MFI Balancing Question

[schmaz356]

I have a '73 911E with MFI with a newly rebuilt pump. I read the attached description on how to balance the MFI throttle bodies and was going to use this method. Note I am going to use a standard high quality vacuum gauge rather than the U-tube system.

I was wondering if someone can confirm the following procedure. Disconnect the vacuum line of each throttle body side one side at a time, plug all of the open ends and at 3000 rpm disconnect and measure each ITB vacuum one at a time. When done with both sides, add all of the values up and divide by 6 giving me an average vacuum. Then repeat the first procedure but this time by adjusting the air bleed screws, adjust the ITBs to the average value. The paper seems to indicate that this is how you get each ITB balanced in the system.

Is this right?

[RSTarga]

That is the procedure described in the CMA fuel injection manual

[Jeff Higgins]

Per CMA, there is no need to disconnect the vacuum lines.

Also per CMA, the first thing to do is to close each air correction screw by turning it in until it is lightly seated. From there, open them up one and one half turns before proceeding. That is your starting point.

Once you have them balanced, set idle speed on a warmed up engine by turning all six of them equal amounts. Turn them in to slow down the idle, back them out to speed it up.

[schmaz356]

First, thanks to everybody for the input.
Jeff;
The reason I was disconnecting the vacuum line was I need to have a place to attache the gauge to. Am I missing something? Also I do have the air correction screws currently at 3 half turns out from seated. I have not previously hear to method two get to an idle value. Right not mine is a bit low with the throttle lever all the way down.
Thanks again.

[ed mayo]

There's no way to access vacuum on each cylinder, as only # 1 and #4 have a vacuum port.

[Bill Verburg]

Originally Posted by schmaz356

I have a '73 911E with MFI with a newly rebuilt pump. I read the attached description on how to balance the MFI throttle bodies and was going to use this method. Note I am going to use a standard high quality vacuum gauge rather than the U-tube system.

I was wondering if someone can confirm the following procedure. Disconnect the vacuum line of each throttle body side one side at a time, plug all of the open ends and at 3000 rpm disconnect and measure each ITB vacuum one at a time. When done with both sides, add all of the values up and divide by 6 giving me an average vacuum. Then repeat the first procedure but this time by adjusting the air bleed screws, adjust the ITBs to the average value. The paper seems to indicate that this is how you get each ITB balanced in the system.

Is this right?

I always used a Uni- Syn

[yelcab1]

You attach the vacuum gauge at the top of each throttle body. That accurately measures the total air being drawn into that cylinder, not accounting for any small vacuum leaks.

[Jeff Higgins]

I use a carb synchrometer. Remove the air cleaner and place it into each stack to measure its vacuum draw. It's the only way I know of to read each individual cylinder. Our host sells these, but any similar tool will suffice.

[schmaz356]

I have one and will handle it that way. Thanks.

[tobluforu]

I have the same a Jeff, works great. I also have the old school one that has a little ball that floats up and down in a skinny glass tube. That works as well if need be.