Lotus Elan

Timing again...

PostPost by: JonB » Fri Dec 29, 2017 10:39 am

Hi

My Big Valve Twink is running like a sack of spuds.

This is partially my fault as I'd closed down the butterflies on one of the Dellortos by tweaking the adjuster. Should have left it alone! Oh well, out with the hand held flowmeter and it's balanced again, near enough. Now I want to check the timing. My first problem is that I cannot see the timing marks on the case. I know they should be visible from the exhaust side of the engine but they are not - I just cannot seem to get a good angle. Any tips? I plan to try and paint the marks for better visibility, or maybe just put a small mirror behind the radiator.

Secondly, I am trying to determine what the timing specification should be. I have a UK spec big valve engine from 1973, which I guess is what Miles calls a "late domestic" big valve with Dellortos.

The book is a little confusing as it first makes a point about distributor RPM being half crankshaft RPM, then shows tables of advance timings with "distributor RPMs" going up to 6500. Which would mean the engine was running at twice that and I do not think that's possible in a road going engine. To further confuse matters, it shows a graph that has crank RPM against advance, and these match the figures given for distributor RPM. So I reckon the advance tables are showing crankshaft RPM despite being labelled "Distributor RPM".

The "which distributor" tables are confusing. I could have a 41225 ("late model domestic Big Valve"), or a 41189A ("all domestic Big Valve"). The second table ("Engine use") states that "All big valve Weber/Dellorto domestic" engines use the 41189A distributor with 12 degrees static advance, so I will go with that.

This means the following advance curve applies (with the 12 degrees static timing added already):

1000 RPM: 12 degrees
2000 RPM: 12 degrees <-- after this point, the centrifugal advance mechanism should kick in
2500 RPM: 14.5 degrees
3000 RPM 16.5 degrees
3500 RPM 19.0 degrees
4000 RPM 21.3 degrees
4500 RPM 23.5 degrees
5000 RPM 26.0 degrees

Hope I got that right.. comments?
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PostPost by: nmauduit » Fri Dec 29, 2017 10:59 am

I suspect the "41189 curve" relates to Stromberg carbs, if you have Weber/Dell'ortos you would probably benefit from more advance (32? total max rather than 26?) : experimenting will eventually confirm the best tuning of your own setup.

If you want to make sure your timing is spot on, you will need to check the true top dead center on piston #1, but to get started you should be able to see the timing marks, looking at an angle from the corner of the rad : take your time to find a comfortable position before getting ready with the timing light, the lighting/seeing angle is not wide, at least on my car.
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PostPost by: JonB » Fri Dec 29, 2017 11:06 am

It doesn't according to the book - Federal Stromberg equipped engines use 41225A.

Anyway I would like to get the engine running smoothly - at present, it is not. Tuning can wait.

I came across this post that looks like I've chosen the right table: elan-archive-f16/timing-for-elan-t11479.html#p49096

At this time I can only verify the timing curve is right once idle advance is set with the timing light because I do not have a box full of distributor springs / bobbin weights or whatever you need to make changes. Therefore, I can only set the idle timing for now.

Hopefully I can see the timing marks from the radiator corner!
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PostPost by: Elseezed » Fri Dec 29, 2017 11:19 am

Highlight the timing marks with a dab of white paint. Set mine to 10 deg btdc.
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PostPost by: JonB » Fri Dec 29, 2017 11:22 am

Elseezed wrote:Highlight the timing marks with a dab of white paint. Set mine to 10 deg btdc.
Les.


Bit confused now.. Are the timing marks before or after TDC (in other words, do they show retard or advance)?
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PostPost by: nmauduit » Fri Dec 29, 2017 11:26 am

JonB wrote:
Elseezed wrote:Highlight the timing marks with a dab of white paint. Set mine to 10 deg btdc.
Les.


Bit confused now.. Are the timing marks before or after TDC (in other words, do they show retard or advance)?


timing is advanced with respect to TDC, that is the spark starts igniting the mix before TDC
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PostPost by: JonB » Fri Dec 29, 2017 11:34 am

OK, so the crankcase marks are BTDC and the crank is rotating clockwise. The Lotus and Brooklands manual do not agree, and Miles's book is also different. What a mess.. I'm going to try Miles's settings first.

To check TDC I will use a thumb in the spark plug hole of cylinder 1 to detect the compression stroke, then a rod to see when the piston is at its highest (I really should get a dial gauge!).
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PostPost by: vincereynard » Fri Dec 29, 2017 11:51 am

You have to remember that the engine has been changed (rebuilt / head skimmed / valves cut etc etc ) and the fuel is different. You don't even know the marks on the pulley or water pump are accurate. Any definite figures and settings are only approximate.

As Les states 10 degrees BTDC is a good starting point.

You may find taking the fan belt off makes locating the marks easier.
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PostPost by: billwill » Fri Dec 29, 2017 4:28 pm

Read my trials & tribulations with Distributor timing, starting here:

lotus-elan-f19/getting-ogu-roadworthy-again-t26101-105.html#p191499

~~~~~~~~~~~~~~~~~~

To see the mark on the crankshaft pulley you probably need to paint a white blob/streak around the notch in the pulley. The fan belt will probably wear it off but two tiny white marks on either side of the notch should remain on the edge of the pulley.

It can be difficult to poke a timing light down there as the radiator is rather close to the front of the engine. Don't try it if you have a mechanical fan blade on the water pump pulley.
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PostPost by: billwill » Fri Dec 29, 2017 4:39 pm

On this picture you can see that I painted the whole front of the crankshaft pully white and then added a red line where the notch is.

Image
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PostPost by: MarkDa » Fri Dec 29, 2017 5:56 pm

If carburation is right but you're unhappy about timing you can adjust for best tickover whilst running by slackening pinch bolt slightly and rotating dizzie direct.
Needs decent access of course!

You do have the correct dizzie?
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PostPost by: p.faurie » Fri Dec 29, 2017 7:00 pm

Give it to a professional.
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PostPost by: JonB » Fri Dec 29, 2017 7:47 pm

Well, I painted the casing and set it to roughly 12 degrees as planned but it still runs poorly (better but still poor). I renewed the air filter as I noticed the revs went up when I took the plenum cover off. I also replaced the O rings in the manifold joints, and renewed the Thackeray washers and nuts. Carbs are firmly in place, but there is some movement (the O rings). Thought that might have been it, but no. Rechecked the throttle balance and reset.

It's a bit of pain in the ass, but I will have to try to see what the ColourTune says. I read that setting a blue flame, then back the mixture off to the edge of yellow is the thing for the Twink. There is also the advice in the manual: idle mixture screws all the way in then back out 2.5 turns then play until you get the highest RPM. That seems pretty arbitrary to me!

I did notice that the idle revs increase when the advance is increased, and I tried to set it to the highest revs, but it doesn't work well under power. I think it is over advancing. If the standard advance doesn't work well, how can I determine what will work (other than trial and error, which I don't like the sound of)?
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PostPost by: sprint62ch » Sat Dec 30, 2017 5:53 am

JonB
At first, Miles' book confused me too. But distributor refers to the dizzy type and, on a new line, rpm to the crankshaft revolutions. And this way, all Miles' explanations make sense.
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PostPost by: JonB » Sat Dec 30, 2017 9:32 am

Yes, I figured that out, but it is misleading in the light of the comment about dizzy RPM vs crank RPM. Maybe I will tell him next time I see him... maybe not!
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