Tuning and synching weber idf

[Brage]

Hi, I just want to know if I have understood everything. To tune the carbs you have to:

1:
- remove linkage
- air bypass screw in
- idle mixture screw 1 out
- idle speed 1 in from touching

2:
- Start car
- Turn idle mixture screw in until cylinder dies out, turn back out until it runs good. Repeat on all cylinders. Some say in a #1 #4 #3 #2 pattern.
- Adjust idle speed screw for good idle rpm.

3:
- Use a synchrometer on the throats closest to you. (The two closest to the idle speed screw). Check both sides and turn the high flowing down to the low flowing carb using idle speed screw.
- I've read all four throats need to pull the same amount of air. Which screw do I use to synch the two throats on the same carb?
- Recheck idle rpm and adjust idle speed screw if necessary while making sure carbs are synced.

4:
- Check sync is good at mid rpm and higher rpm. What do I do if not good? Idle speed screw won't work for that?

Is that it?

[WFBowen]

I’ll add a few things. Be certain that your valves are properly adjusted, then make sure your ignition timing both at idle and higher revs is within specs; check plug gaps, wires, etc. This gives you a clean starting point. Without these steps you’ll be chasing ghosts.

Start your carb tuning with a warm engine. Make sure the float levels are on spec. I’m assuming you have Solex. If so, look between the throats to see if you have adjustable throttle shafts (on later versions.) If so, these carbs can be adjusted for equal suction. If not, it’s safest to live with uneven throat readings. (The “old way” of equalizing readings was for a highly skilled Porsche mechanic to gently twist (bend) the shaft with the motor running using end wrenches on either end of the shaft. Please don’t attempt this.) I was always told to leave the air bypass screws alone.

The bottom line here is to remember you’re working with old technology on probably old and worn parts. Even when new, these motors could never achieve the efficiency levels of today’s engines.
Good luck
Bill

[callard]

Brage asked about Weber IDFs on a 912 and did a fair assessment on how to sync them. Solex carbs (solid and split shaft) don't adjust like Webers which is both good and bad. The Weber bypass screw performs the function of the Solex throat unique split shaft flow function while the Solex solid shaft carb cannot be changed without attempts at bending the shaft. Solex float adjustment is MUCH easier than Weber and there is even a factory tool to aid in the adjustment while on the car. There is also a port on the float bowl to view the float level. Weber requires that you take the top off and hang the carb 90 degrees to bend the float to touch the needle and seat.
Solex carb throttle shafts wore the body of the carbs and allowed the throttle plates to drop down slightly and impact the fuel flow from the transfer ports in the throttle bore. This caused a stumble under acceleration. New Webers don't have this problem and many owners opted to buy the IDFs and manifold adapters due to the price differential vs. Solex re-bush and re-bore of the carb bodies.
All in all, the factory provided a very smooth drivable engine when new but after many tens of thousands of miles and throttle actuations the bodies of the Solex carbs wore and they began to be rough on acceleration. The Weber IDF was a generic carb that was adapted to many different engines for a very reasonable price while the Solex 40 PII was only used on Porsches until 1969. If you have Solex carbs on your 912 today, there are shops that will completely remanufacture them so that they perform as new (read expensive). If you have Webers on your 912, you should keep them and enjoy the car while saving a bundle.

[Brage]

Thank you! The floats are newly adjusted and I did the valves a few years ago so they should be good. I've checked the timing roughly with the mark on the distributor and the wheel on the crancshaft, looked good.

I guess I should loosen the bypass screw on the low flowing throat if the difference is too big for my liking?

[callard]

Do a compression check on the engine. If all 4 cylinders have approximately the same compression, then you can balance the carbs with the air bypass screw. If that cylinder is lower on the compression check you have another problem not carb related.

[Brage]

Just bought a synchrometer. Will soon see what it reads