New syphon fitted now doesn't flush

[sueonmull]

We have just replaced the syphon in our closed coupled Doulton WC so we can have an internal overflow.
The original syphon was marked Caradon bathrooms Ltd and came with the WC which we bought from B&Q 10 years ago. The replacement is an Opella Delchem Multiflush Syphon. Both are 9.5".
The problem is that the flush is no longer very positive and unless you get it right first time it won't flush. Very irritating.
Anyone any advice - have we fitted something incorrectly?
As a secondary problem, the internal overflow only works when the water level is very high up in the cistern, just above the level of the flush handle. Could we replace the siphon with shorter eg 8" siphon so that the overflow level was lower?
Thanks

 

[HERTS P&D]

Change it for an 8", it will work fine.

Andy

 

[sueonmull]

Thanks Andy for such a quick reply. Will the 8" sort out the flushing problem too?

 

[HERTS P&D]

Yes.

 

[Nige F]

Change it for an 8", it will work fine.

Andy

so size does matter

 

[HERTS P&D]

Nige , you are awfull, but I like it.

Andy

 

[sueonmull]

Thanks for all your replies - I'm much happier now.
So yes, size does matter, my husband was relieved to find smaller was actually better.....makes a change!