Weber Issues

[Sisuman]

I have a 3.0 SC engine with stock internals and Weber carbs on a 69 911T. I just got the car running well a few weeks ago and all was fine until last Sunday. The engine developed a flat spot at mid range rpm (about 2500rpm to 4000rpm) with light to moderate throttle, but responded normally with good accelleration under full throttle. This was not an issue until I ran the car hard for a several miles on a twistie road. On the freeway coming home the car ran okay, but not great, and there was a flat spot at normal freeway speed (around 3400rpm). For the past few weeks and even earlier in the day the car ran great.

Any ideas?

Thanks in advance for any help you can provide.

John Dimock

[Fishcop]

Webers need clean fuel... Check for debris in filters. A speck of dirt in a jet or circuit really upsets them.

[1QuickS]

Check the idle jets for a speck of debris on the tip, this is the most likely culprit. The other item that may cause a trouble is a partially obstructed emulsion tube...happened to me when a bit of tissue blocked off a portion of one of the tubes.

Typically air gets past the air filters and causes issues like this. Another source is rusting of the vent tubes in the top covers of the Webers, the rust drops straight down into the fuel well without any filters in the way.

When on full throttle the idle jets are bypassed so any blockage would not be noticed.

[Sisuman]

John & Paul,

Thanks for your insight. I will look into it this weekend and let you know what I find.

John Dimock