Ignition timing scatter

Demetris

Well-Known Member
#1
The last time that i went for a ride with my TC, it felt somewhat lazy so i thought i d' better have a look at the ignition timing. Timing light on the flywheel and it was obvious that at idle, the timing varied at what looked like 3 or more degrees. Taking the average of that, it seemed indeed to be around the TDC, so i turned the distributor a little in order to bring it back up to the required 6 degrees BTDC. Now the car drives better, but i wonder about the timing scatter at idle (at speed it looks to be rather stable). The timing chains must be well worn now, at around 110K miles, and at least the top one feels somewhat slack on finger pressure.
So, do i have to blame the timing chains only, or could wear in the distributor cause this problem?
The distributor is a standard Lucas 25D4, with Lumenition magnetronic fitted.
 

sdibbers

Well-Known Member
#3
My guess would be the springs under the distributor baseplate. I had my dissy rebuilt and recurved a few years ago. Before then it was always all over the place timing wise. It got so bad the ignition would go to full advance above 1500rpm
 
#4
I changed my dizzy for a full electronic version at NEC visit. though rev counter seems to be. fluttering or wondering when idling, everything else seems fine. unsure state of chains and timing gears but until I get other issues or high mileage ( current showing 38K and thought to be correct despite. 6 owners as spent life in London ergo high traffic and little used ) at later date.
 

sdibbers

Well-Known Member
#5
I changed my dizzy for a full electronic version at NEC visit. though rev counter seems to be. fluttering or wondering when idling, everything else seems fine. unsure state of chains and timing gears but until I get other issues or high mileage ( current showing 38K and thought to be correct despite. 6 owners as spent life in London ergo high traffic and little used ) at later date.
I've always wondered what sort of advance curve those off the shelf electronic dissys have? I wonder if they are best for our four cylinder cars.
 

Demetris

Well-Known Member
#7
OK, thanks to all of you for your advice. As i happen to have a new old stock 2200 TC (complete with the famous rev limiter rotor arm) on the shelf, i think that i will just swap it (minus the rotor arm). I presume it won't be exactly what my engine needs, but it won't too far away, and my engine has already the carbs of the 2200 TC.

Steven, the top chain doesn't feel too bad, and there are no nasty rattles either, so i am not rushing to replace the chains, especially as i drive it so little these days. Perhaps one day i will remove the sump to replace the bearings, so i could do the chains, tensioners and oil pump gears in one go. Not that any of these feels bad now, but as i already have through the years gathered almost all the required parts, just for the satisfaction that i will have again an almost new engine.
 
#8
I currently have the distributor for my SC Auto away getting checked out and regraphed. It has an accuspark electronic ignition that I installed a little while back that tey say is working fine.
Although I have got it back yet (next week) they indicated that the main problems were (in order) 1. the rotor button, 2. tired springs & 3. the age of my leads (they ask you to send it with the distributor).
The service they offer includes checking the curve and tweeking it for modern fuel. Its costing around AU$250 (150 Euro) including a new set of leads.
Rebushing the shaft or playing with weights would have been extra.

Scientifically, I'm hoping its a good investment.

Note, I'm also redoing the valve clearances before I put her back on the road I won't be able to report back for a few weeks
 
#9
I've always wondered what sort of advance curve those off the shelf electronic dissys have? I wonder if they are best for our four cylinder cars.
I was told advance curve is set up for each car version they supply .it are complete with auto advance vacuum unit fitted , so not the type with 'programmable' advance which would be better still. ( and cost ££ ) as for performance ..it hasn't made any difference acceleration but does start and run better especially when warm. have a gut feeling initial advance can be set higher than the 6 degree recommended but aware there are many saying actual TDC mark isn't. accurate on some engines . will check that out next season when I het chance.
 

sdibbers

Well-Known Member
#10
I was told advance curve is set up for each car version they supply .it are complete with auto advance vacuum unit fitted , so not the type with 'programmable' advance which would be better still. ( and cost ££ ) as for performance ..it hasn't made any difference acceleration but does start and run better especially when warm. have a gut feeling initial advance can be set higher than the 6 degree recommended but aware there are many saying actual TDC mark isn't. accurate on some engines . will check that out next season when I het chance.
Which supplier are you using for this? most seem to sell the same unit no matter the application.
 
#11
if I recall correctly was "power spark". I bought it few years back at my yearly. trip to NEC show in November. was a full unit with dizzy cap minus HT Leads. ( already replaced) came with a high energy coil as part of kit. slight re-wiring needed on dizzy but instructions came with unit. ending up using one less wire if I recall(white trace?)
 
#12
spark scatter is seldom due to springs or weights. most often it is deteriorating leads and/or plugs. some "restorer"s wipe the grease off the cam which will cause the cam to wear and if their is no oil lower down below the points plate, the bearing in between the top and bottom halves of the rotor shaft can wear badly giving very erratic timing. Not clipping the cap on correctly can also damage that bearing and even bend the shaft giving very erratic timing and variable dwell.

Most race tuners I know leave the weights alone and just change the springs. They generally start with two of the standard soft springs (instead of one soft and one hard) which brings max advance in faster. On a mechanical distributor with the standard two weights and two springs the curve is really three straight lines: being none, springs both working (straight line rise in advance) and finally max advance. The Modern mapped distributors have multiple weigh points giving a curvy curve and possibly multiple options for different inputs such as manifold air pressure, pink sensors, temperature etc.

Personnally i'd just do a full electrical tune first and check for sideways movement in the rotor shaft while you do it. note the way lucas distributors work you can't test the spring action by twisting the rotor. You have to take the plate off and have a look.
 

Demetris

Well-Known Member
#13
A rather late follow up to the subject, but i have just managed to replace the distributor with a new old stock part from a 2200 TC. I have swapped the lumenition guts, fitted a conventional rotor arm, but retained the tin shield of the late distributor. I am pleased to report that the ignition timing is steady now at idle and speed. I have not had the chance for a test drive yet, but the engine feels marginally smoother and perhaps more eager to rev. I presume that the faulty springs (one of them was definitely slack) gave a faulty impression of too much advance at idle, and subsequently not enough advance at speed.
 
#15
Yup
I believe that is how the advance curve is set, one spring controls initial advance, then as the rpm increases the second spring comes into play to add limiting movement to the bob weights as they get flung further out, the further out the big mass gets from the centre point the more spring effort is needed to check it.
 
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