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MG Midget and Sprite Technical - 1275 ignition timing

Just checking the timing before Gaydon. If I set the 1000rpm dynamic timing to 13 degrees I get a 3500 rpm figure of about 24 (with vacuum off). If I set the 3500 rpm figure to 30 degrees( as per Bob) I get pinking when accelerating hard. I put some Shell V-power in and that improved things somewhat. I have a Morspeed phase 2 cam/ 10:1 CR/HIF44/Titan/K&N. I have now set timing back to about 15 degrees at 1000rpm as a sort of compromise. Not quite sure which end to approach from, the 1000rpm setting or the high revs setting. Whats the consensus on this? Original distributor.

Steve
Steve Church

If you get pinking then probably your Comp Ratio is not 10 to 1 but will in reality be a lot more.
However that is not such a bad thing as I have run many times with max ignition setting at only 28, (I am running my turbo at 29 now) which allows more CR.
:-))

What would be ideal for you at the moment is to get it as far advanced as you can without pinking. Sounds like 26 might be the figure, which you already have.
Bob (robert) Midget Turbo

Bob, thanks for that. I had the MG Metro head skimmed to give me 10:1 which I calculated from measurements on the engine and piston. I could have made a mistake of course. I'll leave for now and see how it goes, then maybe increase the advance slowly until the onset of pinking and back off.

Steve
Steve Church

I'm interested in this thread because I have been having trouble timing my midget (MG Metro head etc etc). I found that the mechanical advance was still advancing way up to 5500 rpm so using Bob's suggestion of running at 4000-4500 was still giving too much advance. Admittedly I have a non-standard dizzy but I think running at 3500 will not give you maximum mechanical advance. I set mine to 30 degrees at 5500 rpm (it's scary lying under the front of the car while it roars away at this rate!!).
Chris H (1970 Midget 1275)

Steve

It seems that you have the wrong spec dissy. If you're runnung a high CR, you need less advance in the 2K-3.5K region, i.e. the curve needs to build up more slowly than standard.

I popped a HC/BV head onto my 1293 (276, HS6, LCB, K&N, RC40...), and had detonation in the mid range. If I wound the dissy back 10 degrees it would go away, but then I wasn't getting the benefit of the engine mods...

The solution is a 're-mapped' dissy, like the Aldon AY100V. It has slightly stronger springs to give slower rate of advance.

With this, my timing was set on a RR using mean best torque average at 3, 4, 5, and 6K (swinging the dissy at each rpm to find the peak torque). It turned out be be close enough to 7 BTDC static.

HTH

Anthony
Anthony

Anthony,
If I set the timing to book setting of 13 degrees I get a mid range advance (3500rpm) of about 24-25 degrees which is not as far advanced as Bob's recommended 28-30. I shall certainly consider an Aldon (will look see if any at Gaydon), but I also have a Megajolt system waiting in the wings. Admittedly the Aldon is a bolt in and forget system whereas the Megajolt needs playing with to get anywhere near the right curve.

Steve
Steve Church

I've got the opposite problem, which I think is down to me being a bit of a dummy. Mine is obviously running retarded, like the owner, and I keep advancing it with little response. My question is a simple one, which many of you will probably laugh at, what angle of rotation do the individual marks represent? I have so far read 5 and 4 on various blogs??

Cheers
Neil
Neil Williams

If Megajolt has a 3D map (allows for rpm and throttle posn (to represent engine load, same as vacuum for convetional dissy) it's the best way to go.

Then I can send you 3D map samples for you to put into it.

Otherwise the Aldon is pretty much a bolt-in, time, refine and forget solution.

Neil - most Haynes type manuals will show you a diag. For the Aseries, you have 3 fingers, seen from low/front. Look at your right-hand palm with fingers 3,4 and 5 extended updwards. Now rotate your hand half a turn so they are pointing down. Middle finger (longest) is TDC. 4th and 5th finger represent 5 degress and 10 degrees advanced respectively. This is what the timing marks look like (expect 4 & 5 are same length). You should time static with the crank notch between 'fingers' 4 & 5.

A
Anthony

I assume that the advance figures being discussed are all taken with the vacuum advance disconnected? When I reconnect the vacuum tube on mine this takes the advance reading way over the 28 - 32 range at 3500rpm. Or is this wrong?

Guy
Guy Weller

Sorry I have just realised a couple of things that I missed on this thread.

The 30/32 degrees is the figure we should reach when the mechanical advance is at it's maximum. That as can be seen here can vary depending on the dissy used.

We generally find that by 4.5 K the maximum mechanical advance has been reached and therefore setting this at 30/32 degrees is the correct thing to do.
Chris has done it correctly by setting his figure at the very maximum of his mechanical advance which is a staggering 5.5K

Rereading your first post Steve if you have not reached your maximum advance figure then you need to find this and then set the advance at that point. I thought you had done that at the time but perhaps I was wrong.

As Ant points out if after all this you end up with pinking in mid range then a modified dissy can reduce the advance at that point without hitting the top too much. Being a mechanical device however the setting of that would at best be poor. Which is why we tend to reduce the maximum figure . This maximum advance setting of 28 30 or 32 degrees makes very little difference at top end torque/ power and is not such a big deal. Naturally programmable ignition timing is the ultimate solution.

Bob England

Anthony,
Looks like I need to concentrate on getting the Megajolt going. I have all the bits, just not bolted them all together. I got the pulley machined when I had the engine out last winter. Just need to make up a bracket for the sensor. I must admit that knowing where to start on the mapping was concerning me a bit. It's nice to know that someone has got some maps with which to start.

Steve
Steve Church

Hi guys,
I have adjusted the timing, to what I think is the correct position, and am still getting the same problem. When I accelerate the car goes well up to about 4000 rpm, and then coughs and splutters, which I assumed was retarded timing, as no matter what gear it's in it won't pink. I am now wondering if the bob weights in the dizzy may have stuck, and need freeing off. To further complicate matters it is a Maestro electronic dizzy and an A plus block, fast road cam, MG metro head, and breathing through a 40dcoe with a maniflow manifold so I guess it could be something as daft as fuel pump??

Any help or advice appreciated

Cheers
Neil


Neil Williams

When it "coughs and splutters" what does the rev counter needle do? If it just drops revs slightly, then it may be fuel related rather than electrics. Could be the pump isn't keeping up. Or maybe a sticking valve?
If the needle is more erratic then likely to be electrical.

Guy
Guy Weller

Bounces all over the place like a big bouncy jiggy thing. This is what made me think electrical too.
Cheers
Neil
Neil Williams

yep, sort of rules out fuel probs.
Guy Weller

I think I might have found part of the problem. If you look at the pics you will see the posts that the bob weight springs attach to are bent, to the point where the springs are loose when the engine is not running. As much as it pains me, I am going to have to replace the dizzy for a more standard one. The part number of the dizzy is AAU1529. Does anyone recognise this model??


Neil Williams

This thread was discussed between 19/05/2008 and 21/05/2008

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