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

[60surfer]

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

[edmayo]

What are your carb and timing specs?,,,,port size, butterfly size, are you getting fuel/air specs from the dyno, what are they at point of problem?

[Richy]

Why are you flooring an S engine below 3,000 rpm ? Drive with an AFR meter . I suspect bigger idle jets may be needed . It's relatively straight forward building an engine , now the hard part , tweaking to make it run right .

[lkadams]

1800-2200 rpm seems like low rpm for an emulsion tube change, but I suppose it depends on what's in there now - it may be warranted. Slightly larger idle jets may help as well. If you post what the carb setup is, it may give some clues.

[TEC69E]

Hi Siegfried . . . here are some basic things I would check . . . using a Synchrometer (pelican part # PEL-PMO-STE-BK) make sure each carb is pulling the same volume of air at idle, 2k, and 3k.

Of course, you can only adjust this at idle and if I recall you want this to be around 2-2.5 (all carbs should be the same). Once you've got this set correctly at idle then (using the hand throttle) set the rpm to 2k and check airflow again with the Synchrometer (write down your readings at 2K). Then repeat the same process with rpm set to 3k (write down your readings at 3K). You want the airflow readings to be the same at 2k and 3k . . . if they are not then you need to figure out why. One possible suspect would be the carb linkage. Disclaimer . . . I've never touched a set of webers but have worked/tuned my own PMO and MFI systems.

BTW . . . awesome car!!!

[60surfer]

Hi guys,

thanks for your thoughts so far.
- Port sizes are stock 911S 2.2, intake manifolds and carbs are from an earlier 2 liter 911.
- Distributor is electronic, timing advance curve is programmable and was adjusted on the dyno, 500: 20, 1000: 4, 1500: 16, 2000: 25, 3000: 28, 7500:28, limit at 7500
- air/fuel ratio: see lambda value in pdf (rich mixtures λ < 1.0, and lean mixtures λ > 1.0.)
- jets and settings should be thoroughly adjusted, synchronization should also be ok. Don't know the jet sizes right away, but will check
I agree that further carb tuning could be an option.
What about higher intake manifolds or manifolds with bigger diameter? Weber IDA46?

Thanks,
Siegfried

[edmayo]

I suspect its a misprint, but you show to have 20 degrees advance at 500 RPM (Is that engine or distributor?) then dropping back to 4 degrees at a 1000 and 16 at 1500. Is that correct ? In addition to jetting, need to know venturi size. I autocross a 2.0 S, and there are routinely conditions where I have to give it full throttle at 2,000 RPM, now granted its not "on cam" and is a little flat there,,,,but it doesn't buck and stutter. It can be done.

[60surfer]

No, not a misprint. The guy who programmed the distributor with the car on the dyno explained to me that this was his method to stabilize idle behavior. Especially when headlights/high beam are being switched on, the engine continues to turn at around 800 /min. So that works well.

But the problem is above 1,500 and it's good to hear that you think it can be done
Besides jetting and setting the carbs (I need to find out the current settings including venturi sizes first), what would be the impact of higher intake manifolds or bigger carbs? Could the acceleration pump setting be an issue? I tried to set them according to Porsche workshop manual spec, injecting about 0,5 cubic centimeters into cylinders #3 and #4, assuming that the others in the row would work in the same way. Might have to spend more time here...

I suspect its a misprint, but you show to have 20 degrees advance at 500 RPM (Is that engine or distributor?) then dropping back to 4 degrees at a 1000 and 16 at 1500. Is that correct ?

[Norm01]

You need to call Paul

[edmayo]

I don't claim to understand his reasoning about idle control. I do know that if you followed the Porsche developed advance curve for twin plug that has worked for decades I don't know why it wouldn't work here. Bigger carbs, venturis, manifolds would only tend to make problem worse.