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MG Midget and Sprite Technical - Dodgy dissy?

I'm in the process of undertakinga well overdue tune up and on digging out my timing light I'm finding that with the strobe on the timing marks on the pulley are jumping quite noticeably clockwise every few seconds. This effect seems to become more noticeable at higher revs. I was trying to time 32 degrees before tdc at c4500 rpm and I have of course disconnected the vacuum advance. I'm running pertronix Hall effect electronic ignition in an otherwise standard d25 distributor. Trying to accurately time the bugger is proving impossible and I'm wondering if the mechanical advance mechanism is perhaps on the fritz. If this is the case, what's the best option. A replacement like for like, an overhaul of the existing distributor or something like a 123?
Needless to say before I started all this the car "appeared to be running smoothly and pulling well but I may have become used to it. I do think I'm running too rich and I'm wondering if I've got the carbs adjusted rich to compensate for dodgy timing?
Matt1275

Make sure your plugs/leads/cap/rotor arm, connections to coil are all spot on before condemning the distributor. Repeat the timing check on no 4 as well as no 1. Set the fuelling correct to allow fast idle for checking the timing.

Peter
Peter Burgess Tuning

Thanks Peter, but been there, done that already!
My mark on the pulley seems to sit broadly static under the strobe and then as I said jumps clockwise around the circumference of the pulley by a good inch and a bit and immediately goes back. Just not sure what is causing it. I didn't however try timing off no 4 but if all the HT gear is OK, I'm not sure it should make any difference.
Matt1275

It would be to confirm the 'problem' exists on more than one cylinder. You can even see if the effect is on 2 and 3 if you mark the pulley. You are trying to build up a pattern and work out what the problem isn't.I do not think the mechanical advance can cause a problem as you have, more likely misfire upsetting strobe. Backlash in the drive from crank to distributor can hiccough and show the flickering of timing as the gears and chain play catch up.
Don't assume the movement is 'gospel' and really exists, on many occasions we get similar effects from interference from the other leads or the king lead being picked up in the inductive clamp/wiring for the strobe light. Move things around and suddenly we get a clean signal.

Peter
Peter Burgess Tuning

Peter,
Many thanks. I will go and have another play tonight and see if I can get a "clean" signal. I was hoping at best for worn out mechanical advance and fearing timing chain play or worse, excess movement in the distributor drive. Fingers crossed its "spare" sparks!
Matt1275

Are you using the electronic style strobe timing light ie the type with a dial-in advance setting so it flashes at top dead center?
The reason I ask is that the one I have is cr*p in that it's ability to detect the rpm accurately is poor, so the resulting calculation for "delaying" the strobe pulse is wrongly calculated and jumps all over the shop.
I have to use it with the advance in the gun set to "0 deg" so it flashes in time with the advance set on the dizzy.
Graeme Williams

Needless to say before I started all this the car "appeared to be running smoothly and pulling well but I may have become used to it. I do think I'm running too rich and I'm wondering if I've got the carbs adjusted rich to compensate for dodgy timing?


im wondering if your set to rich to compensate for a vacume leak... esp being at 4500 rpm ...maybe around the intake gasket / spacer blocks... port vac im purly guessing the marker timming advance (actually retard if the mark is moving forward) would occure after the let off of the gas peddle

might be interesting to stick a vac gauge on it and see what the readings say


prop
Prop and the Blackhole Midget

Matt,

Is it possible that your phasing is far enough off that the jump you see is because it is firing the wrong lead?

Charley
C R Huff

Mega X charly

i didnt even think of that but pertronix chips do have a history of phazing... it did it with me
Prop and the Blackhole Midget

..... and the answer was ?
Graeme Williams

I had a similar spark scatter problem on the Frogeye. I was getting about 3 degrees of scatter at tickover ( on the strobe) jumping to a blurry 10 degrees at full advance.
This on a 1330cc Sprite with all new distributor drive spindle + Aldon 100 AY + Pertronix module + duplex + A Plus tensioner. A recipe for spot on timing if ever there was one. So I took the issue to the guys at the rolling road who inspected the various parts and declared them in excellent nick - I told them the distributor cap runs on proper copper core plug leads.... and that was the problem. Copper leads can lead to unwanted signals in cars with electronic ignition and I needed suppressed leads instead. So I walked away with a set of high temperature, fully encapsulated double silicone leads and hey presto, I now have a rock steady reading.
Pollock

Marton: I had a similar problem with the Amethyst unit. I had replaced silicone leads with copper becuase I have (years ago) found them more reliable. The unsuppressed nature of the copper leads totally killed the Amethyst. Interestingly, there was no mention of this in the Amethyst documentation.
Graeme Williams

...ah that may be the answer. Just swapped from silicon leads back to copper! I think I managed an average timing to my satisfaction but I may need to do some fine tuning. See my other thread on sooty plugs!
Matt1275

It isn't mentioned in either the Pertronix or Powerspark blurb, and there's no mention in the Vizard tome either.So I'm sure many people have happily adopted copper leads unaware they were building in a scatter issue at the strobe. Of course Matt may already have silicone leads, in which case he's likely got a snatch and grab thing going on and needs a tensioner.
Fergus

Historically copper leads were much more reliable. THe silicone leads would fracture the core or be very difficult to create a good connection when the screw in plug caps were fitted. THe downside of the copper was a bit of radio interference.
THose of us whose motoring experience goes that far back tend to favour the copper but don't necessarily realise that a few specialist electronic components have crept onto our classics and that these may not be compatible with the old-style copper leads.
Graeme Williams

What about copper leads with suppressor plug caps? that's what I have on my engine.

BTW I found out that the plug caps that screw onto the leads don't work well long term with the graphite impregnated leads. Used to work for maybe a year and then got misfiring, They need crimped connections.
David Billington

David
My new home made leads are copper with suppressor caps. I switched back from graphite cores for the reason Graeme suggests that I didn't have terribly reliable connections at the screw in dizzy cap end. Is the scatter you suggest likely to effect the running or should I just keep calm and carry on?
Now it's all back together and broadly tuned it seems ok if perhaps a little down on top end power. Some further tinkering required perhaps?
Matt1275

The issue here is that home made copper leads - even with suppressor caps fitted, are likely to produce enough electronic noise to mess with the strobe. Although the actual engine will run normally, the problem is that it's well-nigh impossible to set the timing accurately when the strobe is dancing about. So the rolling road guys would always swap to silicone leads for the duration of the test - and put the customers copper leads back on at the end.
Fergus

Fergus
Many thanks. I'm going to swap back to silicones and try the timing light again and see if this gives me a steady reading and swap back when all done. I'll report back!
Matt1275


So I now think my timing is as close as I can get it to 32 degrees at 4500rpm and 12 degrees at 1,000rpm or thereabouts.
I've "retuned" the carbs and given the car a good run and its OK but seems to lack some mid and top end urgency. At revs it feels like it is being held back a little and I'm getting occasional metallic-ish backfires at 70mph plus as if its having trouble breathing.
The dizzy has been moved quite a few degrees anticlockwise from where it was to get the timing "correct" and I think I'm now running with a leaner mixture than before. the only other major change was I adjusted the rocker gear gaps a bit. It now sounds less tappety than before but have I maybe closed them up too much. I used the standard handbook gap settings.

Any clues gents?
Matt1275

12 degrees is a bit advanced for my taste... but not far out of range

the matailic back fire at 70 mph scares me... that could be because its running lean and to advanced

Id try for 10 regardless what the top end is set at... its rare you drive at the 4500 rpm range vs the 1000 to 3000 rpm range

are you sure your timming light is accurate

did you remember to cap off the vac hose that goes from the dissy to the front carb on The carb side of the hose... not the dissy side

prop
Prop and the Blackhole Midget

Hi Matt

What is the spec of the engine including CR? K&Ns? Needle/s?

Peter
Peter Burgess Tuning

Peter,
I've never had the engine apart but its an old Gold Seal unit so I suspect its probably +20 overbored. No idea of the CR, assuming close to standard and I'm running K&Ns with slightly richer needles, can't recall which exactly.
Prop. Vacuum advance was disconnected and I'm pretty sure the timing light is good.
Matt1275

The engine has never been apart and its an old gold seal engine...

Considering you got lots of scatter timing at high revs and some kind of nasty matalic misfire backfiring going on under load yet settles out at idel and its driving sluggish at 70 mph im now thinking its ...

the rubber on the harmonic balancer is wearing out giving some bad back and forth (retard advance) play... because of the rotation of the engine at even constant speed above 70 its pushing the harmonic balancer forward causeing it to retard and lacking power under acceleration its forcing the harmonic balancer backwards making it advance the timing and thus the metal sounding backfire/misfire

other areas would be a worn out timming chain and sprocket and maybe even a wornout dissy... is the plate in the dissy that holds the points solid without slop play ... is there alot of free movement in the dissy shaft while not running .. granted there is some do to reg gear lash ... but alot of movement in the dissy shaft ?

Prop
Prop and the Blackhole Midget

Prop,

A bad harmonic balancer won't do what you mention as it's not in the drive train to the dizzy or anything else. It does lead to a definite rumble in the 5k to 5.5k RPM range on a standard 1275, I've had one and it was quite noticeable as you passed through that range but didn't seem to produce any other problems although continued running in that range could lead to crank failure.
David Billington

Assuming the engine may be somewhat coked up from age, I reckon try 30 degrees max and see how it feels.

Peter
Peter Burgess Tuning

Peter
Thanks for the positive input! I'm away at the moment but I'll try that as soon as I get back.
Matt1275

Pleased to report that all now seems to be well. Had a great run down to the Ace caff and back last night and good to see Geoff, Bill, Gary, Mark and the other usual suspects.
I think the timing marks I put on the pulley may be a few degrees out! I was running way too retarded.
Peter I may be in touch to have an RR session and I also have a MG Metro head which needs some valve work that's been sitting on my bench for the last three years and a Hif44 and manifold that one day I will get round to fitting.
Thanks everyone for all the input!
Matt1275

Matt

I have often thought with timing it is a trial and error exercise. Assessing the car's performance up a local hill, and nudging the dizzy body CW/ACW accordingly is as good a method as any as regards obtaining a reasonably accurate advance. For me, there are simply too many variables in play to go by the book as it were....

You also need to fit that head, the HIF 44 and the manifold soon. They are tremendous!

M Ogden

This thread was discussed between 14/03/2016 and 13/04/2016

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