Boxerworks.com

I have a running dispute with a riding friend. We both use the home made manometer to balance carbs. I have been adamant that both tubes connecting to the carb vacuum ports need to be the same length. My friend has his set up so one tube is longer than the other as he hangs the device to one side of the bike while tuning. Shortest tube goes to the side closest to the meter. I am under the belief that the tubes need to be the same to get an accurate reading. Who is right?

Length doesn't matter (here).

If you suck on a straw, the height of the fluid above the reservoir only depends on the vacuum you draw. Same principle applies.

Mine has two different lengths.

Maybe that's why my bike runs like crap.

Sorry, my friend, your friend is right.

damn! Good job I didn't put any money on it!

What if the difference in length was huge, like one was only 12 inches long and the other was 12 miles long? Practically, seems like there would be a difference. One takes quite a bit more to create the same level of vacuum given the greater volume to evacuate. However, maybe the difference between one 36 inches and another 40 inches might be immeasurable.

Kurt in S.A.

Switch the tubing and see if you can tell the difference.

Kurt in S.A. wrote:What if the difference in length was huge, like one was only 12 inches long and the other was 12 miles long? Practically, seems like there would be a difference. One takes quite a bit more to create the same level of vacuum given the greater volume to evacuate. However, maybe the difference between one 36 inches and another 40 inches might be immeasurable.

Kurt in S.A.

I don't believe there would even be a difference in the 12" vs. 12 mile example, as far as where it would end up, but there would be a huge time lag in how long it took to stabilize.

Oh, there is a difference, all right.
You are pulling a vacuum on different quantities of air (inside the tubing). This is why you should calibrate the scales by interconnecting the sending tubing, then placing them under a shared vacuum and adjusting the scales to match each other.
Now change the vacuum through the range of the scales and note any difference at each end of their range.
You will see that a difference in the tubing length has a corresponding effect on the scale reading. Keep your sending tube lengths as even as possible.

Cheers,
Jon-Lars
Beverly, WA

Since there is basically no air flow, there will be no additional pressure drop on the long side. Each side will almost instantly be at the pressure level of the intake tract that it is connected to.