Help:3.9 (dual fuel) problems

[greendisco2]

Hi , i have a 3.9 duel fuel in a 84 rangie that has started back firing on gas so i started driving on petrol and watched in horror as the needle just plumeted to Empty from seaford to camberwell and back via eastlink.It runs a little rough on petrol when at idle but once going is fine.Any ideas on what i mite need to look at . Thanks Tony

[Lotz-A-Landies]

You should keep the fuels separate and stop them "duelling"!

Is it a 3.9 EFI engine?

Have you checked the little hose on the rear of the plenum (between the firewall and plenum) to see that it has not split? Very common problem.

Regarding the installation, does the PCV hose enter the plenum adjacent the butterfly? If yes it should be re-located to somewhere before and away from the LPG mixer ring.

Diana

[PLR]

G`day Tony

If it`s backfiring on lpg only then i`d suggest probably an ignition problem . Plugs , leads , rotor/cap arm etc .

At night with it dark , have the engine running and look in the engine bay if there are any stray sparks they`ll be obvious , try both fuels .

Presumably it still has its original carbs on it ?

If you have a look at the tailpipe when it`s on petrol and you see black smoke at idle and if at rest you give it a rev and see even more .

The problem will probably be with the needles in the carbs . If you pull the needle out you`ll find they are worn on one side and because of this they can`t seal in their orifices .( they`re on the ends of the pistons below the diaphrams inside the carbs of coarse )

If you find this is the fault it is non adjustable , the only fix is replacement .

Some will say the cause is because the needles run dry , i prefer the rubbish in the lpg is the cause and don`t subscribe to running dry theory .

I think the best way to improve the economy is getting the lpg right and only run petrol when a must .

Cheers

[coops71]

Problems show up on LPG before petrol. I have experienced this with three or four cars with dual fuel. My first experience I had with this problem was with a Falcon with one bad spark lead. It backfired under acceleration on LPG when spark has a hard time getting through denser mixture. The problem eventually went along the petrol mode too. The solution was to get a new lead. It could also be your coil is weak/dieing. I am willing to bet it is a weak spark anyway. Other than this it could be due to a overlean LPG mixture or your LPG converter could be icing up due to coolant air pockets or coolant leaks. Check to see if your converter gets hot after a few km. Possibly a vacuum leak too.

[greendisco2]

thanks people , i didnt mention it was the efi motor, i changed the plugs yesterday and took it for a short run and noticed some blackish matter on the ground behind the exhaust pipe, but it ran ok i will take it to the guru on monday and see what magic he can do.thanks again tony

[It'sNotWorthComplaining!]

It sounds like your MAF may have suffered from the lpg back fires. This will make it drink the petrol and throw black soot out the pipe (over rich). Your plugs are probably going black as well on petrol???