Discussion
Since I put my kit car together it has always seemed a bit rich so I decided to have a go at it today armed with a Gunson colour tune and some new jets. It's a standard pinto engine on twin 40's.
what it was ....
130 main
f16 emulsion
180 air correct
45f9 idle
40 pump
This was recommended by the dave andrews calculator thing which seems to be held in great esteem.
I changed the jets to...
190 air corrector
40f9 idle
35 pump
Which apart from the emulsion tubes brings it inline with the jets speced in an example in the weber speed pro book.
The car seems to run better and the screws are within 2 turns of totally closed and give a nice blue flame. The car has lost its stink of unburnt fuel and doesn't pop and bang on over run.
What is does now is between about 2.5-3.5k on the drive (not tested on the road) it pops from the carb. This is supposed to be a lean characteristic but if I put the colour tune in look at whats happening the pops coincide with yellow flashes so show rich. After 4k the popping stops. This is if i hold it steady in this range. Freely revving sees it run fine.
I think this is about the range for the jets to crossover from idle to main so does anyone know how to smooth it out? I tried bumping the mains to 135 and it made no odds. it used to have the odd pop in this range but its a lot worse now I have the newer jets in. How can I improve that range or smooth the crossover?
I haven't road tested it yet as the throttle cable needs replacing so I don't know if its an issue that will dissapear under load or not. I don't really want to put bigger jets back in as I don't think its right the car smells of unburnt fuel.
Any ideas? I would take it to a rolling road but most people dont deal with carbs anymore and the one locally that does and has worked on another guys car does not leave me impressed. His car eats sparkplugs even more than mine and smells alot worse whilst driving the ecconomy is also even worse than mine.
what it was ....
130 main
f16 emulsion
180 air correct
45f9 idle
40 pump
This was recommended by the dave andrews calculator thing which seems to be held in great esteem.
I changed the jets to...
190 air corrector
40f9 idle
35 pump
Which apart from the emulsion tubes brings it inline with the jets speced in an example in the weber speed pro book.
The car seems to run better and the screws are within 2 turns of totally closed and give a nice blue flame. The car has lost its stink of unburnt fuel and doesn't pop and bang on over run.
What is does now is between about 2.5-3.5k on the drive (not tested on the road) it pops from the carb. This is supposed to be a lean characteristic but if I put the colour tune in look at whats happening the pops coincide with yellow flashes so show rich. After 4k the popping stops. This is if i hold it steady in this range. Freely revving sees it run fine.
I think this is about the range for the jets to crossover from idle to main so does anyone know how to smooth it out? I tried bumping the mains to 135 and it made no odds. it used to have the odd pop in this range but its a lot worse now I have the newer jets in. How can I improve that range or smooth the crossover?
I haven't road tested it yet as the throttle cable needs replacing so I don't know if its an issue that will dissapear under load or not. I don't really want to put bigger jets back in as I don't think its right the car smells of unburnt fuel.
Any ideas? I would take it to a rolling road but most people dont deal with carbs anymore and the one locally that does and has worked on another guys car does not leave me impressed. His car eats sparkplugs even more than mine and smells alot worse whilst driving the ecconomy is also even worse than mine.
1st of all are both carbs balanced and no air leaks?if your sure there are no other problems like those mentioned then from your description that sounds like your air corrector jets are to big to me at that rev point,its hard to simulate running mixtures with the motor not under load i never found any need for a color/tune you can get a nice safe mixture on these carbs just by plenty of road testing,but you will never get as good a result as with a proper Rolling Road Tune
try the 180 airs again this will have no influence on your idle and low part throttle mixture though,failing this try even smaller air correction say 170,s then maybe 140 Mains see how you go 
try the 180 airs again this will have no influence on your idle and low part throttle mixture though,failing this try even smaller air correction say 170,s then maybe 140 Mains see how you go Edited by That Daddy on Wednesday 23 July 12:44
Since A) you don't even mention the choke size which is the most important part of the whole equation and B) think you can do anything meaningful on the drive with a Colortune other than to the idle mixture it tells us that you have no idea what you are doing and that changing jets at random isn't going to help much. Why you would expect anyone else to be able to guess what jets you should try is beyond me.
Take it to the rolling road because whatever your opinion of him he's going to know a lot more about setting up carbs than you do and have the right equipment to do so. If he's really that bad find someone else.
Dave
Take it to the rolling road because whatever your opinion of him he's going to know a lot more about setting up carbs than you do and have the right equipment to do so. If he's really that bad find someone else.
Dave
Main Choke is 34mm. aux venturi's are 4.5.
I don't think that a colour tune and mix screws will do anything more than adjust the idle mix but what I expected to happen was by leaning the jets off would affect the mix at idle. The colour tune would therefore help me to get the engine back to a reasonable level of tune or atleast let it tick over propperly.
The jets aren't being changed at random I have a few different opinions from well regarded source I am trying. I will freely admit I am not the authority on webers. I am just describing what happens now and asking for suggestions in regard to what may aleviate some popping in the mid range. I have a few jets at home so could try out some of the ideas. I was hoping for replies like the daddy who offers advise based on the symptom. I am trying to learn a bit more about how they work but what with mine being the original holbay sort and carbs really going out with emissions rulings information is scant.
I don't think that a colour tune and mix screws will do anything more than adjust the idle mix but what I expected to happen was by leaning the jets off would affect the mix at idle. The colour tune would therefore help me to get the engine back to a reasonable level of tune or atleast let it tick over propperly.
The jets aren't being changed at random I have a few different opinions from well regarded source I am trying. I will freely admit I am not the authority on webers. I am just describing what happens now and asking for suggestions in regard to what may aleviate some popping in the mid range. I have a few jets at home so could try out some of the ideas. I was hoping for replies like the daddy who offers advise based on the symptom. I am trying to learn a bit more about how they work but what with mine being the original holbay sort and carbs really going out with emissions rulings information is scant.
For starters you'll never get it running perfectly or even close to that because the chokes are far too big for your engine. Mind you we're still guessing exactly what engine that is but even assuming it's a standard 2 litre Pinto not a 1.6 then you're only going to have about 115 bhp. 34mm chokes are sized for 150/160 bhp and you won't be anywhere near that. You need to read this.
http://www.pumaracing.co.uk/Chokes.htm
30mm chokes should be plenty and they'll probably want either 115 or 120 mains. Trying to get chokes that are too big jetted correctly is just a recipe for flat spots and awful fuel consumption.
The graphs in the Weber tuning manual which try and predict choke size from cylinder size and rpm are frankly a waste of time and come nowhere close to what Weber actually used on various OE applications or dyno operators find work in practice. For example it would try to tell you that a 2 litre Pinto in race tune revving to 8000 rpm wants 46mm chokes which is absolutely crazy. That's an engine spec that's been built a million times and even good ones don't break 200 bhp or need more than 38mm chokes, maybe 40mm at the absolute outside, in a 45 DCOE.
The closest OE engine I can think of to yours was the Lotus Twin Cam in 115 bhp form which had 32mm chokes but even they caused a flat spot and Weber eventually conceded that 30mm chokes were a better solution.
Similarly although much of the info on the "highly esteemed" Dave Andrews page you refer to is useful a lot of it is flat wrong. It should be obvious that a 1.6, 1.8 and 2 litre Pinto can't all want the same choke size and jetting when they have completely different power outputs. Nor do any of those want a 34mm choke in standard form.
Get the choke sizes right and jetting Webers is actually very simple. Get them wrong and you're wasting your time.
Dave
http://www.pumaracing.co.uk/Chokes.htm
30mm chokes should be plenty and they'll probably want either 115 or 120 mains. Trying to get chokes that are too big jetted correctly is just a recipe for flat spots and awful fuel consumption.
The graphs in the Weber tuning manual which try and predict choke size from cylinder size and rpm are frankly a waste of time and come nowhere close to what Weber actually used on various OE applications or dyno operators find work in practice. For example it would try to tell you that a 2 litre Pinto in race tune revving to 8000 rpm wants 46mm chokes which is absolutely crazy. That's an engine spec that's been built a million times and even good ones don't break 200 bhp or need more than 38mm chokes, maybe 40mm at the absolute outside, in a 45 DCOE.
The closest OE engine I can think of to yours was the Lotus Twin Cam in 115 bhp form which had 32mm chokes but even they caused a flat spot and Weber eventually conceded that 30mm chokes were a better solution.
Similarly although much of the info on the "highly esteemed" Dave Andrews page you refer to is useful a lot of it is flat wrong. It should be obvious that a 1.6, 1.8 and 2 litre Pinto can't all want the same choke size and jetting when they have completely different power outputs. Nor do any of those want a 34mm choke in standard form.
Get the choke sizes right and jetting Webers is actually very simple. Get them wrong and you're wasting your time.
Dave
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