Lotus Elan

%CO observations

PostPost by: tdafforn » Wed Jul 06, 2005 11:38 am

Hi Guys,
Finally had a chance to spend some quality mechanical time with the +2.
Over the winter I stripped and rebuilt the Dell ortos as well as fitting an electronic ignition.
Re set up the carbs at 3 1/2 turns out on each idle screw and have been driving it like this for a few months..
Anyhow thought it was time to check the %CO
Obervations are as follows
%CO at 3 1/2 turns 5.5% @ 750rpm
Lean her out a bit
%CO at 2 turns 4.5% @ 750rpm
anymore turns in and she sounds too weak
At 2 turns %CO is as follows
750 4.5%
1000 5%
1500 5.5%
2000 6.0%
3000 7.0%
All sounds a bit rich to say the least...
Anyone any ideas?
car also stumbles when a snap open the throttle from idle, fine at all other positions?
Cheers
Tim
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PostPost by: type26owner » Wed Jul 06, 2005 1:52 pm

Tim,
Here's a chart that compares CO to the mixture. Target should be about 2% CO for maximum power at all times. Try raising the idle speed up to get rid of the stumble. You're not that far off actually. Does it emit black smoke at high rpms? Other than theory I can't help anymore with the Dellos.
http://www.perfectpower.com/Technical_info/afr.asp

With 10% ethanol added to the gasoline the stoichiometric constant is reduced to about 14.13 to 1 AFR. Folks need to know here in the USA.
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PostPost by: tdafforn » Wed Jul 06, 2005 3:14 pm

[quote] Does it emit black smoke at high rpms?[/quote]
As a matter of fact it does! I took this to confirm that it was running too rich..
The Dellorto manual I have suggests that 5% is good for power, I guess they are a little out
Cheers
Tim
PS thanks for the hint about the stumble, is it worth richening the idle mixture to drop the idle revs in order to allow the throttle to opened a little so that the progression holes are brought into play earlier?
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PostPost by: steveww » Wed Jul 06, 2005 4:24 pm

Interesting that you mention 5% for good power. This is the figure that many people have told me for a carb set up is for max power. Hmm :?
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PostPost by: type26owner » Thu Jul 07, 2005 4:39 pm

I'm just on the greener side of the CO range. Really requires a dyno to pin down the exact AF mixture to generate maximum power. Anywhere in this range will get you within 90% of full power though and provide good tractability if you can keep it there. That's the tricky bit of it of course!

Tim try going a size or two smaller on the main jet and open up the air correctors too. If you're really emitting black smoke at high rpms then by all means let it suck in more air to lean it out somewhat at the high end. You realize that without an AF meter you're have to do it by seat of the pants method only. I could not master that skill very well so I broke down and bought a wide-band O2 sensor which makes the observing part tons easlier.
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