Tuning Carbs - Bench Setting
 

Hi All,

I am tuning and balancing my carbs because my 911T has not been running well and I can tell the Weber carbs are not balanced.

I followed this video to the T for bench settings.
https://youtu.be/p3qwIbRdcvE
Fuel Mixture two turns out.
Air Bypass all the way in.

This is just a bench setting to get the engine running and start the tuning process right?
Well, one side is great and pulls about 5 on the syncrometer . The other side is way off and is pulling 15 on the syncrometer.

https://youtu.be/bZ__x3zXHog

I tried cleaning jets, mixture screws but I am stuck. Any suggestions on how to troubleshoot and solve this issue? I can’t even start the tuning process.

Thanks in advance.

Phil

With at least one of your throttle arms disconnected, you should be able to adjust the idle screws to get even flow. It looks like you can lower the drivers side carb (by unscrewing just a bit) and/or raise the passenger side by screwing the adjuster in just a bit. Seven is a good number to shoot for when tuning.

All throttle arms are disconnected. I have adjusted the idle screws on passenger side but no change. I screwed the air/fuel all the way in and it still pulls 15. Very weird. If you have time, Watch this video to see what I mean.
https://youtu.be/bZ__x3zXHog

Cheers,
Phil

It’s been a long time since I’ve done this, but here’s how I did it.


Carbs on car (914-6 in my case)
Idle fuel mixture 1.5 turns out.
Air bypasses closed
Throttle linkages disconnected
Run fuel pump to fill carbs, check float level with Weber gauge. Adjust float levels
required. Note: fuel level is adjusted by adding,removing baskets under the needle
valve NOT by bending tang on float!
Adjust accelerator pump delivery. On my six the manual was ‘way wrong, too much
fuel by about a third. (If engine bogs when throttle opened but picks up if you close
the throttle they are too rich. If it continues to bog when throttle is closed too lean.
There is only one pump diaphragm for all three throats, so I always checked the
pump delivery on the center throat and let the others fall where they may,)
Start engine, do whatever it takes to keep it running
Roughly balance carbs side to side using center throat. (Doesn’t really matter which
one, but use same one every time.)
When they are in synch begin adjusting the idle mixture screws for smooth idle.
Connect cross shaft linkages
Now set the individual throats. On each carb find the one drawing the most air. Using
the bypass screws set the other two throats to match. Do the same thing on the
other carb.
Disconnect the cross shaft linkages again and rebalance the carbs side to side. Set
your idle speed at this time.
Connect the cross shaft again paying attention to whether you need to make
adjustments to the linkage lengths to keep the side-to-side balance.
Go back and recheck the idle mixture screws for smoothest running. On my car I
always turned them all out about a quarter to half turn more; seemed to improve
throttle response.

I generally did everything with the cross shaft disconnected “blipping” the carbs to clear the engine using a hand on each carb rather than keep fooling with the cross shaft.

Note: side-to-side balance has little effect on performance once off idle. Once the butterfly starts to open the volume of air flowing is such that any differences are inconsequential.

Hope I remembered everything; been a long time.

If you're pulling that much air on the left bank and everything is disconnected, make sure your throttle adjustment isn't too far in. Can you visibly see the throttle plate and compare if left is fully seated and closed? If it's pulling that much air, you'll be pulling fuel / air from the progression port and won't be able to dial your base setting.

Start with adjustment screw and inspect plates.

Thanks for this…I believe this is what is happening. I was using color tune as well and I really can’t tune the left bank so I must be pulling fuel/air from the progression port. Left bank is un tunable right now. Even the air bypass (which I use very cautiously) had no effect.

In the video of the green car he has the mixture screws and air bypass screws in the wrong ports from what I see

^This

OMG!!! All I can say is I didn’t do this. Had my carbs rebuilt by someone. No wonder I have been fighting my carbs for the last month.

Just switch them and should be good to go . Let us know how it turns out

Thank you very much for pointing this out. I ran out to my garage and confirmed that yes mixture screws and air bypass have been swapped. I did a bench setting and things are back to where they should be. Thank you for noticing that!!!