A single point system need not run lean IF its set up properly and was sized to suit the engine. You will never do it by ear, you should get someone with an exhaust analyser to have a go. Systems like this should ideally be checked annually, and will almost certainly drift out of tune anyway in a couple of years as springs and rubbers age.
Most single points have three adjustment places. One (usually the bigger screw) on the vaporiser controls the idle mixture - its a simple needle valve allowing a constant trickle of fuel, because the vaporiser isn't very well controlled at low demand. This is the LAST adjustment to make in seqeunce! Sometimes marked 'min' or 'minimo' on italian made units; screw in to lean off, screw out to enrich.
The other adjustment on the vaporiser controls the fuel pressure under normal-to-large demands. Rarely needs adjusting. Not even present on all makes.
If this is a manual system, without an electronic stepper motor to control fuelling, there is usually a big adjusting screw in the vapour path to the mixer at the throttle - somewhere at either end of the vapour hose, or in the middle, or at a Y-junction. Set this for best mixture at around 3000rpm with the exhaust analyser, then allow engine to idle and adjust the idle screw for best mixture. Then re-check - it's always a compromise of some sort.
If it's an electronic system with a stepper motor, a few can be recalibrated without extra equipment, but you will need the makers manual. Most will need some software from the makers used with a laptop.
Vaporisers have a limited life - maybe 60,000 miles or three years before the rubbers in some get trashed. If that's happened, it just won't set up properly. Buy a new one, don't bother attempting to rebuild, £100 maybe.
Hope yours just needs a tickle, but do try to have it done on an analyser.
Another good reason for a catastrophic loss of power when pushed hard is a restriction in flow of liquid from tank - checked for squashed pipework. Oh, and has the LPG filter ever been changed? Can also be caused by a faulty solenoid not fully opening. This is a fairly dramatic effect - all seems well until fuel demand passes a critical threshold then pweh, it dies. This assumes the tank valves, pipe sizes, etc. were originally specced big enough for this engine. If you don't have this effect like a huge flat spot, this ain't the problem.
Before you do ANY of that give the ignition system a birthday - LPG will show up weaknesses in the sparks that petrol is more forgiving of :)
cheers
Ross Kennedy
GLASS
