Compession Question? 79 650 SR

Just had my 79 KZ650 SR's carbs rebuilt and sync'd at my local shop. They also fixed my battery leads which casued the bike to not start. Now the bike is starting 1st kick and running perfectly. I still have to wait a good five mins to pull off and depending on the situation pull off with some level of Choke engaged. After about 3 minutes of riding I can take the Choke off and its good to go. Acceleration is what I would expect out of a 40 plus year old bike. Just wanted to give some background. 

Here is my current issue. While driving at say 30mph and above, I get these inconsistent little pops (like puffs of air). Like small compression pops. Does not feel or sound like gas or and engine pop. Hope that make sense. I can feel for a split second, a loss of acceleration, but not enough to slow me down. I have not changed cylinder gaskets and am wondering if this is comperssion gasket issue? Got this 79SR after my 78B2 was destroyed by a fire. Ran out and could only find a SR as a replacement. 

Spit back through the/a carb ? Cracked enging to carb boot?
Missng or leaking vacuum cap - if those carbs even have them.
Does the bike have the clean air setup with connections from valve cover to air box ?

nice bike

Valve clearenees check?

Nice looking bikes.