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MG MGB Technical - Tuning and Timing 79B with SUs

Hi Everyone,

I got the bug again and started doing some work on my 79B.

I have a rebuilt pair of SU HS4s installed, new fuel tank, fuel lines, pump, large bore exhaust, original distributor so the fuel delivery is great.

When I bought the rebuilt SU kit it came with K&N air filters. The guy I bought them from I believe said he was going to set them up with the next step up richer needle due to the K&N's.

Now I am getting my tuning somewhere decent following the standard procedure. I have a colortune but its not really helping out. All vacuum lines are plugged up.

My timing seems to be way too advanced and Im curious to the state of my distributor. The car idles about 1k-1.2k but it is shaking way way too much. Im getting a little worried as I dont want to do any damage.

In regards to the advance, Ive read 40 can be normal etc due to new gas etc and im using the standard hi test from the gas station. But it seems to be further past the 40 mark if you were to project one.

I also have an edis system I was going to install. Should I install that first and then worry about tuning the carb from there? I was going to see how well I could get the car tuned before switching to the EDIS so I can revert back to the regular distributor if necessary. Any and all help would be greatly appreciated.

Thank you,

James
JRB Mr

Hi James,

40 deg is way too much. Without knowing which engine you have, set the timing at 10 deg BTDC statically and you will be somewhere near right.
Then go through the mechanical setup of the carbys as described in your manual, starting from the float level through jet centering, balancing and the rest. Now you can start tuning for mixture. Using a colortune does work, albeit tediously, as you need to keep swapping between front and back cylinders.
Once you have the mixture somewhere near right, go back to your timing and set spot on, according to the manual for your engine. Remember that the vacuum advance needs to be disconnected and blocked at the manifold to properly set the timing. After setting the timing reconnect the vacuum advance, then tweak the mixture and idle speed and tighten all linkages.
This will set up the system as originally designed.
Modern fuels do have a different requirement for advance curves, for optimum efficiency, however, this requires re-mapping of your distributor, see this article.

http://www.mgccq.org.au/tech13.htm

Not being familiar with the EDIS, I think that it allows one to dynamically change the distributor curves, which could help you with modern fuels.
I personally have invested in a 123 GB dizzy and run curve C. Good performance.

Before rushing out and changing the needles try running the car first and see how it performs. Others with K&N filter experience may be able to advise you better.

Herb
Herb Adler

Herb

I'm also running curve C with my 123. I was wondering what static timing setting you are using. I'm using TDC at the moment but intend playing with this to determine the effects on fuel consumption.
Geoff Everitt

You have to get the ignition right before you move onto balancing the carbs for air flow and mixture. 40 BTDC is quite feasible for *maximum* advance, some distributors give over 41 centrifugal alone, others 50 with a combination of centrifugal and vacuum. 10 degrees is a good starting point, but for high compression engines that need to be fine-tuned based on actual running experience with the fuel you are using by advancing just short of pinking at any combination of throttle, revs and load. That point will give you the best combination of performance and economy, within the capabilities of what is (conventional distributor) really a pretty crude device You won't be able to run at the original figures with the 'standard' grade of unleaded available in the UK for instance, barely so with the highest octanes. For low compression you may not get pinking, and can advance it so much it will stall on cranking, but should be able to run with the original figures.

The 123 is a nicely engineered but very expensive piece of kit, but the curves are just not relevant today, as witnessed by a couple of suppliers recommended the generic BL unit for the MGB and not the MGB specific unit. Also Geoff planning on departing from the TDC static timing method as recommended by the suppliers of the unit.
PaulH Solihull

PaulH ... you said -

"Also Geoff planning on departing from the TDC static timing method as recommended by the suppliers of the unit."

Is there a word missing or something?
Geoff Everitt

Hi

Geoff,

I have set the 123 up as per instructions, ie TDC. I'm getting around 30 mpg (imperial). I would like to try curve D, but pulling the dizzy to set and then having to retime the engine puts me off a bit.

Paul,

According to the BMC workshop manual the timing is 40 deg max with full centrifugal and vacuum, at 2000 rpm +.
James states that he has plugged all the vacuum hoses, so that 40 deg is centrifugal only. Haynes only quotes advance at "idle" rpm and this varies from 10 to 15 deg BTDC, depending on engine. (5 to 10 deg static).
I agree that you have to get the ignition right before setting the carbys, but I have found that the carbys need to be close to right to get the engine running smoothly enough to get a stable ignition reading. Hence my suggestion to roughly tune the carbys, then set the timing and then fine tune the carbys.

Herb
Herb Adler

OK Herb, thanks.

I was getting about 30.5mpg (summer), 28mpg (winter) - we get seasons over here - without vacuum advance so I'm looking for something better with V.A. I will let you know what I get by playing around with the settings. After almost despairing, I discovered that the dizzy can be removed by simply loosening one clamping bolt. After you've changed the curve and replaced the dizzy, it should only take a couple of minutes to adjust the timing. I turn the engine to the desired position using the fan belt *before* I remove the dizzy.

Incidentally, (to whomever may be interested) although the instructions don't mention it, the optimum static timing with a 123 dizzy *without* vacuum advance is around 15 BTDC.
Geoff Everitt

Geoff - no, I meant that your planning on departing from the standard setup for the 123 also indicates that the original curves may not really be relevant for todays fuels. And surely one of the selling points for the 123 is that you *always* adjust static timing to occur at TDC, then when running it is 'corrected' to the figure appropriate for the curve you have selected. Dynamic timing is measured as the static timing, plus the additional centrifugal advance for that rpm (vacuum disconnected in most cases). If you set the 123 to 15 BTDC instead of TDC then when running it could well have an effective static of 25 and an actual dynamic of 35 at 1000 rpm, depending on curve selected, which *is* way too much for a high-compression engine, almost certainly going to cause massive pinking. Engines to exactly the same spec did vary even when new as to how much advance they could take, and some could take more than spec and get the consequent benefits in performance and economy. But that was on unleaded, on leaded even 99 octane my 73 roadster can't run at the original advance, it only just could do so on unleaded. With any curve the best you can expect is negligible or no pinking at any combination of throttle, revs and load. But unless you are very lucky the selected curve will only be touching the ideal curve at one point, everywhere else will be running at less than ideal. With a 123 you can try different curves and compare the *static* timing for each when set for maximum advance short of pinking, but you have to take the distributor out every time which is annoying.

It's not a good ideal to keep undoing and doing-up the single clamp bolt to remove and replace the distributor or you can end-up distorting the shoulder on the distributor body and it will start jumping out. It's easy to overtighten this bolt, as well as not get the distributor fully seated. You also completely lose the timing while it is out unless you put marks on the distributor and block. It's safer to remove the distributor complete with clamp plate undoing the two bolts, but if you mean only refitting one while getting it in and out to change the curve then that's fine. It also means it is easier to keep the timing as it can only vary within the adjustment range of the slot in the clamping plate. Not sure whether pulling on the fan belt to turn the engine is such a good idea, I prefer nudging it along in 4th gear. Doesn't matter whether you do this before or after removing the distributor, the distributor shaft will only engage properly with the drive gear in one position, the distributor shaft can turn while it is out of the distributor in any event.

I regularly get mid 30s winter and summer, and on a long steady run through France and back got over 40. I've had 30+ out of the V8 on several occasions.

Herb - James didn't state when he measured his 40 degrees, I'm saying 40 degrees *can* be correct, as are you, when the engine is revved. It can be anything from 15 max to 41 max depending on distributor, which originally depended on engine number and compression, year and market. Some American spec of this era could be 35 max centrifugal plus 24 vacuum, or as little as 15 and 14.

Haynes doesn't have the timing 'curves' but the Workshop Manuals and other sources do, I have collected these together at http://www.mgb-stuff.org.uk/curvestext.htm#curves
PaulH Solihull

Paul - thanks for taking the trouble to write such a full answer.

My running at 15 BTDC (without VA) happened like this ...

... a short time after having an unleaded head fitted, successive tankfuls produced 28 ... 27 ... 25mpg so I had to do something. I discovered the 123 was no longer tight so I advanced it, actually about 6deg from where it was. It then ran fine, with only very occasional very slight pinking. This gave better than 30mpg for 10,000 miles. It was only recently I realised that this was set at 15 BTDC!

The best I've ever had was about 34mpg on a steady run, probably an equivalent run to when you achieved over 40mpg. I eventually decided that I ought to be able to improve fuel economy if I had Vacuum Advance, which is the where I am at the moment, trying to optimise things.

I have been running on curve C (32 deg @ 4,400) and I doubt that I am getting even 30 mpg. Performance is certainly as good as can be expected and there is no trace of pinking. One way or another I need more advance don't I?
Geoff Everitt

Hi,

Geoff,

For interest's sake, which version of the 123 do you have, the MG or GB one? I assume that you have the GB, because you are using curve "C", but the MG one will also has a curve "C", as below:-

C - intended for 'stage 2' engines TDC 32 @ 4400.

There are 3 123 dizzys that can be used with MGs, with B series engines:- the MG one, which has the original MG curves programed into it, but as Paul has pointed out these are less than optimal for today's fuels. The next one is the generic Lucas dizzy, the GB version which has a selection of curves that are better, but whether they are optimal..? This is the unit I have, as it was the one recommended by members of my club. Reading the article I mentioned, above, where an MGC's dizzy was remapped, with great improvement in performance, the original curve quoted is very close to the original curve for my GB engine and the remapped curve is very close to curve "C" of the 123 GB. (In my case the dizzy that came with the car was out of a Hillman Imp MK1, according to the part number).
The third 123 is quite new, its called the 123 tune, and can be dynamically tuned using a PC, to adjust the curves, ideal for a rolling road tune.
If it had been available when I bought mine I probably would have opted for it, but since 123s are very expensive I'm not going to do it now.

Herb
Herb Adler

Hi Herb

No, I have the MG-4-R-V 123. I was first running on Curve A (34deg @ 5800, as suggested in the instructions). Curve C (32deg @ 4400) was later selected by Peter Burgess when he fitted my Econotune head.

What does curve C represent on your dizzy?

EDIT: OK I've found it, 36 @ 4500
Geoff Everitt

Lack of vacuum advance could well be contributing to lack of pinking at such a high advance, i.e. you are compensating for it anyway by advancing the static, adding vacuum could end making not a lot of difference if you then get a lot of pinking and have to retard it. My roadster does pink slightly on runs in more hilly territory (this era of engines seems particularly prone it seems and there are cylinder head mods described in Lyndsay Porter's book to hep alleviate it by removing hot-spots), it being set up for round here which is relatively flat. Paradoxically I find I can stop part-throttle pinking by opening the throttle more, which could be in part be due to removing vacuum advance altogether and part the larger cooler charge going into the cylinder. I also have the vacuum advance switched electrically, using an American TCSA solenoid and a switch under the dash, and this will also stop part-throttle pinking under some circumstances, as will simply changing down. But personally I would always run vacuum advance as the vast majority of my use is touring.
PaulH Solihull

Hi Geoff,

I found this article about EDIS, it might help you.

http://www.mgexperience.net/article/mgb-edis-installation.html

Herb
Herb Adler

Hi,

Sorry, the last should have been addressed to James.

Herb
Herb Adler

This thread was discussed between 02/10/2010 and 08/10/2010

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