It's difficult to say, but assuming you've checked all the obvious stuff (fresh fuel, clean air filter, decent correctly gapped plug etc.), then I guess it could be TPS related?

Is the problem rpm specific, or throttle position specific? If it is only at a particular engine speed then it could be a number of things, not least the ignition circuit - coil, plug, HT lead etc. (especially if it�s been damp recently). If it is related to the throttle position itself, then the TPS perhaps should be investigated further. You can get some idea of whether it is the TPS by seeing if it hesitates at higher rpm on a light/very light throttle (the same position as when it occurs at low speeds where you first noticed it).

You really need to get access to a diagnostic tool to check and set the TPS angle (%), but you can check the TPS voltage easily enough with a multimeter. The TPS voltage with the ignition on should be 0.63-0.73V (between the yellow and blue/black wires) with the throttle closed and in the idle position. If it isn�t in spec then you can adjust (rotate) the position of the TPS unit until it is.

You can also check whether the resistance changes across the TPS range from closed to fully open are smooth with no dips, spikes or dead areas. For this you will need to disconnect the TPS connector, set your multimeter to read resistance (K Ohm) and connect to the two outer terminals in the connector. The only actual figure given is the maximum resistance which should be 2-3K, but you should be able to move the throttle across its range and see that the resistance changes evenly and there are no dead areas.

However, I would stress that you should first confirm that the idle is around the 1500rpm mark and the fuel, spark plug and air filter are good. Rule out all the simple, cheap and easy to fix stuff first.

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