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Thread: 2.5 liter short stroke with Weber IDA40: low rev engine bucking/jerking

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  1. 11-02-2017, 05:31 AM #1
    60surfer
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    2.5 liter short stroke with Weber IDA40: low rev engine bucking/jerking

    Hi,

    I just finished restoration of a 2.2S which comes with a 2.2S engine modified regarding displacement (89mm Mahle racing P&C's, 1:10.3 c/r), Weber IDA40 instead of MFI and twin spark distributor.
    I'm quite happy with the engine, delivering almost 220 HP and running very nicely above 3,500 rpm, ok between 2,200 and 3,500, but bucking between 1,800 and 2,200 when putting the pedal to the floor (e. g. when accelerating in second gear at about 20 mph out of a steep turn uphill).

    Measuring lambda values show the problem very well, see dyno diagram attached as pdf.

    However, as everthing is new, and as the car has been on the dyno 3 times for carb and ignition tuning, the problem still remains.
    What has not been done in detail so far is playing with the acceleration pumps at both carbs, so that could be a reason.

    But in case this is not the problem, any experience here what to try?
    For example, which effect would high manifolds or smaller/bigger venturis have?

    Thanks so much!
    Siegfried
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