HELP- Volvo - Can't get more than 4000rpm

HELP please - I've got 2 x Volvo AQ145A on Penta 280 out drives, which run on Petrol and LPG. However, since I have added electronic ignition and electric petrol pumps I can't get above 4000rpm on Petrol or LPG.
I've tried advancing and retaring the engine timing but can't get above 4000rpm on either fuel.
I was getting 20 knots but now can only get around 12 knots. The only other possible change is weight from full LPG tanks which are at the rear of the boat. This weight is around 320 litres of LPG (600lb approx).
When I was getting 20 knots I was only carrying 20% of LPG.
The engines rev freely upto the 6000rpm when not under load / under way at sea.
I am going to try using up all the LPG to at least remove this from the equation.
ANY OTHER SUGGESTIONS PLEASE ???

Regards

 

It seems that the boat is overloaded. Also, it may be trimmed to far down by the stern.

 

What kind of boat is it? The new extra weight is likely the problem. If you intend to keep the LPG you'll probably have to reprop to bring your WOT RPM's up to spec. Suggest relocating the LPG as far forward as possible and/or shift all the other weight you can fwd.