I don't know if this has been tried before, but I was going to buy some adapters or else try making them by drilling a 5mm bolt. I found that I had some spare brake line that measures-up to be close to 5mm in diameter. (I think its the smaller size, nominally called 3/16 inch.) So I cut some small pieces and threaded up one end with a suitable thread die, cutting the thread to just the depth of the die. They seem to screw into the manifold holes with just finger tightening, up-to the extent of the thread which will taper off and make a seal. The other end of the pipe is then just the right size for a plastic hose to slip over.
I tried popping these onto two of my carbs, with a loop of hose with each end attached to these adapters. I had put a bit of fluid in the bottom of the loop, hoping it would act as a manometer to register the difference in vacuum. However, even with what I thought was a reasonable length of hose (the end hung down to the ground from the tank height), all the fluid quickly got sucked into the running motor. Maybe the two carbs I chose to hook to are REALLY out of synch.
Fortunately I had chosen to use some old model engine fuel that my son has had lying around for a number of years - it's also a nice pink colour and looked good in the tube - so it didn't seem to have any affect on the motor
. After that experiment, I've decided to get some vacuum gauges off ebay (got four of the NOSHOK 1.5" Dial Size 1/8" NPT 30" Hg Vacuum Gauges on-order) and make-up a set for sych'ing that way.
I know how a picture is much better at explaining, so here is a picture of one of the adapters hooked-up to a tube. The other end is threaded for screwing into the motor at the carb intake.
