Urgent... need help on setup Weber 46 on a 911 3.2 engine

[Dekens]

Hello,

I need help on setting up a set of webers on my 3.2 engine.
We are using the car for rallying, engine is stock, no modifications, no other cams, only a race exhaust.

Hope someone can help me on finding the correct idle air jets, venturis, idle jets, main jets, emulsion tubes, etc...

Thanks in advance.

[KTL]

How many rpms are you planning to pull, or more specifically in what rpm range do you expect the engine to usually operate? Reason I ask is because the rpms are a determining factor of which main venturis/chokes you choose for the carb throats. Consequently, the size of the chokes you use then determines what your jet sizing will be.

Example being my 3.0L in the '79 racecar. This is a mild build (crankfire single plug 9.8:1 Mahle motorsport pistons, race cams) that puts out around 225 HP to the wheels. It was previously run up to 7500 rpm with 110 octane leaded race gas. I chose to use 93 octane "premium" unleaded. On the dyno I was overly lean when running the following jetting:

Main venturis/chokes = 42mm
E tubes = F24
Main Jets = 185
Air Correctors = 160
Idle Jets = 55
Tall Secondary Venturis

A/F ratio was above 15 and near 16. Much too lean. Threw the biggest main jet I had at it, which was a 200. Still couldnt' get it rich enough as the A/F was still around 15.

I didn't have any smaller air correctors on hand, and it should be noted that air corrector factor is 3x that of main jet. Meaning, going down one size of air corrector for richness is roughly equivalent to going up 3 sizes of main jet. The next best option I had avaiable to me was to choke the main venturis down to size 36mm. By doing that and making no other changes, the engine could now pull less air thru the carbs (induces richness), but at a higher velocity (greater air velocity also pulls/siphons more fuel from the carbs) and thereby richen up the mixture.

Where I then found myself is being a bit too rich. So I had to pull back the main jets down to 180. What I currently sit at is this:

Main venturis/chokes = 36mm
E tubes = F7
Main Jets = 180
Air Correctors = 160
Idle Jets = 55
Tall Secondary Venturis

Note that I changed the E tubes because F24 is not a common choice and I had the F7 on hand to try. Engine behaves fine- good transitioning and no flat spots. However I feel I still have some optimizing to do because I gave away some top end breathing with the choke reduction to 36mm and i'm still a little bit rich. I can go down on the main jet a size or two, or up air corrector one size to lean it a bit (and if the result is excessively lean, I can retard that with more main jet). Or I can throw the 42mm chokes back in, in concert with smaller air correctors that I have since acquired and stay with the larger mains I used to have, go up to 60 idles (because I had chronic popping with the 55 idles and 42mm chokes. See how it's a balancing act?

Point of all that information is there's no one definite jet & venturi combination for you to go with. Based on your engine, my semi-educated guess, and take this with a grain of salt because i'm still a noobie at carbs, would be:

Main venturis/chokes = at least 38 or 40mm
Main jets = dependent on your chokes = try 170
Air correctors = 160
What size secondary venturis do you have?
Idle jets = 60

There's a nice chart from a Weber tuning book I have that plots rpms vs. engine displacement for varying choke sizes, and also an equation to calculate a recommended baseline choke size. I'll look that equation up and post the graph this evening when I get home.

[Dekens]

Hello,
Thanks for your reply.
The distributor has a rev limited finger from a 2.7 RS, which is limited at 7300rpm.

Problem I have with the car is that it takes too much fuel on driving between rally stages and engine running on idle. When we use it on a rally stage, full speed, it looks that the car takes less fuel and engine temperature is OK. Between rally stages engine temperature is very low.

I just took out the main jets and these are 160.
Emulsion tubes I checked these are F1... which I think is very strange for this engine...

It would be nice if you can post the graph from the weber tuning book... thanks in advance.

[jpnovak]

Having been through this process with a few friends tuning webers I suggest you make the investment in a wide band oxygen sensor. I use an Innovate Motorsports LC1 with a self-built RPM adapter.

If you have the RPM vs AFR graph it is much easier to suggest jetting.

Kevin makes some great statements on the tuning process.

Looking at my notes I only have numbers for a stock 3.2 with 40IDAs. Here is the jetting.
Venturis 32
Main jets 140
Idle jets 60
Air Correction 160

Also, I do not suggest spinning the 3.2 to 7500. The rod bolts will be severely compromised at this engine speed. Plus with stock cams you are not making any power beyond 6600 or so.

[Dekens]

This is the setup from the previous owner but he never drove the car

[ttweed]

Originally Posted by Dekens

It would be nice if you can post the graph from the weber tuning book... thanks in advance.

Not the graph you were hoping for, but here's the formula for choke size from BA's book, p. 154:



You might want to read to the bottom and also this paragraph from the top of the next page:



His opinion is that for a 3.0, 40IDAs are more than sufficient, w/ 34mm venturis. The 3.2 is only 33cc more per cylinder, and if you are running stock cams, as you say, HP probably peaks about 6600, as jpnovak has said. Plug that into the formula, and I think you will see that 36mm chokes are probably sufficient, and these could be fitted in 40mm throttle bores and be more driveable than using the 46s, which are probably only necessary on a full race 3.2 with wild cams run mostly at high RPM. Review his comment about "75% reduction in metering signal" w/ 46s.

HTH,
TT

[KTL]

Thanks Tom for adding more info. to the discussion. I agree that the 46 is a bit big for a 3.0L , especially for a street engine. I suspect the only reason my 3.0L was given the 46 is because it is a mild race engine and it spends its time above 4000 RPM more often than not. Below that RPM, it's a bit weak.

Here are the images I said i'd share

[KTL]

The chart with the three curves on it is the one I was referring to as a good baseline to start figuring out your main venturi size. Thanks also Tom for posting that eqn from B Anderson's book, as that is where I must have remembered seeing an equation for calculating venturi size.

I posted the last image because I was looking thru an old service manual for 1970-1976 914s. I stumbled upon the fuel chapter for the 914/6 and thought it'd be interesting to share what the carb setup (yes its a Weber 40 ID) is for that 2.0L six. Reason I thought it'd be interesting to share is because of the emulsion tube the 2.0L uses. It's an F26. So what? Well go up to the emulsion tube page I scanned and look at the first column of tube specs- this column is the tubes for our 40 and 46 IDA carbs. Notice that it says "for very large main fuel jets...." Well that's not the case for the 914/6, as you can see it uses a rather small 125 main fuel jet.

Point being is the e-tubes are a bit of a mystery and you should go with what people have proven to work well under most any condition. F3, F7 and F11 seem to be commonly used tubes for our engines. However I had F24 in mine and it ran fine too...... The e-tubes are still a mystery to me as i'm learning these carbs.

[jmz]

just as a data point. I changed the 34 mm venturi to 36 on my Weber 40s that are on my 3.0. It seemed to really wake up the engine.