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MG MGB Technical - Ignition timing

I have been lurking on the MGA site for a couple of years and have now just bought a 73 BGT Auto. The previous owner maintained it fastidiously, but prior to selling it switched back to (new) distributor and points. The timing seems a little advanced as it's pinking under load. My question is what is considered to be optimum dynamic timing for a standard engine? (or should I just go by the book?)

Neil
Neil McGurk

The optimum should be a degree or two below the number at which it stopped pinking. Just slowly retard and test until you reach this magic number.
Frank
Frank

Neil. 34* with vac disconnected is usually the most a standard B wants or needs. You do get some MG engines that wont ping even when over advanced, but to go any higher is detrimental. As Frank said set it below the ping anyway. The book does not mean much now as fuel has changed a lot. Denis
DENIS4

Thanks Denis, that's what I thought and just the information I needed.
At what revs should it reach max advance?

I am planning to buy a new timing light (Sealey TL93) with timing, RPM, dwell and volts so I'll try 34 and see if that cures the pinking, if not I'll just back it off a couple of degrees at a time.
Thanks,

Neil
Neil McGurk

There are distributor curves here
http://www.mgb-stuff.org.uk/wn_ignitionframe.htm
Geoff Everitt

About 3000rpm on a standard to warm engine. A highly modified or supercharged closer to 2500rpm. As Geoff said you can get information on all the curves but many of them were compromises. Denis
DENIS4

Is it a US or UK spec car? If its a US car and has the vacuum advance run from the intake manifold, the replacement vacuum units offer double the advance of what you need and that causes pinging at cruise speeds. I'd suspect 10 BTDC at idle should work fine though, unless your mixture is really rich. Make sure the point gap is correct as well.
Jeff Schlemmer

Hi Jeff, it's a UK car. I've ordered the new strobe light mentioned above and it should be here Monday. I'll recheck the timing etc. and report back then.

It has a new distributor and has been extremely well maintained since a complete rebuild about twelve years ago, so I am not anticipating any nasty surprises (famous last words!).
Neil McGurk

Jeff

Good to see you jump in. I recently went through the timing game with my 74 MGB GT. In an earlier post I indicated that I couldn't get it right after installing an electronic ignition. What I have learned is this.....I have owned the car for over 10 years and never has the vacuum advance been connected and the car ran just fine with lots of power. I assumed that the distributor was the original as the car was very original when I bought it. I connected the vacuum advance in the hopes of getting more MPG but ended up with less power and a rougher running engine. I then installed the pertronix ignition and the engine started easier and the running was smoother but no power.

It turned out that I had a 45d4 distributor and the lucas number indicated that it was for a landrover. I used a timing light that allowed me to set the advance but just couldn't get the timing right. I finally resorted to plugging the manifold vacuum hose and setting the timing by adjusting the engine to run at maximum RPM (at idle) and then back it off by 100 rpm. I have left the vacuum advance disconnected but plugged and the engine now starts and runs better and has the old power back. I have recently purchased a vacuum gauge as I understand that this could actually allow for a better tuning. I use the "B" every day on the highway to get to work in another town and am getting approx 32 miles to the gallon (imperial measure) so things are getting better but I am shooting for 36.

Brian
Brian Smith

The only real thing you can do is what Frank says, i.e. retard it until it just stops pinking at any combination of throttle, revs and load.

Even then if you normally drive on the flat but take to the hills you may find it start pinking on the hills.

There are so many variables today including worn and incorrect distributor springs, and very different petrol to originally, so that even if you have the original distributor and engine no one else can give you a figure, and you can't even use the original manufacturers figures. A 73 UK auto should have an 18V583 engine with a 25D4 41288 distributor. This was originally set to 10 degrees static and 13 degrees strobe, and gave a maximum *additional* centrifugal advance of 20 degrees at 2200 rpm i.e 30 degrees in total.
Paul Hunt

Thanks Paul, it's the original engine with a replacement distributor. I live in the Lake District National Park, so not so much driving on flat, unless it's a longer journey! I guess the fuel octane rating will play quite a big part too. I have 95 in the tank just now so I'll see if it will time on that. I normally use 97 so if I think I'm losing power to stop the pinking I'll try again with that.
Neil McGurk

Paul, what you're saying is contradictory and this is where people get confused. If a car has 10 degrees static timing, but it strobes at 13, that extra 3 degrees isn't magic. That's subtracted from the 20 degrees of total mechanical advance, leaving 17 degrees of mechanical. Now, we're assuming that the 10 degree cam in the distirbutor is accurate and most aren't, so that could make up for the discrepency. Also, you should figure that the springs in an old distirbutor are fatigued, so 10 degrees static COULD mean 15-20 degrees under strobe. This will decrease the total timing by the corresponding amount if you use a strobe to adjust your timing which means your setting will be retarded at highway speeds. That translates to a lack of power and likely a hotter running engine.

In Neil's case none of this matters since he has a new replacement distributor and we can only guess at its specifications.
Jeff Schlemmer

Jeff - If you look at the spec sheets the original distributor for Neil's engine has 10 degrees static, 13 degrees strobe at 600 rpm, and on the curve it has 3 degrees at 600 rpm i.e. 10 degrees static plus 3 degrees from the curve gives 13 degrees strobe i.e the curve figures are added to the static figure as I said. Further up the curve the same will also apply, i.e. at 2200 rpm there is 20 degrees on the curve which is added to the static as before to give 30 degrees. It's done this way because any one distributor could be used on a variety of engines, which may all have different static and strobe figures, but the curve figures i.e. *additional* centrifugal advance at a given rpm are the same for all.

If by the 10 degree cam you mean the finger that gives the maximum centrifugal advance i.e. 10 distributor degrees or 20 crankshaft degrees, I can't agree with you that most are inaccurate. These aren't like springs, there is nothing to stretch or wear, nor can I image that they are significantly inaccurate at manufacture. I did mention worn springs, but it depends on whether these are weak or stretched as to what effect they has. Stretched springs will start to add advance much earlier than they should, but once the slack has been taken up the curve i.e. additional advance will be as before. Weak springs will mean that the curve will be more aggressive, i.e. the further up the rev range you go the further away from the correct figure it will be, and you will reach the maximum advance much earlier. In practice our cars will have a combination, and so it is impossible to predict. All these factors plus the different grades and composition of fuel is why I said the only practicable way to set timing is to avoid pinking.
Paul Hunt

This thread was discussed between 14/10/2008 and 22/10/2008

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