Carb Screws

[nateweisiger02]

Alright guys, I’m totally new to the carbs on these. I have just left them stock for as long as I have had the bike, just did a deep cleaning and new gaskets. I have done a ton of reading, however I have not found much on the screw with a question mark. Pretty sure the a/f mixture screw is on the bottom, then the idle adjustment is labeled in the pic. But what is this other screw?? I know that on a lot of carbs, air screw is typically on intake side, then fuel screw is on the engine side, but it doesn’t look like these carbs are made like that. I would also like to note, that the “?” screw is turned all the way in… first time touching it. 1978 Cb750k will run but cuts off if I don’t have the throttle pulled a little, has stock air box, and chopped exhaust. I know I’ll have to upgrade jets but just want to know exactly what screws I’m working with here. Let me know if you can give any advice!

 

[dirtdigger]

Its a plug you take out to attach gauges to sync the carbs.

 

[Floshenbarnical]

dirtdigger is correct. If you're going to use a vacuum synchronizer but don't have the adapter, what I did was to buy some 5mm/ .7 thread pitch bolts and drill through end-to-end with a 1/16 drill bit, then add an o-ring (the fuel vent line is perfect for this) and a nylock nut to hold it in place. Then attached the vacuum hoses to those.

And yes, that is the fuel mixture screw.

PS, I just rebuilt a set of PD41a carbs. I think you have the PD42s, which I think are pretty similar. If you're going to break them down there are a wealth of great posts on here and also on http://forums.sohc4.net/ I can link to you but I'm happy to answer any questions you have if you need. Maybe you can learn from my mistakes, lol.