Cranks a lot before starting

Hi, I have VS commodore on LPG. Most of the time it cranks a lot before starting. Trying to work out what to do to sort this problem. Only running it on LPG as it does not drive well on petrol, it stalls and or has flat spot with no power when driving on petrol/will not start on petrol. Drives well on LPG just cranks a lot before starting on LPG.
It has done a lot of kms and have not changed spark plugs for several years, have not changed the leads and friend said the coil pack may need replacing.
In the past have changed fuel pump with a used fuel pump from donor car, also have bought injector cleaner and think it is time to try using this?
On some occasion's it will start first crank and on other occasions it will start with less cranks, if I push on accelerator first or while starting it can also start easier/quicker.
Thank you in advance for any information/help.