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Paul Coleman

How do you set the mixture to the correct level?

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How do you set the mixture on your carbs? I'm not asking which screws to twiddle but how do you know when the mixture is correct? I was wondering if I should invest in one of the Gunson Colour Tunes or is there a better way? I have 3 cars with carbs and so it's something I need to get to grips with. At the moment I have just finished restoring a 1976 Mini 1000 for my daughter which looks as though it's running rich judging by the blue smoke coming out the back.

 

Thanks, Paul.

Edited by Paul Coleman

Lotus Esprit [meaning] a 1:1 scale Airfix kit with a propensity to catch fire

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Colour tune or an exhaust gas analyser but the latter has the issue that you have 4 different venturi to set so it's difficult to tell what any one cylinder is doing once it's all mixed in the exhaust

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What flame colour do you look for..? since the esprit is meant to run a bit rich?

Also I heard running 2 at once is much easier for comparison.

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I use two whenever I'm playing with twin Webers or similar. Amazing how sensitive they are in conjunction with using a bubble flowmeter at the same time.


DanR

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Hi Dan, I only have 1, so it is more art than science. I do this in the evening when it is near dark to get a better look. 2 would be better for sure. I still have the colour chart that came with the kit, Bunsen blue at idle and when you blip the throttle you check for an orange flash. At steady 3500 rpm, it's back to Brunson blue. I heard it is better to run a little rich, so maybe a slightly darker blue is fine. I also have an air flow synchroniser. That said, the garage did slightly change this setting.

Edited by IS
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Easiest way, even with all the tools including a gas analyser is to read the spark plugs. http://www.ngksparkplugs.com/tech_support/spark_plugs/faqs/faqread.asp The colour tells you a lot and you'll soon get a feel for how much to tweek the mixture screws. I've been tuning late 1970's Aston Martin V8 Vantages for years this way and have some of the sweetest running V8s around.

 

Roo

www.jpsesprit.com

Edited by v8vantage
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Thanks all for the replies. I obviously need to invest in a colour tune and learn how to use it. I'll add it to my Christmas list.

 

Do I need the 14mm version for the Esprit?

 

Cheers, Paul.

Edited by Paul Coleman

Lotus Esprit [meaning] a 1:1 scale Airfix kit with a propensity to catch fire

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The colour tune etc will only set the idle mixture and the colour of the plugs are the best indication after a run out but don't let it idle after the run as they will change colour again. I found throttle response was affected greatly by float height too. This is the first thing that needs to be set to do the job properly.

Trevor.


I'll get around to it at some point.

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Can you set float height without taking the carbs apart?


Amateurs built the Ark

Professionals built the Titanic

"I haven't ridden in cars pulled by cows before" "Bullocks, Mr.Belcher" "No, I haven't, honestly"

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Did you mean take the carbs off? You set the float heights by removing the carb tops.

Trevor.


I'll get around to it at some point.

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Hi Trevor/ Andy,

Sorry reread that and i sound a dick. I meant would I need new gaskets etc. I doubt my float height is correct. As nothing else was set right.

Thanks

Chris

Edited by red vtec

Amateurs built the Ark

Professionals built the Titanic

"I haven't ridden in cars pulled by cows before" "Bullocks, Mr.Belcher" "No, I haven't, honestly"

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For what a new gasket costs I would replace at  the same time.

 

I re-set my floats earlier this year (with the help of the infamous Des Hamill book) - easy to do but  a bit fiddly.  I think the float height for my Dellortos is 15mm and one of them was at 40mm!

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Hi,

Think ill take a look at them,

Chris


Amateurs built the Ark

Professionals built the Titanic

"I haven't ridden in cars pulled by cows before" "Bullocks, Mr.Belcher" "No, I haven't, honestly"

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I have to say that although float height is set measuring the the height of the from the face if the carb top, there is a little more to it than that. Although the height can be set that is not always the height of the fuel in the float bowl due to slight difference in weight and size of the floats themselves. I know there was great deal of difference to how the car drives when tweaking these heights as that sets the fuel level which makes a big difference. I can try and go in to more detail if you wish but first I would try the stator setting and see how it goes.

Trevor.


I'll get around to it at some point.

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Ah, the joys of plug cuts...the only way to tell what's going on in there when you're really trying!

First, find somewhere to do them. You need a deserted bit of straight road with good visibility and where you can coast to a stop somewhere, too....that takes a bit of finding. Next, the procedure....you have to run flat out at max rpm for a few seconds, at least! Then...all at the same time.....turn the ignition off ( think about steering locks first!!) with the throttle still fully open, declutch, go to neutral and coast/ heel and toe brake to a stop. Doing this keeps the plugs in the same conditon as they were at max. power, and means that you get a "snapshot" of what was happening inside the engine. The plug chart shown in an earlier post is useful at this point... this is useful, too, although it's not for cars!

 

http://ossaengineering.com/wp-content/uploads/2012/02/sparkplug.pdf

 

And I've found this, as well..

 

http://www.wallaceracing.com/plug-reading-lm.html

 

Back in my days with the supercharged MG TC I used to do these on a road beside the District Line underground in Kew....flat in third...... these days, they'd throw the key away....


Scientists investigate that which already is; Engineers create that which has never been." - Albert Einstein

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I have done the plug cuts as there is two mile straight near me with little traffic on it. The plugs looked the correct colour, but I still have a stumble so will look at float height.

Chris


Amateurs built the Ark

Professionals built the Titanic

"I haven't ridden in cars pulled by cows before" "Bullocks, Mr.Belcher" "No, I haven't, honestly"

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Flat out mixture is controlled by the jet size and is what you are looking at with the plug cuts method and is likely to be correct or pretty close as it was defined at the factory. I found float heights affect the 2500 to 3500 rev range throttle response.

Trevor.


I'll get around to it at some point.

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That sounds the same as mine in the 2500-3500 rpm range


Amateurs built the Ark

Professionals built the Titanic

"I haven't ridden in cars pulled by cows before" "Bullocks, Mr.Belcher" "No, I haven't, honestly"

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Im trying to adjust my idle as I have a stutter about 3000-3500 rpm I think its when Im coming to full boost?

 

Is is something I wont be able to do by adjusting the idle settings alone?

 

Buddsy


 

"Belief is the enemy of knowing" - Crrow777

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I think that's when the jets switch over.


Amateurs built the Ark

Professionals built the Titanic

"I haven't ridden in cars pulled by cows before" "Bullocks, Mr.Belcher" "No, I haven't, honestly"

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I've never had much success with carb tuning.

More than just getting it running, anyway.

I'd just end up tinkering, so if you're like me, I'd recommend carb tuning be left to experts, with rolling roads.

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Im trying to adjust my idle as I have a stutter about 3000-3500 rpm I think its when Im coming to full boost?

 

Is is something I wont be able to do by adjusting the idle settings alone?

 

Buddsy

As Chris says its when the carbs are effectively switching jets and the idle mixture will have little or no effect at that rev range. Float height and more accurately fuel levels on the float bowl will have a greater effect. It's a time consuming and sometimes frustrating job but the results are interesting and worth it.

Trevor.

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I'll get around to it at some point.

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Des Hamil book says you can get over a cross over stumble by making the idle mixture richer to a point. I checked my plugs tonight and Im running too rich already so have set it back to about 3.5 turns out where I think it runs best. Im going to check my timing again. I think I am a little retarded (no jokes!) compared to where I was when my dizzy cam loose. Still in spec but Im sure it ran a bit better before. I think at idle the manual says 12 +-1 degree. I think Im in spec but slightly lower rather than higher. So Im going to check that next...well when I find the spark plug U dropped in the engine bay tonight! Can I find it! I think its sitting in the under-tray somewhere!

 

Cheers Trevor I did spend time doing the levels a couple of months ago but may be worth checking again.

 

Buddsy


 

"Belief is the enemy of knowing" - Crrow777

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