I think it is a 5 millimeter threaded barb fitting for the fwd carb. I have tried all the local hardware stores. I am going to make one of those cheap sync gages w/ the tubing and yardstick.

 

What many people do is order the part that's called a "boost joint" that is already located on the rear cylinder and install it. Then cap the nipple when you're not syncing the carbs. The rear cylinder is used to activate the petcock, so it has a line attached to it.

Boost Joint: Honda Part# 16214-MB0-000

 

I'll see if I can pick one up

 

what he said.... it's quick, simple and easy to get / install. and only a few $. JOINT, BOOST - Search : 16214-MB0-000
$2.66

Im trying to get away from barbed fittings and vac line for the carb sync,, but have had NO luck finding ANY M5x.80 fittings So making my own.

 

100% of what Marquez posted. Boost valve ron ayers.....

 

I know where you can get them from
But I won't tell you.
Had a boost joint already as a spare but also made a carb sync relocation under my seat.
All I have to do is uncap the ports there and plug in a couple vacuum gauges sync done in about 10'minutes.

 

I'll probably do the same, leave a couple of pigtail hoses plugged for future syncs.

 

Yeah, after removing your carbs, or if you happen to have the hands of a small child...otherwise it can be done without removing anything but not without cursing a little...

 

Yup same as I have now.... but I do not like the vac line barb fittings on two ends... so I have -3 AN hose and hose ends.. for the motor Im machining some adapters for -3 AN to M5 x .80. On the working end I have a hose end with o-ring sealed threaded cap.

Those two caps will be under the seat.