title says it all.
if i have a decent team thats not feeding but not getting fed can i carry?
if so what items should i get?

Mundo carry as he pleases

This is my mundo build when im dominating, kinda risky, but its worth it:

MrBoots>Mogs>Atmas>IE

As is said before, this is for a risky, face smashing Mundo, max cleaver, then masochism, with a pint in agony in lvl 4 or 5. Also note this works better in TTL

If you want a safer build (like for SR) you can get Visage*FoN for better healing and ignoring mages, but this would set u out from the carry-ish build.

If you can farm like a god and are getting fed my ultimate build would be: Mercury,Visage,Mogs,FoN, Atmas,IE ... and if its one of those games that really go long you may get a Starks

As for runes Id go with CritChance reds, FlatHealth Yellows, Cdr or Mr/lvl blues

A typical melee carry has a pretty good base attack speed, autoattack boosters of various kinds, a way to SLOW the enemy, and most importantly... a gap closer. They also tend to have at least some reasonable burst damage.

Look at Xin. Charge/Slow, a knockup, and a charge,ult, poke combo does quite a bit of damage. Passive AS boost too.

Nocturne: ultimate gap closer with high damage, antimagic shield, fear as a damage/chase mechanism, dagger as a secondary chase mechanism and more damage.

Most carries have these characteristics.

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How bout Mundo? Well... he doesn't. He has a huge AD booster...one of the best in the game. However, he has NO GAP CLOSER which is a huge problem. he also has no reliable slow. At melee, his cleavers are much easier to miss with ironically. His base attack speed blows, so all AS boosts which are (percentage of his base AS)-based are weaker than normal.
He has NO burst damage at all other than his cleaver.

So basically no. Mundo makes for a lousy carry. I mean...he CAN carry, but I've seen SHEN carry too so there you go. The best way to build him is to make him a Tanky DPS/damage soak. Enough tankiness so that he can survive being focus fired by the enemy and thus perform as a tank. Enough DPS that he cannot be ignored and when he comes rushing towards their Ashe...they have to do something...which is ideally to focus fire him where his massive health and regen can be of the most use.

Theoretically, Mundo can carry.

In practise, you'll find it hard to do so as you're wholly reliant on a skillshot to slow people down so you can kill them. All the good tanky dpsers have a gap closer. Olaf has the same problem. IF you can get in melee range, you can do good damage. But chances are you'll just get kited around.

ive seen mundi tabj carry though....he tanked the whole team and killed them all with cleavers, burning agony and sunfire cape. it was a 4v1 (he killed the first one)

i dont know because the enemy consisted of ashe, jax, soraka abd taric neither of which were very fed.
he just focused the squishy ones while tanking damage easily then ran away while kiting them with cleavers. when they followed he would juike into a bush and when they face checked he would burst them with a cleaver followed by masochism and burning agony. u thubj he had randuins omen.
but the point is is i farm my items real quick and start killing can i carry? (with FATMOGS yeah!)

Mundo can't really carry SR (except against really bad enemies). He can definitely carry TT though.