about the Zenith, the UK didn't have them, and Haynes only talks about setting the mixture using exhaust CO. This can be done with single carb of course, but not with duals unless they already have a balanced mixture, which needs to be done with the lifting pins.

Interestingly John Twist also only mentions using the main tool, whereas Haynes says to use the fine screw, or the coarse nut if the screw hasn't got the range of adjustment required, and only to use the main tool if the course nut is outside the range as well. Maybe this is because Haynes talks about monitoring the changes with exhaust CO, maybe looking for some change in audible engine note is too coarse an indicator. Haynes says to disconnect and plug the air pump port, and disconnect the float chamber vent pipe. Also valves and ignition must be spot-on, no defects in the carb, and no vacuum leaks at oil filler cap, rocker cover, side and front covers, and dipstick.
Paul Hunt 2

Great input thanks.

I do own the Bentley publisher MG75-80 workshop manual which is similar to the Haynes. The air lift method is described in the Moss Motors MG catalogue (John Twist)and is mentioned in one of the older forums. Nonetheless it is not described how the lifting actually is done.

I think I will just keep on trying to find a reasonable setting.. or better go on ebay for a pair of HIF-4. Just a bit hesitant because so many new bits and pieces are needed and I am not sure if the HIF-4 exhaust and inlet manifold would actually fit on my 1980 engine??..

Thanks again for an additional input!

Reto Schlumpf

Reto, part of the probelm you'r eexperiencin gis due to the distributor itself. For best performance, you should have a distributor with 10 -12 degrees of mechanical advance and a vacuum advance with 5-10 degrees of advance (these figures all double when measured at the crank.) In your case, your stock distributor offers 18 degrees of mechanical advance and 12 degrees of vacuum advance. It'll never give you the hill climbing power you want without a recuvre!
Jeff Schlemmer

Hi Jeff,
Thanks for the input.
Could you please explain a bit more in detail what you mean, because it is the first time I hear that the distributor is part of the problem.
Reto Schlumpf

Should've said that on SUs the piston only needs to be lifted about 1/32"/.75mm, any more and it masks the change you are listening for. If the engine tends to stall the misture is too weak. If the revs rise and stay risen while the piston is held up it is too rich. The revs should only rise momentarily as you lift the piston. It can be difficult to judge the correct point, but you can train your ear by *making* it weak and rich until you can just detect the changes mentioned, then position it between the two. With practice you should be able to detect the correct point itself. If there is no lifting pin then removing the air filter to lift the piston won't cause any problems.

Jeff is quite right that the distributor can cause loss of performance, in fact all the ignition functions must be good before you even start on the carbs. First thing to check is that you do have working centrifugal and vacuum advance mechanisms. For the former disconnect and plug the vacuum port as mentioned before, then with a timing light make sure you can see the timing advance as you increase the revs. If you have an adjustable timing light you can plot the curve and compare it to the original for your distributor (e.g. and click on 'Distributor Curves'). Also check for slop or free-play in the rotor by twisting it anti-clockwise against spring pressure, then let it go gradually. Ideally it should return all the way, free-play indictates stretched or missing springs. You should also be able to feel a weakish spring initially, then a stronger spring come into play as you twist further. For vacuum advance suck on the pipe leading to the distributor and watch the timing with the light, and you should see it advance. If you can suck air through the diaphragm is punctured and will be making the mixture weak at idle, although if it is corrected for that it will make the mixture rich at wider throttle openings.
Paul Hunt 2

It's just great to get such detailed advise. Thank you.

Do I understand it right, that on top of the rev depending mechanical advance, there is the vacuum advance which only gives additional advance during acceleration phase? Depending, if the vacuum advance is connected to the carb or manifold it will have different characteristics of additional advance?

The distributor characteristic can be plotted rev related in a chart form.

Are this statements correct?

Thank you,

Reto Schlumpf

Because he won't self promote...Jeff will not say it but he is an excellent source to have your distributor recurved by a true professional. There was a recent thread on the other large forum for MGB owners (MGB Experience) and you wouldn't believe the number of forum members that were happy customers of Jeff's. You can reach him via his e-mail in his header. BTW I have no financial affiliation with Jeff...just a satisfied customer.

Reto - generally the reason for vacuum advance is to give more advance during steady-state *cruising*, which gives better fuel consumption. Less vacuum advance is added during acceleration, but the benefits can still be felt on part-throttle acceleration, although at full throttle there won't be any. This is the same for both carb vacuum and inlet manifold vacuum, the only difference between the two is at idle.

Centrifugal advance can be plotted just by using a timing light and rev counter. Vacuum advance is a bit more tricky as you need a vacuum gauge as well as a method of varying the vacuum, then plot that against advance. I say 'plot' but for vacuum advance there are only three figures - the minimum vacuum that any advance is added, the vacuum where it reaches its maximum, and how much that maximum is.
Paul Hunt 2

Thank you Frank for your recommendation.

Paul, great explanation.
Is it therefore correct to say that total advance at idle would be bigger if connected to carb vacuum port compared to manifold vacuum?
Reto Schlumpf

just read the article about the difference of vacuum advance in Hammer and Spanner. It's clear now.
Great source of info. Wished I would have known about it earlier.
Thank you,
Reto Schlumpf

This thread was discussed between 12/01/2008 and 15/01/2008

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