Welcome. My name is Tommy and would be glad to assist.
Do you have a 2 lead voltage tester by chance? The 18 gauge is too small for the application and the voltage may be dropping when the fan is on causing a low speed. I would not run it that way as it may overheat the motor
Is there any way to run surface molding to wire the fan? Post back and give me a little more information and there may be a way to wire it with the proper wire and connections, thanks.
no I don't hav voltage tester, I can run surface molding to the fan from a power source (outlet) so, I should be running larger gauge wire?
Yes, you need to find the breaker for that receptacle, if the breaker is 15 amp, then run 14 gauge, if it is 20 amp run 12 gauge wire
You can add a thin extension box on the receptacle, the run the wire mold from it to the fan. That will give you the proper wire size and installation.
18 is minimum of 2 sizes too small, the fan operates at a higher current than a table lamp, it needs the larger wire for proper operation. Make sure all connections are tight after running the new wire. Use the screws on the side of the receptacle for adding the connection and do not depend on any push in style connections.
That should get you going well.
Just reply if any clarification or further information is needed, here to assist