ambient temp and weber carb

[yeuporsch]

I've done the search. It seem that my car, 72T with 40 weber, has a moderate popping at idle and deceleration. In the morning with cooler temperature, the car seems to run great. In the late afternoon in Texas, the temp can change about 20 degree or morehotter. At that time, the car seems to pop more under acceleration and deceleration at the exhaust. I think it is a rich condition. Is this correct?

Thanks,
John

[mb911]

Yes thats what I would shoot for first

Ben

[jpnovak]

John, You will notice a change in running with that temp swing. The car runs fine (may be slightly rich) in the morning when the air is cool and dense. when it heats up there is less air creating a more rich condition. What you hear is unburned fuel igniting in the hot exhaust, especially on decel. Perfectly normal for a carb'd car.


where are you in TX? If you are close you should come on one of the fun runs here in Austin.

[Grady Clay]

WOAH Hoss,

Is this a new symptom or has it been going on for some time? Is it changing over time?

Popping is usually a lean condition.
The popping under acceleration is of concern. What color is your tail pipe after driving while hot?

For the trailing throttle popping, the first places to look are vacuum leaks between the carb to manifold or manifold to head. What vacuum lines are there? What is the condition of the carb butterflies and shafts.
Another is the integrity of the exhaust system: leaks in the muffler, leaks in the gaskets, or (dangerous) leaks in the heat exchangers.

The popping at idle can be from the above, from worn carbs, or just a mis-set idle mixture.

Please report what idle, main, and air jets are in the carbs.

Best,
Grady

[jpnovak]

It doesn't say... Popping through intake or exhaust. I was assuming a slight pop in the exhaust. As Grady pointed out, popping though the intake indicates a lean condition and is really bad for your motor.

[yeuporsch]

There is a lot of carbon, black soot in the tail pipe. Some of the plugs are the same way. I think there is a slight exhaust leak at the right bank of the carbs.

Does anyone have the torque value of the carb to the intake?

Thanks,
John

[PorscheRB]

My car does the same thing- '74 with weber 40s...

[Grady Clay]

John,

That is actually good and easier to fix. Lean popping under load can damage an engine. Too rich wastes gas, doesn’t run well hot, and can dilute the oil with gas in cold weather (not a problem in Texas today.)

First you need to fix the exhaust leak. That will probably solve most of the popping. What exhaust system is on your 911?

You will need to confirm there isn’t an intake leak.
I use WD-40 to spray at the gaskets and see if the idle changes. Since it is already so rich, you might use a spray bottle of water.
What is happening here is the propane in the WD-40 will speed up a proper mixture by adding fuel through the leak or will slow down the idle by the oil sealing the leak. The water will just seal the leak. Any change locates the leak.
Unless the nuts are actually loose, tightening probably won’t fix an intake leak for long and you run the risk of damaging a flange. The best solution is to replace all the gaskets (12) with clean sealing surfaces. This is an easy DIY project (see 101 Projects #28 & #29.)

Once you have the leaks fixed, you can set the idle mixture. With it this rich you may use several sets of sparkplugs. While rich you might use NGK BP6ES. When it gets proper mixture use BP7ES or BP8ES in very hot weather and mostly highway driving.

Today there is a thread on sparkplug reading you might find useful:
http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html
tell me my MIS-fortune, read my plugs


You still need to report the carb jets. There is some unusual reason for this to be so rich. What is your gas mileage?

Best,
Grady

[PorscheRB]

John I hope you don't mind my input as well but as we seem to be having the same problem. I get about 10 to the gallon...
Thanks
Ian