Weber - Temp / Mixture question

[Chuck]

Have not been around Weber's long enough to know the answer to this one: Air density and thus fuel / air mixture changes with temperature. Warmer makes a given set up run more rich and colder makes it run more lean. So if a Weber is tuned to run in 60 degree air, will there be a noticeable difference in performance at, say, 90 degree air, where it would presumably be a bit more rich? Or is this just not an issue in real world driving.

I know jets can be changed to compensate for altitude, but does anyone change jets to compensate for ambient air temp?

Once spring finally arrives I proabably will have a real life answer, but given this cold wet spring I was getting tired of waiting. . . . . .

 

[DaveM]

In real world street driving don't worry about the A/F difference at 60 degrees vs 90 degrees. For track use that would be about one step difference in fuel jets ( say 155's vs 145's).

 

[Cliff Beer]

Agree with Dave here - for street I wouldn't adjust for it. Why? Well, many climates with very hot temps also have quite cold temps at night (desert climate). If you lived in 90+ degrees year round, 24 hours a day then it probably would be worth changing out a jet or emulsion tube but there aren't too many places in the world like that (where you'd want to drive a GT40 anyway). That being said, if you lived in northern Alaska, you might want to change jetting to compensate for that...

 

[Scott Calabro]

Chuck,

You may want to have a setup for spring/summer and a setup for fall.

What you are referring to is called density altitude.

Ambient atmospheric pressure at your location can certainly vary with changes in temp and humidity.

Ask any Pilot.

With my Weber IDF setup I notice a difference in A/F ratio throughout the year.

Mostly in the early summer and mid fall where daily temps can swing the largest.

There are charts avialable to help you compensate for this but the best bet would be to install an O2 sensor in one of your exh. headers for an A/F mixture indicator.

Edelbrock has a nice one that is inexpensive.

Besides it only takes a few minutes to change the setup on a set of IDF's!

Regards,
S

 

[Dean Lampe]

Chuck, Don't change anything you can't put back together in the next 3 days. We have a date with a road course! Are you ready go?

 

[Chuck]

Dean:

It is just about nailed. Noticed the carbs seemed just a bit lean above 4500 RPM, not bad, but noticeable. Will try taking the air jet down a step to see if that makes a difference. Since it was 60 when I drove it, thought the decrease in density altitude associated with the rise in temp might make a difference, but guess not.

Will call before Thursday to confirm status.

Scott:

Thanks for the observations. Have often wished one could fine tune the fuel mixture by just turning a knob on the dash of the GT, like one can on a Cessna.

Guess the bottom line is one should tune the carbs and not generally worry about jetting changes unless there are real temperature extremes.

Thanks for the info.

 

[Keith]

Chuck,
I have always been an advocate of the "one jet size too rich is much better than one jet size too lean" theory. You never hear of anybody saying that "it was little too rich" and I burned a hole in a piston. But if I only had a dollar for every time I have heard "it was a little on the lean side" and I burned a hole in a piston....................
Keith

 

[flatrod]

I live in Alaska, and when I used IDA's on my Shelby, they would run great when it was cooler and the sun was not shining, but just 15 degrees different and sunny day, they were way different. I never did get it to run at a compromise.

 

[Rick]

Drag racers who are real sharp on carburetion have a portable Weather Station in the trailer or hanging on the trailer door. It will not only tell them the fluctuating changes between runs in Temprature but also Barometric Pressure (Air Density) and Humidity. For those in the know, all of these things will and do affect A/F ratio's and can be adjusted for, that's what all the current fuel injection sytems do automatically via their electronics and various sensors. (Just not as much fun as the Webers).
You will actually feel a difference with a 15+ degree swing on the temps, the engine will not be as crisp, often times changes will bring back some of the lost power.