250cc or 305cc

Picky · Post by **Picky** » Thu Jul 04, 2013 4:58 pm

Hi,
I'm confussed.
I have a bike with engine that reads CB72E-212372
The logbook (UK) with the bike says 305cc?

I know the bike has been restored in the past, do I assume that the Log Book info is correct & the engine has been bored out or a bigger 'top end' from another machine has been fitted?

Is there any way (without taking the top off the engine) I can find out.

jensey · Post by **jensey** » Fri Jul 05, 2013 12:27 am

Hi,

Welcome to the forum. Maybe they replaced the cylinder, you can see that because on the front (base) of the cylinder there is printed 250 or 305. If it says 250, it's only guessing I believe (or pull the head).

Jensen

G-Man · Post by **G-Man** » Fri Jul 05, 2013 1:56 am

Picky

If you take the head off. 250 bores will be in the range 54 - 55 mm, 305 bores will be 60 plus.

Alternatively you may be able to get an endoscope-type camera in the plug hole. 305 pistons have a squish area around the edge.

G

Picky · Post by **Picky** » Fri Jul 05, 2013 3:20 am

Thanks for your replies.

Jensen - it does read 305cc at base of cylinders so they must have 'married' a 305cc top end to a 250cc bottom end. I assume it's OK to do this!

G-Man · Post by **G-Man** » Fri Jul 05, 2013 8:15 am

Not a problem. The balance may be a little out as they use different cranks.

G

Phil-UK · Post by **Phil-UK** » Sat Jul 06, 2013 8:32 am

Hi

What size carbs are on the bike ? If 305cc you should really have the 26mm carbs.
If no carbs look for replacements and if no joy ( not as common as 250cc 22mm in the UK ) you can go British.

Phil