Starting problem

[Zodiac]

The bike is a low mileage 2003 Sachs ZX125. It has a Yamaha DT125 engine. Starting is a pig. When it's cold or has been standing for 2 or 3 hours it won't start. The battery eventually runs flat and it needs a booster battery to help out. After a lot of turning over, about 2 minutes, usually with a lot of throttle feathering, it will start. When it's running it's great and ticks over fine.
The spark plug was changed and re-gapped and tested for sparking, carb is clean, air filter is good, fuel is good (no water). Any ideas?
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Zodiac

[27cows]

Fuel cap air vent blockage, possibly. Maybe damage/wear to the throttle slide or needle, or incorrect float height.
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The RXS100: vehicle of choice for Chuck Norris

[Zodiac]

Hi 27cows

Thanks for your thoughts. Carb is fine in all respects. Already tried it with the fuel cap breather tube removed. No difference. I'm wondering if the spark delivery might be weak, i.e., coil or cable to plug. Plug sparks OK when its placed on the exhaust but compared to a car the sparks seem weak. Bought a new plug on Wednesday and at first thought it was a dud but the local Honda service guy where I bought the plug said it was fine. So maybe I'm dreaming up new problems.
Still at a loss though.
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Zodiac

[27cows]

A failing coil normally lets the bike start OK, but then things feel weird when you're moving, particularly at low revs. An alternator failure usually is like a switch - works, then doesn't, at all. CDI box is a possibility. Might be a good idea to test everything with a multimeter. But most likely it'll be something simple, like HT lead corrosion or water in somewhere.
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The RXS100: vehicle of choice for Chuck Norris