Simon: Sorry, but this sounds more like a base engine issue. Can you duplicate the backfire? If so, put a vac gauge on the intake to see what it's doing. You may catch a flutter at idle, which means a valve is hanging open. How long has it been doing it? How many hours on the unit? I suspect maybe a bad lifter or something of that nature. That's why the lighter the oil the better - even 5W20 or 0W20. Never more than 5W20 on these engines.

check the trim valves. it could have 2. they are a very common problem with the pro 5. we are a doosan dealer and have had many problems with them they are a part of the woodward fuel system for tierIII emission.

No not yet had dealers from Ability handling but they spent 2 hrs with laptop plugged in but didnt know how to work it, oil grade didint cure it.

Have you updated the SECM (software) as this was a fix on one of the releases

we had similair issues using 15w40 ,started using 10w30 as per doosan book seemed to cure it

Hi this truck doesnt have a distributor it is all electronic, the grade of oil i am using is just a good 15/40 do you think the grade is too thick.

Have you checked the distributor operation?
I think that it has a centrifugal regulator of the avance, inside the distributor.
If it is ok, check the ancle of the avance or loose the distributor and change the ancle a little while the engine is working, in order to hear the sound.

But if it has smooth sound and it back firing just some times, it may has spontaneus compustion at 40 degrees Befor Top Dead Center. It happens when the pistons has a film of carbon and it is very dangerous for the engine. If this is the reason buy some engine and injection cleaners. put a bottle of that stuff in the gas tank. Do not do that with propan!! only with petrol.
Any question feel free to contact me

what grade of oil are you using in the engine