How do you fix a puncture on a Mountain Bike?

Step 1. Find the puncture
Using the valve as your starting point, closely inspect the tread of the tyre to ﬁnd the cause of the puncture.
Pay close attention to the sidewalls (the non-treaded portion on the side of the tyre where the pressure and size information is printed/moulded) to make sure there are no tears or holes.
Remove any glass, grit or other debris that you spot. A small flat-head screwdriver or awl is the best tool for this, but be careful not to dig too deep. A small set of pliers can also be useful for removing embedded thorns.
Even if you ﬁnd one possible cause of the puncture, continue checking the tyre thoroughly until you get back to the valve because there could well be more.

Step 2. Remove the inner tube
Let the air out of the inner tube and push the valve up into the tyre, unscrewing and retaining the valve lockring if ﬁtted.
On the side of the wheel opposite the valve, slip a tyre lever under the tyre’s bead and a further tyre lever about roughly 5cm away.
Pull the nearer tyre lever towards you, lifting the tyre’s bead over the edge of the rim. Continue until one bead of the tyre is completely free of the rim then pull the tube out.
Remove the tyre completely from the rim – with most tyres this can be done by hand unless they are exceptionally tight.

Step 3. Locate the puncture on the inner tube
If you are just going to replace the inner tube, move to step 6.
If you want to patch the inner tube – and you ought to, it’s not that hard! – inﬂate the tube and listen for air escaping.
If you’re struggling to find the hole by listening alone, try passing your lips over the top of the tube.
If the hole still can’t be found, re-inﬂate the tube and pass it through a bowl of water, or rub the tube down with soapy water, until you spot escaping bubbles. Be sure to dry the tube before proceeding to the next step.

Step 4. Prepare the inner tube for patching
Select an appropriately sized patch – if in doubt, err on the side of caution and use a bigger rather than smaller patch.
Roughen the surface of the tube around the hole with sandpaper (usually included with any good puncture repair kit).
Ensure that any moulding marks on the tube are completely flattened down because these can cause issues when glueing. Thoroughly brush off any rubber ‘shavings’. Cleanliness is key when repairing a puncture.
If you’re using pre-glued patches – such as Park’s popular GP-2 patch kit – you can now patch the hole. Thoroughly press down on the patch to ensure it’s fully in contact with the tube.
If you’re using a ‘traditional’ glue-on patch kit, start by applying a generous drop of glue – or rubber cement by its proper name – to the tube and spread this across an area slightly larger than the patch you intend to use. Allow this to dry.
Similarly, apply a second, thinner layer. Once again, allow to dry – when the glue is dry, its surface will change from a shiny to a matte finish.
The key to ensuring a good long-lasting puncture repair is patience, so don’t rush this step.

Step 5. Patching an inner tube
Firmly press the patch into place after removing the backing foil – cleanliness is also key to a good repair, so leave removing the foil until the very last moment.
If there’s a thin cellophane backing on the patch, it can be left on. It’s good practice to dust any stray glue with chalk, talcum powder or fine road dust to prevent it from sticking to the inside of the tyre casing.

Step 6. Inspect the tyre casing and rim tape
Before refitting the tube, thoroughly double/triple-check the inside of the tyre casing – there’s nothing more frustrating than going to the effort of patching a tube only to puncture it again with a stray thorn you may have missed.
It’s also good practice to check the rim tape. If a hard plastic rim strip – often found on cheaper bikes – is torn, it leaves a sharp edge that can easily slice a tube.
Likewise, if your rim tape has slipped, it can leave eyelets or spoke holes exposed, which can also puncture a tube.
If you have persistent problems with your rim tape puncturing your tube, try swapping it out for new tape tape 


Step 7. Refit the tyre
After repairing the tube and thoroughly checking the tyre, reﬁt one side of the tyre – one bead, to use the correct term – to the rim.
Slightly inﬂate the tube (just enough to give it structure) and reﬁt it to the wheel, putting the valve through its hole first.
Starting at the opposite side of the rim to the valve, use your thumbs to lift the tyre’s bead over the rim. Work your way around the rim until there’s just one small section of the tyre left.
Push the valve up into the tyre and then, using your thumbs, ease the remaining section of the tyre’s bead over the edge of the rim.
If the tyre is particularly tight, avoid the temptation to use a tyre lever to push the last section of the tyre onto the rim – you’ll almost certainly pinch your inner tube doing so.
If you’re struggling to pop the tyre onto the wheel, try putting the tyre on the ground, holding it in place with your feet and rolling the bead back toward you – heavy gloves really help here. This takes a little practice but should work with even the most stubborn tyres.


Step 8. Check the repair and inflate the tyre
Check that the tube isn’t trapped between the rim and the tyre bead by working your way around the tyre, pushing the bead into the well of the rim.
If the tube is trapped, try ‘massaging’ the tyre to encourage it to seat properly. Pumping it up a small amount may also help to seat the tube properly.
Inﬂate the tyre to a point where it feels soft but has maintained its shape. Then check that the moulding mark around the tyre follows the rim evenly all the way around.
If not, deﬂate a little and ease any high spots down and pull low spots up until the bead is ﬁtted evenly.
Inﬂate fully and check once again that the tyre’s bead is still seated evenly and that the tyre isn’t lifting off the rim at any point, then adjust your pressures to suit.