Engine turns over but wont start. While trying to start it, it gave a backfire, I changed the spark plugs plus oil hoping hat would fix it but no such luck. Any suggestions as to what could cause or to fix issue.
To run, an engine needs 3 things - the correct fuel/air mixture, compression, and spark. If any of those are missing, it won't start.
When you try to start it, does it sound like it normally would (i.e. not turning significantly faster or slower than a normal start)?
Does it have enough of the right sort of fuel? (sounds like a silly question, but you'd be surprised!), and can it get through? Are there any filters that might be blocked?
The mention of a backfire suggests to me that it's most likely spark that's missing. You've changed the plugs, but have you checked the coil/coil pack and HT leads(wires)?
To check, take one of your old plugs, and connect it to one of the plug leads, then lay it on top of the engine (so that the metal side of the plug is sitting on a grounded metal bit). Get an assistant to try to start the engine, and watch the plug for a spark. DO NOT touch it while you do this - you could get a nasty shock.