Does engine run more rich in hot weather?

[David C.]

I notice that my 73 has noticeably more of an unburned hydrocarbon smell at idle in the hot weather. Also during hot weather, I experience more exhaust popping when letting off the throttle during upshifts. By hot I mean Florida summer heat around 90 degrees.

When the weather moderates and gets 10 - 20 degrees cooler, I notice less exhaust smell and less popping. The car runs and idles noticeably better.

My question is does hot weather cause a rich running situation since the hot air holds less oxygen, therefore combustion requires less fuel? Something our old tech carbs cannot adjust for so well.

[David M.]

Re: Does engine run more rich in hot weather?

Heat and heat soak will cause a carb'ed engine to run less efficient as the air charge is not as dense. Unless the carb is tuned for such conditions. Rarely does a carb'ed car run right at a 14.7:1 Air Fuel Ratio. The fuel isn't being atomized as efficiently as it would be with a cooler intake charge. Less air means you're on the rich side of stoichiometric AFR or below 14.7:1 , So yes, rich.

Is your heat riser system still intact? If so consider disabling it. Especially in Florida. I can explain more if interested?

Are you using ethanol blended gas? That's a big no no for anything with a carb, from your car down to a chain saw. Today's fuel properties, even non ethanol are different from yesterdays. Today's fuel boils at a lower temp than the "good stuff" did. The 73 carb and timing settings dont exactly apply today. The float level should be lowered a 32nd or two. This minimizes hot soak "burp over" on a Q Jet. The accelerator plunger needs to be compatible with today's fuel blend.

The timing needs to be fine tuned for optimal operation on today's fuel(all 3 events).

Have someone go through the carb, blue print it if you want a good bang for the buck. Someone highly recommended such as Cliff Ruggle and others that specialize in Q Jets.

Have the distributor re-curved and checked for general health. Discuss an adjustable vacuum advance "can" with the tech. If it has points Id dump them just for the sake of not having to deal with the maintenance.

Id also suggest you do a tune up... at minimum check all of the consumable items, fuel filter, plugs, cap, rotor, wires, carb accelerator plunger etc...

By optimizing the timing curve the AFR and freshening things up you will be rewarded by a better smoother running engine without the pop.

Exhaust popping could be... too rich, too lean, too much vacuum advance(at idle), bad dwell setting, an exhaust leak or a vacuum leak or a combo of the above.

How do the spark plugs and exhaust tips "read"?

Hows the engine op temps look?

I know more info than you needed or wanted.

[Duke W.]

Re: Does engine run more rich in hot weather?

Higher air temp means lower density, so the basic answer is leaner, but likely the problem you have is percolation, which causes a richer mixture, especially driving around town in stop and go traffic. This is a common problem on vintage carbureted cars running E10 gasoline.

The situation is exacerbated by the emission oriented spark advance map with ported vacuum advance. This can be easily changed by converting full time vacuum advance and optimizing the centrifugal advance and initial timing for the antecedent non-emission engine.

Search the web, my name, San Diego, Corvette, download and read the tuning seminar.

Duke

[David C.]

[David C.]

[Domenic T.]

Re: Does engine run more rich in hot weather?

My opinion is that hot air is less dense and you will run rich. The same as if you are driving in cities like Denver, or even worse Aspen. People don't like this but I am going to use a example. Gasoline aircraft have to lean their mixture when taking off in hot air, or as they climb. Introducing hot air in the carb from the carb heat box that is there in the event that the carb starts icing, will cause about a 150 RPM drop. It was also said that the power loss is about 7%. All that said, there is a mixture control to lean the mixture on airplanes so they do not run rich in hot weather or high altitude.

Dom

[Duke W.]

Re: Does engine run more rich in hot weather?

It's difficult to compare automotive and aircraft due to both the big difference in engine architecture and operating conditions. Typical light aircraft engines are air cooled boxers of four or six cylinders. The carburetor is usually somewhat isolated from the engine itself by a manifold fabricated out of tubing rather than castings that bolt directly to the engine structures, which allow a lot more heat transfer.

Idling time is usually low and once airborne there is lots of air flow through the engine and avgas has a lower distillation curve, so excess richness form carb bowl percolation is rarely an issue as it is with auto engines that are slogging along in low speed urban traffic in hot weather.

Despite the fact that lower air density means the carb runs a little richer, hot weather low speed percolation can cause way excessive richness to the point where the engine may stall. That's why AFB carburetors have a hot idle compensator - a bimetallic strip located between the secondary venturis that opens up a calibrated orifice when inlet air is very hot to restore the A/F ratio to more nominal values. The additional air should cause an increase in idle speed, but I never noticed this in my 340 HP SWC, and always wondered if it every opened.

Apparently most fuel system engineers didn't think much of it because I don't know of any other type carburetor that has such a device.

Duke

[Terry M.]

Re: Does engine run more rich in hot weather?

It's difficult to compare automotive and aircraft due to both the big difference in engine architecture and operating conditions. Typical light aircraft engines are air cooled boxers of four or six cylinders. The carburetor is usually somewhat isolated from the engine itself by a manifold fabricated out of tubing rather than castings that bolt directly to the engine structures, which allow a lot more heat transfer.

Idling time is usually low and once airborne there is lots of air flow through the engine and avgas has a lower distillation curve, so excess richness form carb bowl percolation is rarely an issue as it is with auto engines that are slogging along in low speed urban traffic in hot weather.

Despite the fact that lower air density means the carb runs a little richer, hot weather low speed percolation can cause way excessive richness to the point where the engine may stall. That's why AFB carburetors have a hot idle compensator - a bimetallic strip located between the secondary venturis that opens up a calibrated orifice when inlet air is very hot to restore the A/F ratio to more nominal values. The additional air should cause an increase in idle speed, but I never noticed this in my 340 HP SWC, and always wondered if it every opened.

Apparently most fuel system engineers didn't think much of it because I don't know of any other type carburetor that has such a device.

Duke

I believe in their original configuration 1969 and 1970 Rochester Q-Jets have a hot idle compensater.

[David M.]

Re: Does engine run more rich in hot weather?

What I am referring to as far as disabling the Hot Air System is blocking off the crossover ports in the intake or in the heads then eliminating the thermo stove flap in the exhaust manifold. I chose to weld in the special cast plugs made specifically for this purpose. I welded them into the intake verses risking the heads. Easier to reverse the mod on an intake verses heads(mill out plugs to remove). This eliminates the hot exhaust gasses from passing under the carb. You will need to modify the choke to lighten it up(divorced) or it will take forever to "pull off". No worries with electric as its not dependent on intake temp to expand/contract the bi-metal choke spring.

I have preformed this mod on several muscle era cars including my 68, 400, 442. I left everything on the exterior of the engine intact except the thermo flap. I simply removed it and welded up the holes. I did used another set of manifolds so I didn't harm the correct numbers matching units. The Vettes got headers, Edelbrock intake and carb and no heat soak problems to date.

On a Flight car you may want to refrain, thou the internal mods are not detectable You could get creative and modify the external parts of the thermo valve to look like its present. Not sure what this would do for a PV. Again if 100% originality is a concern tread lightly.

With a driver it will completely rid you of percolation, hot start problems(fuel related) and reduce the sluggish throttle response feeling when the ambient temps are hot. As an added benefit the paint will never burn off the intake or discolor an aluminum unit. I like the idea of less exhaust restrictions as well. Yes it will now be a bit "cold blooded" but I dont care I never drive in cold(winter like) weather.

[Duke W.]

Re: Does engine run more rich in hot weather?

My experience blocking the heat riser with .010" stainless steel shims on my 340 HP SWC was unsatisfactory. It suffered from severe startup stumble until the engine had be running for about 20 minutes in mild weather, so I removed the shims by loosening the manifold bolts and grabbing the ends of the shims with needle nose pliers and pulling them out. I installed the shims with the ends above the junction in anticipation that I might have to remove them and didn't want to pull off the manifold, again.

My current recommendation to those who want to try this is just block the drivers side. This will allow some heat to migrate up from the pass. side and will also allow remote choke coils to operate, although full pull-off will take more time.

Some also remove the valve from the heat riser assembly leaving the shaft and spring intact, which doesn't make the change visually detectable, but it should still be caught during ops check and PV because judges should check exhaust flow on initial start-up, and with the heat riser functional the right side flow should be considerably less than left side flow.

You can also buy shims that have about a 1/2" hole in them which will reduce exhaust flow through the crossover, which reduces heat transfer.

Duke