goes lumpy and lacks power

[webbie1993]

was out on my honda varadero 125 today, was running perfectly fine, then went to accelerate out of 30mph zone, got to 50mph, then it went lumpy, and started to lack power. almost feeling like something was blocked? the bike is new to me, but i dont think the last owner had it serviced often. could this be a simple fix? or is this going to cost me losts of money? the chain is very slack, and am getting it reaplaced, could this be responsible?

thanks in advance

[shippy]

what do you mean by 'lumpy'

[Ariel Badger]

Main jet comes in at 3/4 revs, take it out and clean it.
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Bikers make great organ donors, get 115 on your licence today.

[webbie1993]

By lumpy. I mean it feels almost like the power goes up and down. The sort of feeling you get of you tap the front brake every now and again. Sorry if this doesn't really make sense. Only way I can describe it.