Standard Twink Timing

[simonknee]

Hi all,

Bunch of background follows 'cause you'll only ask anyway...

Replaced my entire ignition system. Lumenition unit failed and it snowballed. Now have new 43D4 dizzy from PM, modded by Aldon to their 103TC spec. New lucas coil. Replacement lumenition bits. New lucas HT leads. A shiny timing light with advance dial. In fact only the NGKs and the sensor in the lumenition remain.

I am 90% that the engine is a standard Weber TC with the 'B' cams. DPO kept it non-SE last time he rebuilt. Cams under the cover confirm this (no grooves). The old dizzy was a 41489 which isn't correct for any TC according to the list in Miles Wilkins. Plus it had: a. stuck advance weights b. worn springs, c. play in the shaft. Oh and I always got black plugs in the three years I've had this car. Remember I run this daily through London though.

OK back to the plot...

Fit it all up and get it running at 12 BTDC.
Test drive and find loads of power past 4000 rpm that has been missing for a while.
Around the 2000-2500 mark experience occasional spit or backfire and stumble.
So I start to advance the timing.
A bit more.
A bit more.
I am now at 20 BTDC!!! yes really!
There is no stumble or backfire or spit.
There is no audible pinking.
It idles better.
I *think* it is starting to loose the top end power but that might just be because I've got more low-mid end in comparison.
I do hear a 'grumble' (as accurate as I can get, sorry) when pulling in 4th from very low revs e.g. 1500-2000. Is this knock? I've never known what knock sounds like.
I just did a good high speed run for several hours today.
Checked plugs and for first time *ever* 3 & 4 are grey brown, 1 has a little grey, 2 remains soot black. (so suspect slight carb imbalance).

I am waiting for the Aldon engineer who builds these dizzies to return from holiday so I can have a chat. I want to find out which of the 23D4 specs they copy (if any). The guy I spoke to so far says that the 103TC dizzy has 24 degrees of advance at 5500rpm. A check with my timing light shows 12-13 degrees of (extra) advance at 2500 (approx). According to Wilkin's data this could be a 41189 but with a different curve.

Does the above sound normal or would a true 20 BTDC never work?
Could my timing pulley be in the wrong place? Perhaps someone took the 10 degree mark as 0??
Is the screwdriver in the plug hole accurate enough to check this?

Any other ideas? Can anybody explain the gaps that are left in Miles Wilkin's book when it comes to the 40953 since the advance curve shows a max of around 36 degrees total advance but he says that no twink needs no more than 25 degrees total. In fact if you look at the difference between the 40953 and the 41189 the biggest difference of all is in the range where I had stumbling problems. If I was to set a 41189 at 20 BTDC then it would result in a curve similar to a 40953. There would still be an 8 degree difference at 2500 but I wouldn't ever get more advance over the range. Can anyone confirm the advance curves that Wilkin's shows, certainly my Brooklands manual contradicts at 2500rpm?

Dispite all this I am definitely getting somewhere, though I suspect it'll end in a rolling road but then that's another kettle of worms.

[type26owner]

****Actual dizzy advance + intial static advance = total advance relative to TDC****
What's the confusion? We run 40 degrees of total on our BRM which we used to rev to 9500 rpms back in the late sixties. The actual is also 40. Guess what the intial works out to be?

Long reach dial indicator thru the spark plug hole. Rotate up to TDC from both directions but note the crankshaft angle when stopped say .06" shy of TDC. Halfway between those two angles is TDC. Good chance the pulley index mark is off a lot. Mine was retarded by 8 degrees when I measured it.

Give up on the photo diode replacement for the points. Everyone who's used them for a short amount of time finds that out including me. I had multiple failures on my 62 Corvette. The Pertronix hall effect switch is the trick way to go to make the Lucas dizzy super reliable. In fact most of the vintage racing organisations in the USA allow and ENCOURAGE the replacing of the original points as a safety related improvement. I'm just waiting for the improved version to be offered now. Where if you forget and leave the ignition switched on while the engine is not running the added microprocessor in the hall effect senses this and shuts off the currect flow so the coil does not burn up. This very thing happened to us at the racetrack about four years ago. Not a good day! Just installed this new technology intelligent type hall effect switch in the vette.
-Keith

[pereirac]

I have a replacement Aldon dizzy and Lucas electronic ignition on my Sprint. I had the timing set up on a rolling road many years ago and it was decided 20 degrees BTDC was about right.

Peter Day of Daytune recommended the Lucas electronic ignition to me in the 80's as it's a hall effect unit so no optical sensors to get dirty:-)

[type26owner]

I know when I've power tuned the Elan's advance too much cause when I release the clutch quickly without giving it much throttle it detonates in protest. The grumble sounds like you've got detonation happening. Absolutely do not allow that to persist or serious damage occurs very quickly evidenced by micro aluminum beads showing up stuck on the spark plug's ceramic insulator from the melted pistons. Pinking is not so bad but should also be avoided on a road car. Many race engines are allowed to pink or ping intentionally. We let our BRM do it. That's where the power peaks with the high lift/long duration cams we're running.

Tune the advance by ear and ignore the numbers. Published numbers are mean values and as such are useful as a starting point only.
-Keith

[rgh0]

If you look at the advance curves in the Wilkins book there are 3 basic groups of curves

1. Retarded ignition for emissions reasons (19 degrees maximum total advance 41225A curve).

2. High maximum advance curves with total advance above 30 degrees that appear to be required due to the low volumetric efficiency of the Stromberg head ( 33 degrees total , 40953 curve) or the low volumetric efficency of the Standard cam engines ( 36 degrees total, 40953 curve)


3. Middle range maximum total advance curves between 24 and 29 degrees for the SE and Sprint Weber head engines. Less maximum total advance required due to better volumetric efficiency with the cams in these engines.

Ignoring the emission driven retarded ignition the other 2 sets of curves are basically correct for the types of engines they were used on I believe.

However a little fine tuning be ear and seat of the pants or on a dyno for the exact condition of your engine and the type of fuel you use is always needed.

A couple of directional comments based on my experience in this tuning.

a. Most of the curves ( except the 40953) have the maximum advance coming in too late. If you want to play with the springs go for lighter ones that bring in the maximum advance around 3000 to 3500 rpm). I believe Lotus like most manufacturers were conservative with the advance curves to minimise risk to the engines from poor fuel or driver abuse.

b. If you have to set the static advance up around 20 degrees to get the right maximum total advance then you dont have enough centifugal advance in the distributor and you need to modify it. Try to aim for a maximum static of less than 15 degrees other wise you just going to overstress the starter when the engine kicks back when starting and you will have to much advance in the 1000 to 2000 rpm range and end up with pinging and detonation on initial acceleration


By the way my competition Elan engine running 12:1 compression ratio, 300 degree cams and using Avgas needs 32 to 33 degrees maximum total advance for maximum power between 7000 and 8000rpm. The ignition runs 12 to 13 degrees static advance and has 20 degrees centrifugal advance that comes in a straight line between 1000 and 3500 rpm.

Rohan

[pereirac]

Avgas!! I suppose that's one way round not being able to buy 5 star leaded fuel!! It's about ?1.05 a litre at my local airfield, expensive but not much more that buying premium fuel and an additive.

[mike+2s130]

im running a +2 130 big valve.the dizzy is a 41189a.most books reccomend 12 degrees static.because of lower octane fuel i run at 10 degrees static.the car runs smoothly with no flat spot .pulls well through out rev range.no pinking.starts fine .hope im not tempting fate here.best regards mike.

[Gra-ham]

Another slant on things!
My +2s is running Dellortos and 'C' type cams (single groove). I removed the dizzy recently and found that it is a 40953 for a Stromberg head! THe car has a bonnet bulge and the wipers park on the right. Perhaps these are LHD parts.
At some time past the internals have tried to force their way out leaving three protrusions and one hole in the bowl! The internal cam is marked with a 12" ( 12 degrees advance on the camshaft) which I think is correct for the 40953. I set the timing around 10 degrees BTDC, use 98 octane and it seems to run ok. I have a lot of timing scatter at idle.
Any comments?
Graham

[steveww]

If you have a lot of scatter it is not helping. The dizyy is most likely worn out and needs rebuilding. I did a rebuild myself making one good unit out of two tired units. I only did this as I had the bits laying around any way. I would suggest you get in touch with either Aldon or H&H.

The problem with Aldon is that they never appear to have anything in stock and I have always ended up getting the bits elsewhere, you may have more luck than me here. I have been told very good things about H&H and I would give them a call first 01384 261500

[hatman]

Now please be tolerant with me if I've inadvertently grabbed the wrong end of the stick here - I am, after all, just an enthusiastic numpty - but I do seem to remember that the static timings were always supposed to be no more than a starting approximation for getting the timing spot on. The real fine tuning was supposed to be done on the road; pick a measured distance, preferably on your own private road, keep making runs over it with your welly hard down while checking the time with a stopwatch and the setting that gives the shortest time without causing pinking is the optimum timing for the engine, depending on what fuel you're using at the time (plus, to an extent I suppose, ambient conditions).

[rgh0]

"Hatman"

Your absolutely right that static timing is just a starting point. You really need to set it finally on a dyno or by doing the sort of acceleration runs you describe.

The maximum advance point and when it is reached in the rev range is much more important than the static setting.

Rohan

[type36lotus]

Well as we are talking about timing. Any advantage to changing the advance springs? And if so, to what? As well as where to get them.

[rgh0]

The optimum maximum advance point in the rev range depends on mltiple factors -cam shaft, carbs, fuel type, compression ratio, other engine mods. So hard to be specific. However most manufacturers advance curves are conservative and more advance sooner can generally help.

For a Weber head with C or D type cams running on premium unleaded fuel. I would aim to bring the maximum advance point to 4000 rpm down from its original 5000 rpm point. However the advantages are relatively small and proably more important to actually get the right maximum advance set for your car and your fuel which for the same engine will be in the 25 to 30 degrees range.

Rohan

[paros]

I too have been messing around with distributors, engine QED 420 based and used for some racing in Greece. Originally had 5 degress static when bought car after rolling road session and with a distributor giving 10 degrees intternal advance, and thus 25 degrees total advance at crank.
did not run well at low rpm and flat spots etc
Fitted Aldon 103TC this year and used 12 degrees static, dizzy had something like 26 of advance and ended up with 38 degrees at full chat. Overheating was race sympton but lot of power [ subjective judgement ].
It transpires Aldon do 2 dizzys with a race and road advance curve developed from tuning twinks on the dyno - I had the road advance.
I have thus gone back to old dizzy and 13 degrees static and just over 32 degrees at 4500 and have very smooth engine, no flat spots etc. Most tuners suggest with Octane boosters 32 degrees static [ Aldon booster being best ], I think for next race a little more advance might help at top end without detonation being problem - I know 38 is too much for this engine and 33 OK but therefore perhaps 35.
As said before stop watch on calibrated bit of road is the answer and plug cuts at 7000 to see what it looks like!Last race was noticably down on power with only 30 degrees adavnce and for example using second gear before one corner where previously with 38 degrees Iwas into third gear. Lap timews were marginally faster but I was driving like a lunatic and shredded one tyre in practice in 7 laps!
By the way rotor arm failed at last race - went open circuit. Have since heard of lots of problems with Lucas based caps and arms - anyone got any ideas of bullet proof replacement bits?

[lotusanglia1965]

:ph34r: Just to stir the pot a bit more; how many out there have tried 3D mapped ignition,with the twincam? These systems are not vastly more than a complete dizzy-based overhaul.I've seen excellent results on two 4.2 e-types i worked on,these have similar problems coping with lower octane fuel.
Martin