Bike won't start

My son has a Honda CBR250. It's been out of rego for a while and he's only started it every week or so just to keep the battery charged etc. Now when he tries to start it, the bike tries to kick over for a while and then eventually backfires. A friend suggested it may be the CDI unit. Does this sound right? How can my son test the CDI unit? If not CDI what else could it be?

duoble ditto on chucks comment, also if it happened all of a sudden, (experianced here) check and make sure you didn't suck a mouse into your carb, or have a wasp nest built there., other than that gas.

If he's not riding it much then starting it every week or so is not really good for the bike.
To keep the battery charged use a battery tender. Starting the bike and letting it run a few minutes will actually run the battery down. It takes a more than just a few minutes to recharge the battery after starting. 15 minutes at hi-way speeds at least.
The short run time won't be long enough to burn out the moisture in the oil. As bike runs ring blowby collects in the oil. Condensation collects as the engine cools, this collects in the oil. It takes a while once the engine gets warmed up to burn out these contaminates in the oil. If not they turn to an acid and can eat away the internals.
For storage, warm it up, change oil, Drain the carb, top up tank , add a fuel stabilizer.
Can't think of the name right now, They sell an oil that you spray into the carb while cranking the engine to coat the cylinder walls and valves with oil. This protects the engine from rust.
A properly prepped bike will not be hurt by storage, even up to two or more years.
When you want to start riding it again it should take just turn the gas on and statring it up.
The fuel stabilizer will keep the gas fresh up to two years when done right. Topping up the tank prevents rust in the tank. Draining the carb keeps the carb from gumming up.
The battery tender keeps the battery charged.

I agree with all before and have this wicked bit to add - the "friend" with the suggestion about the CDI unit is 99.99% the last person I'd ever ask again any question to about motorcycles (and, really, I wouldn't ask for bicycle advice from him/her).

A bad CDI is likely to give a zero-spark indication, which you can test without much trouble - but before even going there, replace the gas. And get a Battery Tender (or similar charger/maintainer unit) to trickle charge that battery, instead of just starting it up occassionally.
:cheers:

Tnx guys for all the advice. My son has replaced the fuel and cleaned the carby but problem still exists. No mice or wasps.

I'm passing on your other comments. Really appreciate your input.

You say he cleaned the carb, but did he check the jets?
How did he clean them?
And did he clean the needle and seat or replace them all together.
And check the adjustment for the float?

The new Ethanol gas is stoppin them up like you poured Cement into them.
Have you looked into the carbs when you twist the throttle and make sure its spraying gas.
And have you checked the spark to the plugs?
And when you check the plugs after trying to crank the bike do they have fuel on them ?
Have you checked the fuel lines also and the filter for the tank,
Look in the tank for water.
And is fuel coming threw the petcock.

Is the bike getting spark? Does he know how to check if he is?