Author Topic: Carb Jetting Query  (Read 4138 times)

H150GTT

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Carb Jetting Query
« on: 12:40, Mon 18 October 2010 »
Guys

I have been tweaking at my cars fueling a bit lately.
My carb is set up as follows
120 main
0.9 a/c
1mm first stage
1.4 second stage.
Boost is 22 psi.

I think I am on the limit for the carb without changing the main jet. But I still want more boost. (yes I am greedy lol)
I am getting around 12.2 afr @ WOT right through the revs.
I dont really want to mess with the off boost fueling as its damn near perfect so was wondering has anyone had any luck with reducing the a/c jet further than 0.9 rather than starting to increase the main jet? Also has anyone ever tried increasing the second stage past 1.4mm although I know this is not recommended?
If increasing the main is the only solution then I will leave the car as it is as its really nice to drive.
Cheers guys

markey mark (bd)

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Re: Carb Jetting Query
« Reply #1 on: 16:47, Mon 18 October 2010 »
Whats size needle jet you got mate?
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5teve L

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Re: Carb Jetting Query
« Reply #2 on: 18:45, Mon 18 October 2010 »
Prob find if you haven't already, that you will need a larger needle jet soon, as Mark is getting at.
Also you may have to go larger on the main if you want more boost, 125 or 130. Afraid it's a matter of time if you are running over 220bhp... My old car would lean out over 5800rpms on a similar spec to you.... was emptying the carb with a 2mm needle & couldn't get a larger one to seal...

H150GTT

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Re: Carb Jetting Query
« Reply #3 on: 00:26, Tue 19 October 2010 »
Am running a 2mm at the minute. I got it off this guy
http://myworld.ebay.co.uk/carbs_and_parts/
He supplies up to 2.5mm I think..
Steve would the larger one not seal due to the fuel pressure or would it just not seat properly after it was drilled?

Havnt had any leaning out problems yet. Was thinking earlier that I have only tested it on a long flat road. I am going to take it to a road were there is a long climb and see how it goes there. Having the hold the revs for longer may show up some fuel supply problems...
If all goes ok then I think I will just settle for what I have then.. To be honest its running a lot better than I though it would have.
Cheers for the help guys

5teve L

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Re: Carb Jetting Query
« Reply #4 on: 09:18, Tue 19 October 2010 »
Mine was drilled, I will be trying a 2mm & larger from that link though, although I think I am going to be hitting carb empty probs... which you will prob find if you up the boost & rev over 6k... sounds like you are prob on the limit now but try a little more boost if you can, I'd be doing a wot run in 4th if you can on the current spec, try to rev it as high as you are going to be running it.
 Ideally you want a stethescope or det cans as well running that sort of boost on std timing & std comp, just to be safe.

fishead

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Re: Carb Jetting Query
« Reply #5 on: 12:34, Tue 19 October 2010 »
what turbo you running
what cam
22psi from where ?

yes i run smaller a/c than .9 an bigger enrich than 1.4, but it definatly needs tidying up as the off boost is awfull

H150GTT

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Re: Carb Jetting Query
« Reply #6 on: 12:42, Tue 19 October 2010 »
Its the BB250 Turbo. It is a T25 with a .49 rear housing.
The cam is the BB GT spec cam which is similar to the Piper 285 Cam.
Running 22psi manifold.

Whats is your carb spec?
Are you suffering carb emptying issues at all?
cheers
W

Ash-Lee

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Re: Carb Jetting Query
« Reply #7 on: 16:06, Tue 19 October 2010 »
For reference i run

120 main
90 A/C
140 2nd stage
200 needle

At 20PSI manifold i'm barely nudging into the 12's but i plan to run 22-24PSI.

James5

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Re: Carb Jetting Query
« Reply #8 on: 18:26, Tue 19 October 2010 »
My Spec is a little different to your's Ash

I run
2.0 needle
120 main
1.0 1st
1.42nd stage
1.0 a/c

I run 22psi manifold ok hitting 12.4 @ WOT but I am sure a .9 a/c will help lower it a bit but all seems good at the moment.

H150GTT

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Re: Carb Jetting Query
« Reply #9 on: 23:28, Tue 19 October 2010 »
Thats pretty consistent.
Think everyone who listed is running similar sized turbo's and cams too..

fishead

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Re: Carb Jetting Query
« Reply #10 on: 17:16, Wed 20 October 2010 »
my carb is very very different
im on a 1.7 needle an emptying is definatly not my problem at the mo
an i gotta say im suprised by the specs above
also personally id be looking for 11.8 - 12 with a good helping of boost, mine has let go at 12.8, maybe not because of that but who knows, it was ok all day on the rollers at high 11s

markey mark (bd)

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Re: Carb Jetting Query
« Reply #11 on: 17:37, Wed 20 October 2010 »
Quote from: "fishead"
my carb is very very different
im on a 1.7 needle an emptying is definatly not my problem at the mo
an i gotta say im suprised by the specs above
also personally id be looking for 11.8 - 12 with a good helping of boost, mine has let go at 12.8, maybe not because of that but who knows, it was ok all day on the rollers at high 11s

Agree that i'd like to see the AFR's around the 12 mark, for me 12.5 is getting very close to the leaning out mark at high boost no harm in keeping it at around 12.0 to keep it safe.

At 23psi my campus was just flickering on the 12.2 mark, with the new mods i think i may need to play with the spec again.
I'm running same spec as James but i think i'll poss be dropping ac to 0.9 and seeing how it goes, if turbo likes working at the pressure i'll be setting it for 25psi
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H150GTT

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Re: Carb Jetting Query
« Reply #12 on: 21:23, Wed 29 February 2012 »
Sorry to resurrect an old thread but was sitting thinking about the carb issues I was having with the carb emptying in particular..
Was thinking has anyone ever tried to piggy back another fuel bowl onto the carb. Was thinking of something of similar style as already on the carb with needle jet and float arrangement. You could Tee the fuel supply line off to it. Was thinking you you could possibly join the 2 fuel bowls together at the bowl drain point. The bowl would also need an air vent on top Tee'd into the carb top.

Its only a rough idea and maybe has a major flaw that I haven't realized yet but am just throwing it out there to see if anyone has seen it before? I am aware that the bowls would have to be at the same level to prevent the carb from overflowing. Only other problem I can think of would be the fuel overflowing while cornering right hand bends.
What ya's think?? It worth a try?

hughesrobert13

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Re: Carb Jetting Query
« Reply #13 on: 21:56, Thu 01 March 2012 »
theres only one way to find out if it will work  :wink:
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