Weber Tuning

[bumble]

A question for all you Weber Gurus out there:

I have just completed building a 2.7L engine built to the following specs:

1. 9.5:1 C/R
2. 911 S cams
3. Standard 2.7L valve sizes but heads & manifolds ported & careflly matched.
4. Case mods for air flow as per Bruce Anderson.
5. 1-5/8" exhaust headers into Super Trap 'mufflers'
6. Standard single plug ignition
7. Weber 40 IDA 3C caburetors with 34mm chokes

In other words, nothing radical...

On a chassis dyno, the engine makes good power (about 180hp at the wheels at 6000rpm) but has an annoying hole in the power curve from 3000rpm to about 4200rpm where the power curve is completely flat. At 3000rpm, the fuel/air ratio falls vertically from about 14:1 to less than 11:1 (i.e. VERY rich) until at 4200 (i.e. the exact range of the flat spot) it starts to lean out until at 6000rpm it is over 14.5:1 (i.e. way too lean) at which point we shut the engine down! Because we didn't have a box of compatible jets to hand, we had to call it quits and think about the problem.

Sooo... my question is - does anyone have a similar setup and if so, can you tell me what emulsion tubes, main jets and air correctors you run? We currently have F3 emulsion tubes, 170 main jets (and I can't remember what the air correctors are).

Our gut feel is we need smaller main jets and smaller air correctors but are unsure of the effect of the various emulsion tubes available.

I would be very greatful of any input from those of you who may have direct experience of this before we start changing things...

Kindest regards,

Mark Lamberton
Sydney
Australia

[ttweed]

Originally posted by bumble
Our gut feel is we need smaller main jets and smaller air correctors but are unsure of the effect of the various emulsion tubes available.

Mark-
I am no Weber guru, but I am in the exact same boat with my PMO-carb'd 3.4 motor. This seems to be a common pattern with carbs- when the main jets kick in, the A/F ratio goes rich at first, then leans out as the engine RPM increases. I think this is why Porsche changed to the IDS carbs on the 2-liter "S" motors, with an extra high-RPM fuel enrichment circuit.

I would caution against going smaller on the main jets, since that will only lean you out further on the top end. I think the key is in the emulsion tubes, since they are what control your "fuel curve", so to speak. I am trying to find out which ones to try next myself (I am using F2 tubes at the moment), that will give me less fuel delivery at the mid-range and more at the top. Unfortunately, the people with the specific knowledge of these details are few and far between. It is mostly a trial and error deal because of the differences between individual engine specs.

My mechanic has been talking to Richard Parr at PMO, and experienced tuners like Jerry Woods, Bruce Anderson, and Steve Weiner may have an idea or two, but it is not an easy question, and may involve accepting compromises. That is why EFI is used on most modern racing setups- fuel mappings can be optimized precisely throughout the RPM range by modifying the injection pulse.

Good luck,
TT

[keitho64]

Here are a couple of links to others doing a conversion to Weber's on a 2.7.

What a difference...

Weber jetting for 2.7

Weber question

I have a 2.2S and have the following setup:
30mm venturis
60 idle jets
125 mains
180 air correctors
f1 emulsion tubes


I want to go to 32mm venturis and see what that does.

From your symptoms I would thing the mains are too large, but you need to find out what the air correctors are. I think the answer lies in the main, air corrector and emuslion combo.

Another option is to send your specs to PMO and see what they recommend.

[rs911t]

But, I have a similar setup: 2.8L with 10.5:1 compression, S cams and heads (cut). Currently, the Webers have:

36 mm venturi
195 air corrector
150 mains
F3 e-tube

I believe I'm running lean up top and plan to install 180 air correctors this weekend. Other than that, I pulls like a banshee from about 2,000.

My impression is the F3 tube is the common performance setup on Porsche engines. The 170 main seems big. I'd call Steve at Rennsport Systems or Richard at PMO for guidance.

Greg

[cstreit]

170 Main is pretty darn big for a 2.7

I have a hopped up 3.0L with 46mm webers and run well at 165 mains.

I had a HUGE flatspot too. Richard An from PMO recommended I create less restriction in the exhaust, because I wasn't getting good scavenging, and therefore was getting an airflow "stall" at that speed. A switch to less exhaust restriction got rid of most of the flat spot.

Try faxing PMO.

[bumble]

Thanks to y'all for the replies - some great info here!.

We've gone back to square one - checked the carbs thouroughly and got a bucket load of jets. Our current thinking is that the hole in the power curve is so extreme there are factors other than jetting involved as well.

The first is that to measure the A/F ratio, we ran with the end-plates off the Super Traps to insert the O2 sensor and that the lack of back pressure will have a significant effect - so will have to modify the headers to insert the O2 sensor :-(.

Some have commented that a big fat spark from a MSDI can help significantly. I have actually built an MSDI with an in-car keypad to modify the advance curve to whatever is required. I wanted to get a baseline from the stock ignition before proceeding with this - didn't want to throw a spanner (er... wrench?) in the works without having a known point from which to work.

I'll post the results of our further dyno tests to (hopefully) add to the knowledge pool.

[TimT]

Its not the main jets,when you get a flat spot in that rpm range its when the carb is transitioning from the idle circuit to the main circuit. Thats where the emulsion tubes help. The air correction jets also affect the way the emulsion tubes work, so the combinations are many.

try F3 emulsion tubes
145 air correction jets
135+ mains.

[bumble]

Thanks Tim, that's pretty much line ball with our current thinking...

Cheers,

Mark
Sydney

[rs911t]

As of today my 2.8 is running (very well) with:

- 180 air correctors
- 150 mains
- 60 idle jets (could be bigger)
- 36 venturis

Pulls smooth and steady from 2000 - 7200.