Planning a dyno session soon and wanted to know if you can
change jets on a holley without removing the carb?

 

There are ways you can do this. Some company sells a block that goes between your metering block and the main carb body. There are two screws on top that you twist to change the jet size. I haven't heard anything about these positive or neative.

Also, holley sells quick change fuel bowls. These replace your fuel bowls, have two big screws at the front. You unscrew these and stick a special 'screwdriver' in to remove the jets without completely taking off the bowls, though you still need to drain the fuel. I tried this kit and it wasn't any good. The concept is solid, but the screwdriver tool was poorly manufactured. Testing this tool off of the car on a spare jet, it wouldn't fit in the jet. It would just scraped up the inside. I wasn't even going to attempt to try and hook onto a jet in the fuel bowl blind.

The best thing you can to is buy or make yourself a catch can to make draining the fuel easy. Then make sure you have some type of flexible tubing (braided, rubber) going to the bowls. That way, you wouldn't have to disconnect the fuel line for every change. That'll make things really easy and really cheap.