Rebuilt 3.0 engine issue, Hi temps on one side Options

[scrz914]

I have a recently rebuilt flat six engine that I’m going to install in my 914. I built a run stand so that I could do the initial break in and then address any other issues. Start up went well with the expected popping through the Weber IDAs and some backfires through the exhaust. Once warm it ran better. One of the first checks I did was to make sure all plugs were firing by using a temperature gun directed at the exhaust coming off each cylinder. This is where I discovered a huge disparity between the two banks. One side, 1,2,3 cylinders had readings over 500 degrees whereas 4,5,6 was a little over 100 degrees.
My first check was fuel pressure. 3 psi on both sides which is good.
Then I sprayed brake cleaner around the manifolds. Good.
When I sprayed points on the throttle shaft on 1,2,3 there was a noticeable change in idle. It became smoother and increased a little.
My question: could a vacuum leak at the throttle shaft account for the high exhaust temperature? If so, what are my options?
I don’t have much carburetor experience so any expert advice would be appreciated.
Thanks all.