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My 1983 Spider Veloce 1.6 has a problem of hesitation/stumble on acceleration, when the pedal is pushed hard and this occurs at less than 3,500 RPM starting at around 1800 rpm. If I press the acceleration pedal very slow, the issue is not there.
My car has Weber 40 DCOEs 106 and 107A with the following jets:
1) 55 F 21 jets for idle
2) F41 for emulsion tube + 132 main jets
3) 30 acceleration pump jets
4) 85 F 9 starter jet air jet start 150
Since idle is set at more than 1 & 1/2 turns from sitting (max recommended) it means the car runs with a lean idle mixture that is compensated by the tuning the idle mixture screw to enrich the mixture. I think I should try 60 F9 jets to compensate for being too lean. At alfa BB someone reported moving from 55F21 to 55F17 one step richer might be sufficient. Des Hammill (his book on Weber and Dellorto build and tne is must read) recommends go up (richer) with idle jet size at 5 increments like 60, 65 and 70 if idle screws need to be turned on more than 1 1/2.. mine is about 10 turns !
Other suggestions for the issue, I read for flat spot in acceleration (up through 3500 RPM) tbd (to be determined) besides Idle jets too small are:
1) Progression air correction jets too large->right now mine is the minimum at 30 so I am not sure. But Redline Fuel Management suggest to go to a 55 jet for the acceleration pump to increase duration and increase fuel volume. Also Des suggest that "acceleration jets should be large enough to remove any trace of hesitation or stumbling when the accelerator pedal is depress"
2) Floats set too low->checked to 8.5 mm at jet closed and 15 mm total drop per spec, still an issue after calibration to spec for my 40 DCOE Weber (also changed the needle valve jets with the new ones of the Redline kit)
3) Try adjusting mixtures at 1800 to 2000 RPM-> this won't work as you compensate the lean idle by acting on the mixture and make the engine running rich with not completing combustion
4) Exhaust header too big->to check with spec as it was replaced in my car by previous owner
5) Valve clearance set too tight (advances timing)-> to check
6) Ignition over-advanced-> to check
7) Weak spark->changed spark plugs, still an issue after change (btw spark plugs are somehow black means the engine run rich mixture by previous owner)
8) Check the fuel squirt of the progression pump when pressed in each carburetor and the stroke length (11 mm)-> to check
9) Check the throttle position at idle below the progression holes-> to check Note Redline fuel management says that this is 95% of tuning problem cause as the the throttle plate when the the idle screw is more than 1 an 1/2 turn (like my case) is exposing (means already using at idle instead during the progression) the progression holes causing to start the engine to stumble off idle at 1800 rpm
10) Check or replace the emulsion tubes-> to check
11) Check the fuel level in the carb bowls -> to check
Anybody in the forum had a similar issue with their Alfa with Weber 40 DCOE and solved the root cause of the issue?
My car has Weber 40 DCOEs 106 and 107A with the following jets:
1) 55 F 21 jets for idle
2) F41 for emulsion tube + 132 main jets
3) 30 acceleration pump jets
4) 85 F 9 starter jet air jet start 150
Since idle is set at more than 1 & 1/2 turns from sitting (max recommended) it means the car runs with a lean idle mixture that is compensated by the tuning the idle mixture screw to enrich the mixture. I think I should try 60 F9 jets to compensate for being too lean. At alfa BB someone reported moving from 55F21 to 55F17 one step richer might be sufficient. Des Hammill (his book on Weber and Dellorto build and tne is must read) recommends go up (richer) with idle jet size at 5 increments like 60, 65 and 70 if idle screws need to be turned on more than 1 1/2.. mine is about 10 turns !
Other suggestions for the issue, I read for flat spot in acceleration (up through 3500 RPM) tbd (to be determined) besides Idle jets too small are:
1) Progression air correction jets too large->right now mine is the minimum at 30 so I am not sure. But Redline Fuel Management suggest to go to a 55 jet for the acceleration pump to increase duration and increase fuel volume. Also Des suggest that "acceleration jets should be large enough to remove any trace of hesitation or stumbling when the accelerator pedal is depress"
2) Floats set too low->checked to 8.5 mm at jet closed and 15 mm total drop per spec, still an issue after calibration to spec for my 40 DCOE Weber (also changed the needle valve jets with the new ones of the Redline kit)
3) Try adjusting mixtures at 1800 to 2000 RPM-> this won't work as you compensate the lean idle by acting on the mixture and make the engine running rich with not completing combustion
4) Exhaust header too big->to check with spec as it was replaced in my car by previous owner
5) Valve clearance set too tight (advances timing)-> to check
6) Ignition over-advanced-> to check
7) Weak spark->changed spark plugs, still an issue after change (btw spark plugs are somehow black means the engine run rich mixture by previous owner)
8) Check the fuel squirt of the progression pump when pressed in each carburetor and the stroke length (11 mm)-> to check
9) Check the throttle position at idle below the progression holes-> to check Note Redline fuel management says that this is 95% of tuning problem cause as the the throttle plate when the the idle screw is more than 1 an 1/2 turn (like my case) is exposing (means already using at idle instead during the progression) the progression holes causing to start the engine to stumble off idle at 1800 rpm
10) Check or replace the emulsion tubes-> to check
11) Check the fuel level in the carb bowls -> to check
Anybody in the forum had a similar issue with their Alfa with Weber 40 DCOE and solved the root cause of the issue?
