DT 4.5 Miss firing in forwards but not reverse

Hi, I'm new to the boating scene and have just bought a little 4.5hp suzuki engine but I'm having a problem with it. As the title says its missing in forwards only and not reverse. I've had the gearbox off and everything seems to be moving freely in both forward and reverse. I've taken the propellor off and ran it in the water and it doesn't miss. 'm guessing from this that there is a problem when the engine is under load, although I'm unsure as to why it wouldn't do it in reverse if this was the case because I thought both forward and reverse would be the same ratio therefore putting the same amount of load on the engine.

Any help on this would be much appreciated.

Thanks in advance, Steven