Carbs going rich

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Tony Z
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Carbs going rich

Post by Tony Z » Sat Jun 03, 2017 8:06 am

Engine specs
2332, 10:1 CR, 86C, 48IDFs with 40vents, CNC Wedge ports and big beef manifolds.

Over the years I have been slowly improving on my jetting and have now settled with a combo which works best.
Needless to say, this combo can still have some improvements.
Which is where I need help.

Around 3000rpm to 4000rpm the engine runs pretty rich, hitting 9.5:1 AFR at times. This sometimes causes misfires.
Then from 4000 to 5000 it sits steady at 12.5:1
Then from 5000 to 6000 it slowly leans out steadily to 14:1

I have tried heaps of jet combos and have kept a basic log of what each setting has given me, but now I need help from basic Weber tuning to something a bit more advanced... That or I need to bite the bullet and go for fuel injection.

Jetting is currently 57idles, F11 emulsions, 155 mains and 160 airs.
Going smaller on the mains helps the mid range richness but everywhere else it goes too lean.
I cant get smaller air jets to richen up the top end.

I have tried F11, F2 and F50 emulsions which is all the emulsions that I have. Buying emulsion tubes gets very costly very quickly.
O yes, this is a street car so mid range rpm is important.

I have read that F2 emulsions are supposed to lean out the mid range more than the F11. But this isnt what I am seeing. The F2 makes the mid range even richer. Above 4000 they behave nicely, but I struggle to get the engine above 4000. And once again, leaning out the main jet means I cant get rich enough at the top end. Even using 145 mains, I still get 9:1 AFR at around 4000.
With the F2s I just cant get close enough to warrant spending more time on them. The F11s are better.

The F50s are just plain crap in my car - I cant get close with them. F50/165 main/200 air I go as lean as 17:1 around 5500rpm, with 4000rpm at 9:1. Swapping to F50/165/160 helps the top end, but I still lean out around 14:1.

FWIW, I hit 131kW on the wheels today at a Subaru dyno day. So the car is making power.
I am now considering advancing the timing over the rich areas but I dont like creeping up on the timing - call me chicken???

Any advice or suggestions would be greatly appreciated

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Marc
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Re: Carbs going rich

Post by Marc » Sat Jun 03, 2017 3:59 pm

Tony Z wrote:
Sat Jun 03, 2017 8:06 am
...I cant get smaller air jets to richen up the top end...
Clarify, please. They're unavailable to you, or you've tried them and they don't richen the top end as expected?
I have some 120 A/C jets; if that's too small a #54 drill is 140.

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Tony Z
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Re: Carbs going rich

Post by Tony Z » Sat Jun 03, 2017 7:57 pm

I havent been able to find anyone selling air jets smaller than 160

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Marc
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Re: Carbs going rich

Post by Marc » Sat Jun 03, 2017 9:21 pm

PM me with your address and I'll send you four 120s to play with. Can't go as a letter so postage would be US$13.25; I have a PayPal account, you can send me whatever you feel is fair.

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Tony Z
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Re: Carbs going rich

Post by Tony Z » Sun Jun 04, 2017 1:20 pm

I'll pm you Marc

madmike
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Re: Carbs going rich

Post by madmike » Mon Jun 05, 2017 8:44 am

Them 40 vents aint helping :roll: try some 38's
check fuel pressure too , what type of fuel regulator your running?

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Tony Z
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Re: Carbs going rich

Post by Tony Z » Mon Jun 05, 2017 9:59 am

I've got a holley regulator. Set to just under 4psi, 3.5 I think
The webers come stock with 40 vents, why would you say they are too big?

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Re: Carbs going rich

Post by FJCamper » Mon Jun 05, 2017 12:02 pm

Hi Tony,

Just because a 48 Weber comes with 40mm vents out of the box does not make them right for your engine.

Venturi size is predicated on RPM's, and the presumption is with an 48 IDA you're going to be turning 7000 RPM's or more. All top end.

38mm vents are a good test choice. They allow more torque (better a/f mixture) at lower RPM's.

Also, there is more to fuel pressure than meets the eye. The actual issue is fuel level depth in the float bowl, which is of course related to fuel pressure, once too much pressure sinks the floats. If the float sinks, you get obvious trouble as that carb floods.

But if the float doesn't sink, but just allows the fuel level to be a bit too high, your emulsion tubes are now deeper in fuel than normal. You have fuel entering some emulsion tube holes as if you were going faster, and that screws up your air/fuel mixture. A float bowl fuel level depth that is slightly too low has the same effect. Of course, too low a level is a float level adjustment adjustment problem.

But either way, you'll see crazy a/f reading as your RPM's rise and fall.

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Tony Z
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Re: Carbs going rich

Post by Tony Z » Mon Jun 05, 2017 1:46 pm

When looking at the weber charts, they all say that a 40mm venturi suites an engine with 583cc cylinders with peak power at 6000rpm. My power peaks at 6300rpm.
The graphs show a 38mm venturi will feed my cylinder to a power peak of 5700rpm.

I understand that theory and practice are different, but one has to start somewhere.
Lets not forget that I am using IDF's which are pretty much street carbs and are used on street (performance street) cars.

I am wondering if it wouldnt be beneficial to try my luck with drilling the emulsions to try to lean out the bottom and mid range, then use larger mains to richen up the top end

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