3.0 SC/ 40 IDA Dyno Tuning

[Eagledriver]

I suspect you will be very rich with the current set-up, but there is no way to tell for sure except by getting an O2 guage or dyno time. You can get a clue by looking at the soot in the exhaust and at the plugs. Get a wide band O2 guage in there. I put one in my race car last year and it has proved very useful in setting up the carbs. You can get them for around 200 dollars now. My cockpit video captures the display so I can see what happened mixture wise during the session.

-Andy

[KTL]

Andy,

I was hoping for rich so we'll see when I take it back to the dyno next week. I did take notice of the exhaust and after the venturi swap the car sounded sort of "fat." We adjusted the mixture screws and it smoothed out a bit but still sounded deep. The tailpipes do show blackening, so I think that's a good sign.

I do have a wideband O2- i've got the old bar graph Halmeter AF30. Funny you mention the video too. Mine's mounted where the radio would be and you can see it on the video. It provides a rather stable reading while at pit speeds and during the warm up lap. Once the race started, the display began to fall backward to lean after just a few turns on the track. Then it showed nothing but a power-on signal....

[VFR750]

Are you overheating your O2 sensor?

Seems odd that it was reading OK and went away after time. That might mean heat build up and it is affecting the reading. Does it go back to "normal" after a cool down?

Did you do a free air calibration of the O2 sensor? Makes a significant difference in accuracy as it affect both the ability to measure O2 and unburned fuel.

[KTL]

I was thinking overheat of the sensor too. But the old sensor gave a reading all the time- albeit completely inaccurate.

My Halmeter has no calibration feature. Instructions say just plug it in. However knowing that other AF meters do a calibration, I hooked up the sensor and keyed the ignition on to pre-warm it before installing it in the exhaust pipe. I also let it cool off before running it in place.

[cay-ahe]

FWIW, I have been very happy with my AEM wideband sensor not too expensive but is very close to the Dyno sniffers. Very simple, no calibration by the user just plug and play.

[TimT]

5-4-3-2-1..

the Daytona Sensor schill hasn't popped up yet?

anyway...

The Halmeter is a narrow band sensor, not good for much except "yes/no" indications of mixture..

Per the OPs post.. I have to check my notes for what combo I have on my 3.2L 914/6 w/Weber 40's .... through many iterations I have a E-tube/jet /venturi combo that is fat enough under load, and leans out to good power at high rpm..

Also do what hundreds of other people have done regarding that 300 RPM block... Just stick your foot on the loud pedal and drive through it..

[KTL]

Thanks for the tip on the Halmeter. I'll get a wideband to keep a better eye on things. I saw the AEM sensor and what's up with it being noticeably cheaper than other brands? One thing I like about the AEM is it's analog to match my other inaccurate gauges.....

[cstreit]

Originally Posted by 1QuickS

Some more thoughts:

1) Optimize advance for each 1000 RPM and check that your distributor provides something like that.

2) Check fuel float levels on your engine when running and be sure fuel supply pressure is 3.5 psi at the carbs. Each float must be shimmed individually using the external fuel level gauge to check . If fuel levels are not matched then the main circuits will not be activated simultaneously for all cylinders which could provide a power dip.

Can you elaborate on point 2 there? Never heard that before and don't understand why. As others have said, thanks for the great tips!

[1QuickS]

There are four fuel floats for the pair of carbs for 911 engines, two per throttle body. The fuel well between cylinders 1 & 2 supplies fuel for the main and progression circuit for cylinder #1 and for the accelerator pump for all three cylinders. The fuel well between cylinders 2 & 3 supplies fuel for the main and progression circuit for cylinders 2 & 3. A similar arrangement is used for cylinders 4 through 6.

The main circuit is activated by enough air being drawn througn the main and auxiliary venturis so that the pressure drop associated with that velocity sucks fuel from the fuel bowl up through the emulsion tube well and out the passage-way that ends into the center of the auxiliary venturi.

If the fuel level is low in one fuel well due to a mal-adjusted float then it will take MORE air velocity than a correctly adjusted fuel level to draw the fuel into the auxiliary venturi. The rusult will be some cylinders transitioning to the main circuit while others are still on the progression circuit which is loss of power or flat spot.

Paul Abbott
Performance Oriented

[cstreit]

Paul,

thanks. I run Weber 46's that I've customized a bit myself so I'm quite familiar with their general operation. However for some reason I think I read more into your statement than you made. What I took out of it was that even a very slight variation would cause loss of power, but went back and read your comment and you didn't say anything like that.

Mine are adjusted to within 3-4mm of each other but just wondering if a little extra effort would yield some results.

[aws]

Chris: I have for years connected the float bowls of the individual carbs together with a rubber hose connected to fittings where the drain plugs go in the float bowls. This at least equalizes the float bowls of each individual carb.
aws

[Eagledriver]

aws,

I would think that linking the float bowls together would be the best way to ensure they were almost never at the correct level except when the car is parked. Under acceleration most of the fuel would go to the back bowl and under braking most of the fuel would go to the front bowl. If you've ever seen a video of a brake fluid reservoir when a car is on the race track you'd see why this would happen.

-Andy

[aws]

My main motivation when connecting the float bowls was to equalize fuel flow to the bowls because one bowl feeds 2 main jets and the other only 1. Truth be told the fuel condition in a race car carb float bowl at full bore is probably more of a froth than a liquid. (A good reason Richard Parr of PMO does not like solid engine/trans mounts) I had never considered the effects of g-forces on the float level--- although I have never had a mixture or drivability issue in years of doing this.

aws

[KTL]

Regarding the 2 jets on one bowl, i've seen a larger needle valve used on that bowl. My carbs are using 200 valves on the 2 jet bowl and regular 175 valves on the single jet bowl.

[1QuickS]

Hi Chris,

The bowl supplying one main jet also supplies fuel for the accelerator pump so there is a parity of fuel demand from both bowls.

The loss of power due to unequal float heights is hard to quantify, let's just say that unless they are equal then the system won't be optimized. The Weber tolerance on fuel level setting is 20.5mm to 21.0mm (0.020 inches) from the top edge of the carb throttle body at 2.5 meters of water which is 3.56 psi. I routinely see equal shims used for setting float heights but the the floats NEVER have uniform geometry, the little tabs aren't set correctly with the fulcrum pin and the top of the float body. Also, an off-road excursion may bend the brass tabs from the fuel sloshing so it is prudent to check your vehicle after such an event, not only for structural items but float levels too.

Lateral forces due to acceleration loading (lateral and longitudinal) will slosh the fuel in the bowls, theoretically the floats have their center of buoyancy so that regardless of fuel movement the float height isn't adversely affected, sloshing is another matter. Fuel bowl dams help keep the delivery to the mains from being exposed during lateral accelerations.

Paul Abbott
Performance Oriented