It's the starter spinning. The Bendix (I think that's the name) is spinning but it isn't throwing out and connecting with the gear on the crankshaft. That's what the intermittent grinding/starting is about. Run a 12 volt wire directly to the starter to check. See'in you just had the starter put in, it may be a loose wire at the starter.

 

Is there any chance it could be the starter relay? Or is it likely that the mechanic that fitted the new starter motor may not have put a wire back correctly. I want to mention it to him but don’t want to offend him by saying it’s his fault. I messaged him earlier and he said it could be that the starter relay is stuck on and the starter motor is spinning all the time??

 

Read my reply. Says it could be a loose wire. It also explains what the starter is doing. IMO: if the starter is spinning there shouldn't be a wire issue. I've had a new starter that was bad.
Ask him. Accidents happen. Chit happens. Nobody's fault.

 

Ok I’ll let him know. Do you know what may be wrong with the foot safety brake? When I pull up the lever it’s loose and not clicking the safety catch down to press the little silver button in to start the bike. I know it starts by holding the button in but it can be a bit awkward for my one 6year old and would love to get it fixed. Cleaned the carb on the other bike and it started first time. Thanks

 

I'll see if I have any parts for that brake switch.
Can you post a close up pic so I can see what is/isn't there please.

 

 

Now't wrong with that, as far as I can see. The foot brake needs adjusting so it lets the parking brake lever move round but puts some pressure on the parking brake mechanism, so you press down with your foot and can just get the parking lever to move round, if the foot lever goes too far, the parking brake can spring off without needing more foot pressure on the pedal to make it do so.

 

I thought that. How do I do that? I thought the 2 adjusters on the back were for the left and right front brakes?

 

Sorry my mistake, I was thinking of my kingquad brakes. I’ve tried moving the adjuster on the back which loosens and tightens the foot brake pedal but it’s still not making the safety lever lock into place. The lever is just moving freely up and down and not locking that U shape catch into the groove on the pedal.

 

I agree w/merryman. Tighten the rear brake.