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MG Midget and Sprite Technical - Dynamic Timing 25D4 with vac


Hello all.

I use a Lucas 25D4 with Vac and magnetronic ignition.

Purchased a timing light with rpm indicator plus auto advance. I've moved the TDC mark to a convenient place on the engine and re-marked the timing mark on the pulley. So now I can time from above the engine on the dizzie side. As I do not know how well the vac advance or mechanical advance are performing I followed advise on this site & timed at 30 degrees at 4,000 rpm (vac tube removed and dizzie end plugged).

I've now got a flat spot (engine falters) at 2500rpm. Does this suggest the vac and/or mechanical advance are faulty?

One thing I noticed with the strobe was that most of the time the pulley mark was consistent but did wander occasionally anticlockwise. Is this another clue but maybe more to do with general wear?

NB - after setting the timing I also weakened the mixture on my HIF44. Following a trial run I re-adjusted to the stronger original 1 without success.

Any thoughts would be appreciated.

Gavin.
G Rowles

What's the advance at the problem speed? (Again, with vauum disconnected)
Paul Walbran

Without the vac connected, the advance should increase with revs, quickly as low revs increase, then tailing off at 4 to 5K. The advance should increase and decrease with revs (as engine blipped).

With the vac connected, the behaviour is more complicated ...generally, the advance will be higher... if you blip the throttle, the advance will reduce with lower manifold vacuum, then increase with revs, and increase further as throttle is lifted off, gradually going back to initial reading as the engine goes back to idle.

I always time static - 7 deg BTDC when engine switched off. This is a definite set of conditions that's easy to replicate, and doesn't reply on dissy advance etc to set.

HTH

A
Anthony Cutler

Anthony - but it does assume that the distributor is behaving as it did originally. As distributors age, wear take place between the second spring and the post as it takes up and in extreme cases can be so worn that take-up doesn't occur until nearly full advance. This changes the advance curve quite a bit, and with distributors with a light primary spring such as the MGB you can end up with very little happening in the way of advance after idle if the advance is set dynamically at idle.

Alternatively, if the advance is set statically and the spring/post has bad wear, then early advance is too rapid and some form of pre-ignition occurs around 2000-2500 RPM - again especially on MGB's which have that really light primary spring.

So yes, static is a good method, but periodically check that the advance is doing what it should and make some adjustments to bring it into line if not.
Paul Walbran

As Paul says, distributors can wear significantly. As such, advised settings for advance become idealized, and a guide only. Personally, I have found the only way is to set it to static as a good starting point and then rotate CW/CCW the body gradually according to how the engine is behaving under load. Fortunately I have a nice long hill close by, so a few excursions with a 7/16 spanner for the distributor body clamp bolt proved successful.

As regards HIF44 mix, do note the mix screw is a pretty fine adjustment and controls the mix all through the range. It is therefore essential to get the mix right before fiddling with ignition settings. Again one can use colourtune or similar, but personally they are too vague for me. I prefer adjsuting throttle stop to about 2500 and setting the mix screw to provide the highest revs from there on in - then screwing in a tad - to richen slightly. All explained in SU manual - on web somewhere....
Mark O

Thanks for comments.

Paul. I'll check the advance at 2500 & see if this conforms.

Anthony. If above is inconclusive I'll set statically as you suggest.

Otherwise, I may have to take the dizzie out & inspect for wear or if unsure just replace the springs/posts. This is unlikely to happen prior weekend ahead but will advise how I get on.
Gavin Rowles


Mark - thanks for your input on HIF. Whole lot easier adjusting this aspect 1st so I'll give that a go.

I did use colourtune but will try your/SU manual option.
Gavin Rowles

Gavin

I also tend to err very slightly rich to prevent overheating and pinking. Set it as per manual and wind in about a quarter turn...Give it some beans up a hill and listen for pinking/misfiring.

Have to say that correct mix setting cured my pinking problem almost immediatly, and that the dynamic ignition timing/setting was a secondary factor - although it did improve matters slightly thereafter. You must know though that I have 123 electronic ignition.

Cheers

Mark
Mark O

Gavin, if there is wear on the spring hook and post, closing up the hook is a good cheap option. Get the centrifugal advance spec from the manual and plot it on a graph. It will be a pair of straight lines. The intersction of them is where the second spring takes up. Compare this with the full amount of advance and that will tell guide you to the point where the hook should take up.

For example, std MGB (sorry, it's the only one I can remember offhand and I'm too lazy to dig out the midget details)
The bend in the graph line occurs after 4 deg distributor advance. Total distributor advance = 10 deg. So the second spring should take up at 4/10 = 40% of the movement between the stop and the post which limits it.



Paul Walbran

Yes Paul, agree... once the springs become weaker, then they will allow more advance to come in at lower revs... or even be sloppy around idle.

Once I set static, I listen carefully for pinking around 2-3K under load in 4th and advance/retard accordingly.

Much easier with 3D mapped curve and application!

A
Anthony Cutler

"""As I do not know how well the vac advance or mechanical advance are performing I followed advise on this site & timed at 30 degrees at 4,000 rpm (vac tube removed and dizzie end plugged)."""


I see huge potential problem based on what you said....

Your not quite set up correctly based on what you wrote....the dissy does not care if its blocked off or not at the advance box

But you do have to block off the vac hose that goes to vacume port on the engine... if you leave this open then you have a vacume leak and a bad one at that

So id recommend blocking off the vacume pipe from the disconnection at the dizzy and see what that does

Prop
Prop and the Blackhole Midget

Can electronic ignition be statically set? I have the SimonBBC kit and the old staple of using a lamp across the "points" doesn't work. Is that just with this set up? Is there another way?

Prop: I read what Gavin wrote and assumed he blocked off the flexible back to the carb at the dizzy end. I'm sure he wouldn't be blocking off the dizzy end. I'm sure if he had we could chat about it at the MASC meeting on Sunday. Make a change from him reminding me about front wheel bearings (eh Gav!)
Graeme Williams

Graeme on these hall effect modules you can often hear a 'clack' inside the distributor as the spark jumps. I'd set the crank to 7 degrees BTDC - turning it clockwise to take out mechanical slack and first retard the distributor, before slowly advancing it (ignition on now) till you hear the 'clack'. Wear rubber soled shoes.
But given the advice above I'd get it checked at full advance. 28 - 30 degreees is about right for super unleaded. This should allow you to give it the beans and not have any pre ignition or running on.
F Pollock


Prop - thanks for spotting this error.

I'll block the carb this time & check the timing. Hi Ho - your never to young!

Graeme - don't tell anyone at the MASC meet. I've got a nice set of bearings for you.

G Rowles

Have had many Midgets but am now firmly V8 B's. Why HIF44? I assume this is for performance gain or simplicity (no balancing necessary). With your mentioning mixture alteration you are assuming the needle is correct! Also have you checked the vacuum unit is actually working and the connector ends are not split.
Allan Reeling

Static timing on electronic: I fit the cap, fit a spark plug to no 1 lead with its body earthed then slowly roate the distributor clockwise until spark occurs.
Paul Walbran

Gavin: I'm looking forward to Sunday! Should have my one pair of trousers out the wash by then!
Graeme Williams


Hello folks - an update.

1. Timing. I removed the vac advance from the dizzie & plugging this end to ensure no leak at the carb I re-timed using the strobe this time at a more sedate 1000rpm & 13 degrees advance. This did not seem to make any difference to where the mark appeared on the pulley. My thoughts were either I have a duff strobe or I had incorrectly re-marked the pulley. I assumed the later & this was the case. So after removing the old mark & inserting the new mark I re-timed at 1000/13 again. This improved the performance but there was still a hesitation at about 3K rpm.

2. Mixture. I was pretty sure I had messed this up & a quick check with a colortune confirmed that the mixture was weak on quick throttle. So, working on the assumption I had got the timing pretty well sorted I decided to concentrate on this. Locally I have a little one way circle going up & down a shallow hill. I progressively increased the mixture & drove up & around this hill until I had got it much improved - although coming down the hill noted a little too much popping from the exhaust. Having retarded the timing 10 clicks (about 1 degree I think) on the vernier I set off for the 25 mile trip to the Kent MASC meet today. On the way I retarded A further 5 clicks & this seemed to be about right (exhaust popping on overrun). Car was still a little hesitant but at constant speed or light acceleration OK.

After a beer and lunch out came 1 plug which was a nice mid tan colour. I thought it should have half a turn more to enrich & despite advice from AN other (you know who you are) the run back was a complete success & car pulls very well. Plugs still look as should be but have backed off a quarter knowing I can add it back if required.

Lessons learnt.:-
1.When tuning change one element at a time.
2. If re-markin TDC on the pulley - do it accurately - mines still out by about a degree - but you all know what a pig it is to see the marks underneath (my excuse)

Thanks all for your comments & advice. Gavin

G Rowles

This thread was discussed between 22/09/2013 and 29/09/2013

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