Engine diagnostics

On my 1978 kz750, I kept having trouble getting idle dialed in after a carb rebuild. Finally noticed there was a single vacuum hose connected to both intake ports on the engine, which got us thinking to check the engine. So we removed the hose and replaced with individual caps.

Ran the bike a bit, then did a compression test - right cylinder at 150psi, left only 70psi. Doesn’t seem like a bad gasket would cause that much deviation Is this most likely a complete engine rebuild/replace?

Side note - would the vacuum hose connecting the intake ports be a way to “cheat” the low pressure issue, or is that a known workaround? Or totally unrelated? Thanks!

I don't know that bike at all but I suspect the "vacuum hose" should just be a vent to atmosphere.  Carburetors need a pressure differential between the bowl and the venturi to operate correctly.  As for your compression difference, if you did the test correctly and got only 70 lbs on one side I think you need to do a bit more testing to discover why.  It might be a valve seal or a burnt valve.  Or it might be a ring issue.  One way to tell is to drop a spoonful of oil into the cylinder and then redo the compression test.  If it remains low it's likely a valve issue.  If it rises significantly higher it's likely a piston/ring issue.

There is lot's of things that can effect the compression ( not all are terminal) the wet test already suggested by HRM is a good start. Depending on the tooling and equipment you have you could build a cylinder leakage tester (adapt an old spark plug shell to take an air line fitting) put the bike in gear with the cylinder at TDC on the firing stroke, get someone to sit on it and hold the brake on then connect an air-line and listen, hissing in the air filter is an inlet valve, hissing in the exhaust is an exhaust valve and hissing in the crankcase through the oil filler is piston/rings.
It could be a simple as incorrectly adjusted valve clearances 

thanks for the suggestions! someone else mentioned valve clearances also. i’ll get my buddy to help me test a couple of things out. i’m currently on the hunt for another engine so i can do a swap out, that way i can keep the bike on the road while i take on the engine project and work at my own newbie pace. i’ll update how it’s going once i start looking into this engine.

i should add that i’m also planning to make the bike off-rod capble while i’m at it, starting with tires. anyone else done this with their kz twin and have suggestions for suspension, front end or other mods?

My 19,000 mile 750 that I just bought had compression readings all over the place, none of them great. It turned out to be the valves had never been adjusted since new, or so I believe because there were no witness marks on anything. They were either way out of spec or my thinnest feeler gauge wouldn't slip under. Adjusting the valves put everything right.
A valve clearance check is mandatory if it hasn't been done to schedule.