1. Try adjusting the load-block. For propane 5-7 turns is a good starting point. It may take more or less turns than that. If the adjustment is off by even one turn, it can cause this issue. Most likely you will need to turn the adjustment counterclockwise if you are having to hold the primer button in.
2. Make sure there are no vacuum leaks.
3. Make sure that you are using propane tanks that have definitely been filled at least 4 times. New tanks will have air in them that will cause this issue.
4. If you have a snorkel kit make sure the probe of the snorkel is not kinked inside of the carburetor. Also the end of the snorkel probe should end exactly in the center of the carburetor where the brass gasoline jet comes up through the center of the carburetor. If it doesn’t please let us know.
If you have the adapter kit make sure the arrows on the air flow sticker are pointing to the carburetor.
5. If engine compression is low it can also cause this issue. When engine compression drops so will engine vacuum. Even if the engine runs fine on gasoline it may not run properly on propane since the KN regulator requires a certain amount of vacuum to open. If the engine is mechanically sound the most common solution for low compression is to adjust the valve lash. Most engine manufacturers recommend doing this every 12 months or 300 hours whichever comes first. However, we have seen even brand new engines need the valve lash adjusted.