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MG Midget and Sprite Technical - Timing scatter

Hi all,

I'd be grateful for your advice. I have a 1098 Mk2 midget which is very original. I was trying to set the dynamic timing for the first time over the winter and I noticed some pronounced scatter, especially at higher rpm.
I have since had the distributor rebuilt, and have also changed the timing chain but the scatter persists (Not the end of the world as the new distributor mean the car runs better and a new timing chain cover has significantly reduced my oil output!)
Any advice as to what else could be causing this - do you get wear on either the distributor shaft or cam gear? If the former, is this something you can amend with the engine in the car?

Thanks in advance, James
J Fettiplace

It could possibly be wear in the dissy drive, which is easy to replace in-situ. The camshaft is another matter.

Are you still running with points?
Dave O'Neill 2

Hi James,
if the distributor was rebuilt was it with the Distributor Doctor, if so you probably got one of his proper red rotor arms.

Dave is already on to CB points - new parts from other that the likes of Distributor Doctor can be sometimes be poor to crap - springs, plates and particularly points, condensers/capacitors, rotor arms and dissy caps. Then you can have faults with new and NOS or wear and tear on existing parts, HT leads and plugs.

Then of course you need to test and confirm the testing equipment.
Nigel Atkins

Thanks both,

Yes and Yes - it is on points and the dizzy was rebuilt by Dist Dr (which i can recommend).

But back to the points, the scatter is much more pronounced than other cars i have had with points so I don't think these alone can account for it.
J Fettiplace

Combination or permutation of any of the above.

I think I've had timing gun flicker on the timing marks below with different electronic igniter head and fully electronic dissy (1275) with different loaned guns but I didn't trust the guns particularly.

Have you tried starting at the tappets and going through the set up again - that the full, total, stop limits of the next to nothing I know about such things.
Nigel Atkins

Can you feel sideways wear in the dizzy spindle - try waggling the 4lobe cam from side to side. Wear here results in variable points opening and closing, affecting the spark timing and resulting in scatter.
Given that you have a rebuilt dizzy you shouldn't have any wear there, but it's worth checking.
GuyW

Yes the dizzy drive can wear so worth checking. HT leads all ok?. I am a fan of points (sorry Nigel!) and I have no scatter at high rpm since Aldon rebuilt the distributor
Bob Beaumont

Bob,
you're forgiven but I will worry about your journeys. :)
Nigel Atkins

Someone here on the BB was suffering scatter recently and it turned out the shaft that drives the distributor had a badly worn gear which engaged the camshaft. IIRC you need a 5/16" UNF bolt about 3" long to screw into the shaft and pull it out.
David Billington

James,
thinking on a bit of a tangent from what Guy put - everything nice and tight, the dissy clamping bracket, (dissy cap to body, dissy plate, points) other things I can't think of now.

What about all wires and connections HT and LT(?).

Could you get interference from something else within the environment of where the test is being conducted(?).

ETA: was that the post about the bolt being one from a the LA damper fixing (on a B/Spridget?).
Nigel Atkins

My money's on the shaft. I don't see how HT and LT leads could affect timing.
Bill Bretherton

Backlash in the drive is almost certainly the problem. It is very common in B series as the camshaft gear wears quite a bit as it has a lot of load from also drives the oil pump as well as the distributor. I have eliminated scatter, sometimes as bad as +/- 20 deg, so many times by renewing the camshaft that a new camshaft is a default setting in a full rebuild rather than just refacing the lobes.

In A series. The oil pump load serves to damp the camshaft so although there is no tensioner lash from the timing gears is less likely. So
Paul Walbran

So we'll worth having a look at the drive spindle gear
Paul Walbran

Nigel,

Yes I believe it was and I still don't think the thread to pull the dizzy drive shaft is 3/8" UNF which the damper bolts are, but I'll stand corrected if it is. Anyone with a spare shaft to check?
David Billington

Bill,
I doubt HT and LT too, especially as most possible but if anything tracks back to the dissy cap or dissy and after checking for most likely you can go to possibilities and outside possibilities.

I had a (V8) car cut out whilst going along and then start and run fine, second time it happened I decided it needed sorting fast, I couldn't trace the cause myself. It turned out to be one HT leads was faulty, very annoying as I was going to change the set at the next service anyway and it cost more than a new set of leads to discover this on the diagnostics (Sun machine IIRC). I couldn't understand how the this would happen but rogue sparks under the cap might have tripped up the ECU. Whatever, new leads solved the issue.

I've found a number of times the problems have been the "it won't be that". Sod's Laws always applies to me it seems, I was very evil in past lives so I must suffer in this one.

Nigel Atkins

David, you are correct.

Dizzy shaft needs 5/16" bolt, damper is 3/8".
Dave O'Neill 2

Hi James,
Did you find the problem?
I had the same issue with my race TC. TCs have bushes in the dizzy shaft. Bushes have to have some radial clearance, and I don't think any clearance is ideal at the top of a dizzy. Rather than just reduce the clearance with new bushes, I got rid of ALL clearance by replacing the top bush with a small sealed ball race. I had to have a small stepped ferrule made up to fit the shaft because there was no ball race available with the correct ID. I also had to have the dizzy body machined out to take the OD of the ball-race. It seems to work well. I kept the bush at the bottom end as that doesn't seem to wear and doesn't influence the timing much. I have to use points because of Historic Racing regulations.

Cheers,
Bob Schapel
R L Schapel

Thanks everybody. I'll change the shaft and let you know how I get on.
J Fettiplace

This thread was discussed between 24/07/2019 and 29/07/2019

MG Midget and Sprite Technical index

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