Join Date: Aug 2013
Location: Sale, VIC, Australia
You're on the right track with your ideas of the problem.
I'd probably discount carburetors as the problem though, as generally they would cause the bike to run crappy at cold as well as hot.
My guess is it sounds more like valve clearances need adjusting. This is generally presented by the fact that it gets worse as the bike warms up.
When cold, all the metal parts of the bike have "shrunk" as their material cools, allowing the clearance between your valve cap and cam-shaft to widen. As the bike warms, and the valves and other parts warm up, they all expand slightly.
This causes the gap between your valve cap and cam to close up, often to no gap at all. The effect of this is that your cam begins holding your valve open slightly all the time, instead of creating a good seal against the head.
This allows fuel and air to escape at low revs, lowering compression and causing mis-firing. When you hold the revs up, you're giving the gasses less time to escape, hence why it still runs at higher revs.
If you're mechanically inclined, you can check your valve clearances by removing your fuel tank and rocker cover, and checking the clearances with feeler gauges. Each cylinder should be at top dead center when you check its valves, and the cam lobes pointing upwards or outwards from the head.
You're measuring the distance under the cam lobe, between it and the valve cap.
Normal clearance is usually accepted as Inlet - 0.003" to 0.005" (Thou) and exhaust as 0.005" to 0.008". Any smaller and you're going to have problems, any wider and they just get a bit noisy.
I'd suggest taking the bike to a mechanic to have shims swapped, unless you are familiar with removing cam-shafts yourself, and re-setting up timing.
Hope this helps.