1992 XJ550j, Not Idling and Reving High When Choaked

Hello everyone, I just purchased a non-running XJ550j. The seller assured me that all it needed was a battery, but what can you do. I can somewhat get it to start but soon after it'll die if I don't give it continuous throttle. Another issue it’s having is if I have the choak on it’ll rev up to around 6000 RPMs.

So far, I’ve checked all the spark plugs and they are all getting spark and aren’t fouled. After I moved on to the carburetor and everything in it looked brand new. I checked all floats and needles, and everything seemed to be in good condition. I checked all the gas levels in all the floats with a clear hose and they all had gas. The last thing I did was a rough bench sync of all the carburetors which helped a little. It went from not running at all to now it can consistently start. I know it needs to be synced with a vacuum gauge, but I need it to idle first. And yes, I did adjust the idle screw.

The only thing I can really think of is maybe there is a vacuum leak somewhere I haven’t noticed, there is something deeper wrong with the carb that I need to investigate, or there is something wrong with the timing. I have a feeling the timing might be off, but I would like to exhaust the other avenues before going down that road.

Questions

What are some things to investigate?

Is there any place other than the two vacuum lines and rubber carb boots where a vacuum leak happens?

What are some other signs that could narrow it down to a carb issue?

What gauge setup is everyone running for carb synchronization?

I’ve read a few other post and I can’t necessarily find anything that fits my situation directly. I look forward to hearing everyone’s opinion on this issue. I’m currently waiting on my service manual to show up, so if you tell me anything specific to look into, please include the reference. A good video would be a help too.

Thank you!