The Effects of Heat on Carbureted Engines

CFS Carbs

The radiant heat of an engine and its exhaust system, and the heat in the air heated by going through a radiator, can be absorbed by fuel system and intake system components, if they are not protected(insulated). These components can include the carburetor, fuel lines, fuel pump, fuel pressure regulators, and air ducts from the air filter to the inlet of the carb. As these components get hotter, they transmit heat to the fuel and/or air in them. As the fuel gets hotter, its fuel density(specific gravity) drops, and the air/fuel ratio gets leaner. This is because carburetors do not sense fuel temperature or compensate for changes in fuel temperature, with the exception of the buoyancy of the float in the fuel. As the fuel's density drops with increased fuel temperature, the amount of the float below the fuel level needs to increase slightly due to the drop in the fuel density. The result is a slight rise in the fuel level, assuming that the fuel pressure remains the same. That higher fuel level will slightly richen the carb, and the decreased fuel density will make the mixture leaner. The effect of the lower fuel density on the carb's fuel metering circuits itself, is the bigger factor, so the net result is a leaner mixture. The effect of these heat sources also can increase the temperature of the air getting to the inlet of the carb. A rise in that air temp, causes the density of the air to decrease, which has the effect of richening the fuel/air mixture. The effect of the heat on the fuel temp, is usually the bigger factor, which means that the overall effect of the increased fuel and air temps is usually a leaner mixture.

As fuel temperature increases and air/fuel ratios get leaner, the engine and the exhaust system usually generate even more heat. So, over time, the heating of the fuel and the heating of the engine can get into a vicious cycle where the temperatures of both continue to climb, each causing the other to experience progressively higher temperatures as time goes on. The results of this vicious cycle can lead to increasingly sluggish engine performance, possible cooling system over-heating with fluid loss, engine damage due to overheated oil, and excessively lean air/fuel ratios. This is more prevalent in longer duration types of competition, like oval track racing.

Insulating fuel system components can minimize the effects of , and even prevent, this vicious cycle of heating. The insulation must protect the fuel system components from radiant heat generated by the surfaces of the engine and the exhaust system, and from the hot air bathing around those components. There is a variety of materials that can be used for this insulation, and some of them are readily available and not expensive. To help make engine performance safer and more consistent over a wider range of weather conditions, insulating the ducting for the air to the carb's inlet is recommended. Pulling air from outside the side shields of a tractor, can really help with that, except in really cold weather. So, insulating the surface of anything you don't want to absorb heat, and transmit it to the fuel or air flows, is time well spent. Care should be used to make sure the insulation does not interfere with external moving parts on the carb or the throttle linkage.

To help keep underhood air temps under control, installing header wrap on the engine's exhaust manifold or header, can be a big help. Make sure that the material of the manifold or header can withstand higher temperatures, because the header wrap will reduce the heat that is being radiated out of the surfaces, and will make the manifold/header itself run hotter.

Norm Schenck/ Competition Fuel Systems/ Birch Run, MI/

This post was edited by CFS Carbs on 11/29/2023 at 08:22 am.
What is the affect of altitude? Yes I
know lower power because of less air. What
do you do to carburetor adjustments, idle
and load?
Yes, higher altitude has thinner air, so the carb needs to meter less fuel. The engine makes less manifold vacuum at the higher altitude, so the low speed/part throttle circuit is pretty much self compensating. However, the main metering circuit is not self compensating. With the Holley main jets, which I use in the updraft carbs I build, normally changing 1 step on jet size per 1000ft of altitude change, takes care of that circuit.
If you went to a pull in Denver, and had your carb well tuned for Ottumwa, that would be a change of about 4500 ft in altitude. That would require changing the main jet 4 or 5 steps smaller(Holley jet steps), probably more likely 5 steps. This assumes that Denver and Ottumwa were in the same weather system, which is possible, but not guaranteed. That s why lots of racers have a small portable weather station that they look at, wherever they compete, even if they compete at the same track most of the time. Here in Michigan, we get some wild swings in barometric pressure, big enough to require jet changes, if we are looking for max power on any particular day. Norm/CFS

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