[fbarrett]

Friends:

This is my first post to this forum, but I've owned a 1965 Giulia Spyder for about 14 years. A few years ago my mechanic installed a pair of 45DCOE Webers on the 1600-cc engine, which has no other mods except higher compression. It runs fine at 5,000 ft altitude but not above 8,000 ft, where it runs rich and fouls plugs at low rpm. It's also hard to start at high altitudes (8,000 to 12,000 ft). First jets we tried were 180 mains and 120 air. Then we went to 190 and 115. Current jets are 200 and 110, respectively. All without much change--plugs are sooty, starting is getting harder.

Questions:

1) Is the stock Alfa ignition system capable of firing the plugs (NGK BP7ES) if the mixture is too rich? When starting at high altitude, the plugs seem to get wet, but the engine won't fire unless they are perfectly clean and dry. I'm considering a Centerline electronic distributor. Any experience there?

2) Is a 45DCOE too much carburetor for a stock 1600 engine? I don't know what venturis are installed.

3) I drive the car at altitudes from 5,000 to 12,000 ft, and I'd like it to function throughout that range even if I do have to change jets. The more I think about this problem, the more I think the ignition system is inadequate. What say you?

4) Is there a Weber 45DCOE jet size vs altitude chart somewhere? Any other appropriate archival posts would be helpful, too.

Thanks for your help.

Frank

 

[101 Alfa Mike]

I wouldn't say the 45DCOE is too much carb, but it sure would help to know what size chokes you are running. Chokes as small as 32 mm are available which is what I'm running in my '65.

The Weber Tuning Manual has an altitude compensation table. It says if your normal main jet size is 1.25 you should reduce it to 1.20 for operation at 4,900'-6600' elevations and to 1.15 for 6600'-9800' elevations.

 

[Gary Williams]

The simple carb selection software at the following site will allow you to make choke and jetting decisions based on your engine, driving style and preferences: http://greend.com/carb_software.htm

The software shows a 1600cc engine that makes peak horsepower at 6000rpm needs a 30mm choke. If you play with the software, plugging in different peak power rpm or choosing performance versus flexibility, you get different results.

You can get Weber parts from www.piercemanifolds.com and other suppliers.

This webpage has excellent information from the guy who developed the carb selection software: http://members.aol.com/dvandrews/webers.htm

 

[Alleggerita]

From the Redline website:

Tuning Weber Carbs for High Altitude
Note...From Sea Level to 2500 Feet there will be very little change.

This is a question we get asked all the time. There is no exact answer or formula to altitude jetting Sidedraft or Downdraft Weber carbs. Here are the steps you need to take in order to Re-Jet your carb for High Altitude operation. This applies to both Single and Dual Sidedraft Carbs and Single and Dual Downdraft Carbs. Bear in mind that there is nothing you can do to stop the power loss you experience at high altitude. That is just a function of the lack of Air to mix with fuel for combustion. Jetting the carb properly to get the correct fuel mixture is the key to maximizing the available power.

Step-1: Find out what is in the carb now ! Do not guess and think that you have the jetting that supposedly came from the Weber factory. Find out what Main Jets, Air Jets and Idle jets you have in your carb. If it is a progressive carb make a note of what what jets were in the Primary and what jets were in the secondary. The Primary is the barrel that opens first. If you have Sidedrafts or Downdrafts with removable venturis then find out what size venturis are in your carb(s).
This is CRITICAL. You cannot tune a carb without knowing what you are starting with.

Idle Jets: You will normally need to drop 1 Idle jet size (Meaning a step of 5. 50 to 45) for every 2000 feet above 3000 feet of altitude. This assumes the car ran properly at seal level. If you have No Sea Level Reference then you just have to get a range of jets smaller than your starting point.

Main Jets: You will normally need to drop 1 main jet size (Meaning a step of 5. 150 to 145) for every 1000 feet above 3000 feet of altitude. This assumes the car ran properly at seal level. If you have No Sea Level Reference then you just have to get a range of jets smaller than your starting point.

Air Jets: You will normally need to Increase 2 Air jet sizes (Meaning a step of 10. 150 to 160) for every 1000 feet above 3000 feet of altitude. This assumes the car ran properly at seal level. If you have No Sea Level Reference then you just have to get a range of jets Larger than your starting point.

Flat Levels anf Fuel Pressure: No changes to these for High Altitude operation.

There is no shortcut or magic formulas. You just have to work through it. The guidelines above are starting points based on correct seal level operation. You will just need to invest in a range of jets to properly tune the engine at the altitude you are operatin at.

 

[fbarrett]

Guys:

Thanks for your helpful suggestions. I'm slowly building an excellent collection of Weber jets--and experience!

Frank