cold winter carb weber adjustment?

[Turbo Den]

hello,
i've tried searching the forum on cold winter carb adjustments, but no good result. so here's the story and question.

this is my 1st yr with a P-car, and 1st yr with carb.
it's 1978 3.0 SC w/ PMO carbs. run very well in Vancouver summer 15-25 degrees C' / 59-77 Fahrenheit.

my brother came visit last week, so i insured the 911 to show him how good a 30yrs old porsche is still a porsche. but i failed to impressive him totally. cos, the car wasn't running properly.

warmed idle is 800-900 rpm, normally 1200 rpm.
engine CANNOT rev past 3-4k rpm, normally rev happy to 6500 rpm.

question: what would you do, if any, to adjust the carbs for cold weather?

i tried full gas tank, long warm up, 1-screw counterclock wise on each mixture adjustment screw for more fuel.

thank you.

[Steve@Rennsport]

A few questions,......

1) At what temperature is it manifesting these symptoms?

2) Is it lean or rich? Without knowing this, you have no way to know what to do,...

3) Precisely,....what does it do when trying to get past 4K? Will it go beyond 4K with wide-open throttle?


Warm idle should be 850-925 RPM. 1200 RPM is too fast and this affects the fuel delivery and throttle pickup.

[john walker's workshop]

icing is a normal phenomenon. it causes dying and rough cold idle until the engine heat builds enough to keep ice from forming. shouldn't affect 2500 and up though.

[Turbo Den]

Originally Posted by Steve@Rennsport

A few questions,......

1) At what temperature is it manifesting these symptoms?

2) Is it lean or rich? Without knowing this, you have no way to know what to do,...

3) Precisely,....what does it do when trying to get past 4K? Will it go beyond 4K with wide-open throttle?

1) cold temp is 2-4 C/ 36-39 F'
2) assuming my summer mixture is perfect, i assume cold weather makes the engine run lean. am I correct?
3) it will not go past 4k rpm. as i step WOT, rev builds slowly to 3500-4000rpm and stop going up.

[Brent Lineker]

I have 40 IDA's on mine and have richened them up just a touch for our winter weather.I turned OUT the mixture screws slightly less than 1/2 turn and now runs perfect.

[Steve@Rennsport]

Originally Posted by Turbo Den

1) cold temp is 2-4 C/ 36-39 F'
2) assuming my summer mixture is perfect, i assume cold weather makes the engine run lean. am I correct?
3) it will not go past 4k rpm. as i step WOT, rev builds slowly to 3500-4000rpm and stop going up.

As John suggested, carburetor icing is quite normal on these cars when operated in cold, moist weather. Its exacerbated by the lack of intake air pre-heat and cold manifolds (needed for summer operation). When the carbs do ice up, the main venturi becomes obstructed with ice that can severely restrict engine RPM.

Once its melted by engine heat, normal operation is resumed, BUT,....BUT if the intaker manifolds and carbs remain cold, the ice won't melt. I would suggest removing any insulators between the heads and manifolds, if they are present for a start.

Indeed, cold weather makes the car go leaner so a review of the jetting is in order.