I wouldn't think your jetting would change based on air temp. Air density will affect how much fuel is drawn out of the venturis. If the air is cold and dense, more fuel naturally will be drawn out and visa versa for warm air.

 

In the heat/ and humidity at the track I loose 2-3 tenths and mph as I see a lot of cars do. You can jet and play with timing and get some back, but unless you're hard core bracket racing I don't feel it's worth it when you know as the day cools down you'll pick back up.

 

From David Reher...

A carburetor is an amazing device. Yes, I know that carburetors have been around forever and that fuel injection is the wave of the future, but I'm still impressed by how well a carburetor can adapt to changing conditions. A carburetor operates on the principle of pressure differential. When the pressure in the venturi is lower than the atmospheric pressure, the gasoline in the float bowl enters the airstream going into the engine. This beautifully simple system is self-compensating: When the barometric pressure or engine airflow changes, so does the fuel delivery into the engine.

Some may think that the self-compensating characteristics of a carburetor are adequate only over a small range of barometric pressures. The fact is that we run the same jets in our Pro Stock engines at sea level as we do at Bandimere Speedway in Denver. For years, I just knew that we needed to change the jetting when we went to the Mountain - until the year we tested there with our sea-level tune-up. To my surprise, the engine ran best in Denver with the same jets that we use in Gainesville and Houston. This is possible because a carburetor only reacts to pressure differential, regardless of absolute altitude. We might change air bleeds or squirters to tune the setup to a particular track, but the jets in our Pro Stock Pontiac's carburetors seldom change.

http://findarticles.com/p/articles/mi_qa4078/is_200309/ai_n9246051

 

That's interesting. I've never been racing at anything other than sea level but I've always read in all the carb manuals for autos, boats, atv's, etc. that the carb needed to be recalibrated/adjusted for changes in elevation.

This is out of Holley's instruction booklet.

Carburetors are calibrated at sea level. Decrease the jet size primary and secondary, one number for every 2000 ft.
increase in altitude.

 

Twice I've loaded up a perfectly running car on the trailer in Austin, TX in mid June (100F), hauled it up to Detroit (mid 50's), and it barely ran. Once was a carb (need bigger jets for the cold), and once was EFI (speed density) that wasn't tuned as well as we thought. The EFI one ran at steady RPM OK, but we hadn't touched the table for acceleration enrichment adjustment versus IAT.

A wideband O2 sensor will tell you the real story.

 

We road race in Mexico and run from sea level to 8400' with the same jets in the carb. We do use a smaller squirter and idle feed restrictor at higher altitudes.

 

I run 2 to 3 tenths faster on hot days.Hotter the day the faster the car.But i'm running lean i'm sure.A 468 with a roller comp cam 286r witha 750 cfm.

 

That's a first!!! Put some jet in that carb. Just because it's a 750 on a 468cid doesn't mean it has to be lean. It'll work on there. Jet it.

 

Cool thread! Hoping for more responses.
One time I towed my ski boat (with my Camaro) across the continental divide @ 11,500'. Couldn't even tell we were above sea level. (Original Holley on my L78)
Then last year I was hauling a two ton camper over the same altitude entering Yosemite from the east. Couldn't tell. I was tailgating CARS. Yep, turbo Duramax.:hurray:
OK, OK. It's still close to the same subject.

 

"That's a first!!! Put some jet in that carb. Just because it's a 750 on a 468cid doesn't mean it has to be lean. It'll work on there. Jet it."

It is 88's and 91's with 50 cc second pump.I think i need more LoL.I noticed it used to surge at cruise 2500 rpm and i jetted it alot more by 5 sizes.LoL.

Funny at the track are writing their numbers slower at the track and i'm lower mine as it gets hotter.LoL

 

Has someone played with the bleeds on it? Sounds like the high speed air bleeds are out of whack. What model 750 is it? Surging at cruise may mean the idle bleeds were enlarged also.

If the bleeds are out of whack you can pull the jets out and not get enough fuel to it. Just a thought....

 

RT -.026/.071/ .015/.043
60 1.747/1.659/1.675/1.613
330 4.883/4.772/4.766/4.712
1/8 7.505/7.379/7.360/7.306
MPH 93.89/94.24/94.81/94.90
1000 9.759/9.633/9.603/9.54
1/4 11.656/11.540/11.503/11.44
MPH 118.65/117.98/118.27/118.77
time of day 10:32/11:12/12:50/1:52

 

RT -.026/.071/ .015/.043
60 1.747/1.659/1.675/1.613
330 4.883/4.772/4.766/4.712
1/8 7.505/7.379/7.360/7.306
MPH 93.89/94.24/94.81/94.90
1000 9.759/9.633/9.603/9.54
1/4 11.656/11.540/11.503/11.44
MPH 118.65/117.98/118.27/118.77
time of day 10:32/11:12/12:50/1:52

What kind of tires are you running? The reason I ask is that your et's dropped, but your mph didn't go up enough to justify. Which makes me think either the track got better as the rubber built-up or your tires got stickier as the day got warmer. If it's the latter that made the difference heat your tires a little more early in the day to find out.

 

i'm running drag radials .And i never messed with the air bleads,everything else but not air bleads.It ran lean at top end you can see it at the end of this video http://www.youtube.com/watch?v=0s8gkVlU9-w .Stock ones for that carb with the choke still on it.

But back to air temp changes i always thought you needed to change jets leaner as it gets hotter.As far as i knew you change them up to 3 jet sizes down.And its air quaility not just temp.

 

Hot air by itself will change the amount of fuel used. When the air gets hotter, there is less air mass going through the venturi. Less mass flow creates less vacuum, which in turn attracts less fuel.

 

But thats not a lean condition, it's a property of the air density. For a given mass of air flowing through the venturi, the carb will deliver the appropriate mass of fuel to match. Power loss from a hot charge is simply a matter of less dense air filling the cylinders, but the charge will still be at the same fuel to air ratio as it would on a cold charge. There isn't much you can do about it other than forced induction to make up for the change in density.

 

I didn't say it was a lean condition, I merely stated that the amount of fuel attracted into the venturi is proportional to the vacuum.

 

I started to say "I think that's what you're saying", but didn't. Consider my comments supporting yours. I'm hearing folks elude to the need for a jet change for temp and it's not true as you pointed out.

 

i run a mild 406 and use a race demon 750 and run in the VA mountains at elk creek dragway. the altitude is almost 3000 my tag weatherstation says the DA is 4200 during day and 2600 at night due to the cold moist air. I run 82 frnt 85 rear with no spacer. Other than tractioin changes my car is with .020 with a 30degree temp drop. I go to Farmington Dragway in NC the altitude is 600 with the da at around 1000 and i jet up two drill sizes not jet sizes. (Check barry grant or holley for a jet drill size chart). I run from .03 to .05 faster. By my experience the leaner I run in heat the less change I see. If im rich then more change with colder temps at night. I test n tuned for 8 weeks to get it right on the money. Won two races and semied twice. By the way i rum super pro against rails tube cars w big slicks and alcohal. I run 11 wide goodyears stock(4) leave springs on mixed pumped and race gas. No two motors are exactly alike so keep a log and find your combo. Hope you can use this info.

 

Oh on the air bleads.Umm its too old to change them.its a 4779 - b carb.They are just little guys in there.And the choke housing still on it with door and all.

So i guess i need another base.I'm still thinking pro-systems carb.

 

The 4779 shouldn't be requiring such large jetting. Have you been having this carb since new? Any chance someone else may have been drill happy with it? Have you run it on anything else with any problems?

 

Temperature affects air density, the hotter the temp the less dense the air becomes so your mixture becomes richer requiring you to lean it in order to obtain performance equal to that at sea level at standard temp. The same applies to altitudes higher than sea level.
Most performance data is computed at sea level at standard temp, which is 15 C or 59 F.
I think you can contact Holley tech or view their frequently asked questions section online. They can recommend how many jet sizes to change when you deviate from sea level or std temp, but you will definitely need to run a smaller jet size.

 

Temperature affects air density, the hotter the temp the less dense the air becomes so your mixture becomes richer requiring you to lean it in order to obtain performance equal to that at sea level at standard temp. The same applies to altitudes higher than sea level.

How does the mixture become richer? Where does the fuel come from?

 

The air will have the same percentage of oxygen, roughly 20%. Hot air is less dense, so the oxygen per unit of volume will be less.
The vacuum in the venturi is directly related to the density of the air and its velocity. Less dense air will create less vacuum in the venturi, and less fuel will be attracted into the carb.

 

...and less fuel will be attracted into the carb.

For all practical purposes it's the same amount of fuel, the air charge is less dense and that is what makes the mixture richer. Now its true that fuel also expands with temperature but this is negligible when adjusting mixture for altitude or temp. And though it's negligible, this is why many racers try to keep the fuel cold by running it thru a cool can.
Is that what you meant to ask me initially?