CARBURETOR ADJUSTING 101
by Patrick James
**** The information given in this article is intended for PRO-SYSTEMS carburetors only. Other manufacturers tuning and jetting ranges can vary dramatically from this supplied information, so follow your original manufacturers guidelines for tuning your specific design.
One of the biggest mistakes made by a tuner of a high performance street engine is not tracking an engines varying needs.
Most high performance street cars make a lot of under hood heat and engine transferred heat. As a result, this engine ingested, super heated air, can climb as high as 60-100 degrees higher than the outside ambient air.
But what you’ll find in the spring and fall is that as outside air temperatures drop from the 80-90 degree range down to the 60-70 degree range, underhood and engine transferred temperatures will fall much more.
So instead of following the outside ambient drop they fall exponentially. So a 15 degree drop outside can be as much as a 50 degree drop to the engine ingested air.
It also works the other way, with exponential rises.
So in June on a hot summers day, your engine is driving around drawing in 80 degree weather, ingesting 150 degree air and in the fall you are driving around in 60 degree weather ingesting 90 degree air. Thats a 60 degree variance!
Obviously you can see why the vehicle manufacturers construct those air ducts that go to your fender well or grille area to reduce this variance.
Unfortunately, the first thing folks typically do is remove those ducts and add a chrome air cleaner. They then tune the car for the weather that day, drop the hood and off they go for months at a time.
Then as it warms up outside, the plugs start to foul, the exhaust is getting rich and changes have to be made.
Typically from cooler air to the warmer summer air ( on non-hood scooped or fresh air ducted engines) you should reduce your main jets about 2-3 sizes on all four corners and possibly increase your idle air bleeds by about .006-.010.
Air Pans are a great way to duct fresh air to your engine. See our “Racing Parts” section for a great Air pan to really help offset this variance.
This is because there is not as much oxygen available in the warmer summertime environment. If you don’t pull that fuel out, you will lose twice as much power, as now the engine is running rich and creating slight misfires and incomplete combustion. The power is lost due to the lack of oxygen molecules, but if you keep pouring coal on the fire, its gonna go out. So you have to pull some coal back.
If you have an air density gauge that corrects back to a Density Altitude number, most Pro Systems gasoline fueled programs will require a drop of one main jet for every 1100-1300 feet in increased Density Altitude.’
Don’t forget to adjust your mixture screws too. Make sure every corner of the engine is happy, pull the plugs out and get them all similar in color at an idle.
Typically you simply, turn the idle mixture screws in until the engine hesitates or requires you to squirt fuel in it to restart and then turn them back out until those problems go away.
Tuning a performance street car for its best performance, even if you have outside air ducted to the carburetor is a constant job (or at least seasonal).
The ducting will help even out the seasons a little, but the engine transferred heat is still hanging around, causing troubles and requiring a little bit of attention from you.
So be sure to pick up a screwdriver and show your ProSystems carbureted car some love, at least once in the Fall and again in the Summer.
Thanks for reading.