20psi carb fuelling

[BluntyR5GTT]

right guys iv done a few searches and not really been able to find a real conclusive answer on this, i know all cars are diff etc but i would like to see what any of you guys have done on your carbs at this psi and what afrs you get etc.

im thinking of using a 125mm main, 1mm air corrector, 1.2mm 2nd stage and 2mm needle how does that sound ? also am not sure if the 1st stage needs altering ? whats the standard size of 1st stage ?

[5teve L]

You need to do it trial & error as every car is different, if your car is in good health & making good power you may need the 2mm needle.
My old carb spec
120 main
.9 a/c
1.4 2nd
2mm needle
That fueled for 24psi manifold up till 6k where it just started to lean off as it was still emptying the carb bowl.

The basic car spec was 285 cam, T25 with .49 rear, uprated fuel pump, slightly lower C/R & I had to slightly retard the TDC or it would det.

For 20psi I'd try: 120 main, 1mm or .9 A/C & a 1.2 or more than likely a 1.3 2nd stage, always start smaller. If it starts to lean off at high revs go larger on the needle. This is a rough set up & you need to do it using wideband really bud. Took me days to get mine right, carb on, carb off, carb on, off, you get the idea... while you are testing join the carb water hoses together with a bit of suitable tube, saves loosing water ;)

Also try not to get any swarf in the carb, get a couple of cans of carb cleaner & blast it out a few times, remove the idle jet as well while you do it, also if you can get the emulsion tube (it's the part the A/C jet screws into) out it will help clean it, I used one of the side carb screws as they fit in pretty well, then a pair of pliers & pull it out, lever against the carb body .

[djinuk]

id be sticking with that standard main for as long as possible blunty, otherwise your going to be rich further down the revs.

My carb spec at 18psi on standard engine with t25 .47 is all standard other than a 1mm a/c and a 1.2 2nd stage.

[BluntyR5GTT]

Thanks for the help fella. Think iv got a good idea of how were going start and see how we go from there.

[James5]

Right my 5 runs the following spec also note this is a high comp build:-

120 main
.9 air corrector
1.0 1st stage
1.5 2nd stage
2.0 needle jet
De choked
Thinned butterfly
Standard venturi
This spec sees me fuel perfect for 22psi manifold any higher and I suffer with det so I am needing to look into water injection myself to suppress the det I am already fully retarded on the tdc due to the use of the vnt turbo. And I am not wanting to take engine apart and skim the pistons tops to lower compression.

[BluntyR5GTT]

Cheers for that steve i have a rough idea now where to start and see how we go iv got an aem wideband kit here as well.

[BluntyR5GTT]

I think im going start with standard main. Standard 1st stage 1.2 2nd stage and 1.0 a/c then see how it goes from there and make changes accordingly.

[BluntyR5GTT]

Oh and 2mm needle of course.

[djinuk]

indeed,, just remmber that 2nd stage is a fixed bush so you cant resolder it if you go to big :).. i think you have a spare second carb body though dont you, just incase. Do you have an afr kit
?

you may not need the 2mm needle yet dude.. go minimum first and see what it does.

[BluntyR5GTT]

yes mate got an aem kit bought from scoff, so all good on that front.

i think ill need the 2mm needle as ill be pushing over 20psi by summer ;) i just wanna get it set up at 20 then its just a matter of prob enlarging the 2nd stage a bit more when i up the boost again.

[djinuk]

you will know if the float bowls emptying as the reading will very suddenly go lean. Check the Articals/tuning section on here dude, some really good info on carb jets and afrs etc.

[BluntyR5GTT]

I will do mate am goin get stuck into this with justin as i wanna learn as much as poss. Also must say what a great club this is

[Brigsy]

For 1.5 bar manifold on my setup

i need 130 main
.9 air corrector
1.4 2nd stage
2.1mm needle (Biggest i have got to seal, and this is not enough really on a long pull in 4th?!)

The afr's are bordering on 12.1/12.3 @ wot. Every engine is different and mine seems very fuel hungry.

[Alastair]

You can get a lot more flow through the needle valve if you up the fuel pressure, although the bigger you go the less aditional pressure you can run, and the more likely your carb is to leak or burst a seal...;)