Drive lumpy at slow speed

[leopard_pagan]

I work on a base where the fastest you can go is 15mph! Been no problem in the past but last 2 months i have found hat the bike is becoming lumpy at that speed.
I can't seem to get a happy balance on the throttle to cruse at lower than 20mph even off base when filtering.
It feels like I am gunning the throttle then shutting it off in quick succession. While its doing this the engine sounds like its running smooth.

I have adjusted the chain
Adjusted the throttle to remove any play in the cable
Fitted new Cush drives

These did improve it slightly but it still does it.

I will be putting carb cleaner (in the fuel tank stuff) thru this weekend to see if that helps but I doubt it.
Am looking to replace the Cush drives as well... When I find a set cheap enough.

Any further ideas?

[The Novice]

As a temporary measure I packed the play in the cush drive rubbers with a cut up bike innertube. I say temporary, mine is still like it a year on as i cant find a decent set.

I got slated for doing it but as a bike mechanic at a main dealers told me to do it I thought i would try it. In fact he also mentioned cutting plastic from a milk bottle to do the same job, but i went with innertube as i had one lying around.

It got rid of the play in the wheel anyways. Try it, its free.

[leopard_pagan]

Well if it fixes the problem I know to get them replaced.

[leopard_pagan]

Cush drives did not fix the issue.

Anyone got any ideas?

[dunkenb]

Maybe crap in the fuel filter? What do your spark plugs look like colour wise?