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LPG Tuning Variations

The basic tuning procedure is the same for LPG as it is for petrol. There are a few areas that need to be modified or at the very least compromised if you plan on running both fuels regularly. Most people convert to save money and therefore rarely throw the switch back to petrol unless they have to, ie running out of LPG or no LPG service station in sight. If this happens and the ignition advance is modified solely for LPG, then caution will need to be excercised at low RPM to avoid detonation. It sounds like a good idea to modify everything just for gas but you must at least in a carburetted vehicle, switch over to petrol once a week, otherwise the carby gaskets dry out and eventually shrink and when you finally do switch to petrol for whatever reason, fuel will leak out of the carby causing a potential fire risk. My recommendation is try it and see. If the detonation can't be avoided then get the advance trimmed back so the detonation doesn't occur on petrol. Adjusting the actual advance curve of the distributor is done by adjusting the tension on the primary (weak) & secondary (strong) springs. However the total advance of the distributor needs to be retarded. Since this is acheived by mechanical stops internally, the distributor cam needs to be removed from the distributor and modified. This generally requires some silver soldering / brazing. If this step is not done and an attempt is made to increase the tension on the secondary spring to retard the advance then an adequate curve will not be achieved in the low to mid RPM ranges.

IGNITION TIMING.

LPG burns slower at low RPM or small throttle openings. It burns faster at high RPM and WOT. This means that you need more centrifugal advance at low speed but less at high speed compared to the advance curve of a petrol set-up. The problem with this is, when you run on petrol the advance is to much at low speed and can cause detonation. So a compromise must be reached if you plan on regularly changing from one fuel to the other.

 

HT LEADS.

LPG is a dry gas and mixes easily with the incoming air into the combustion chamber. LPG does not conduct as well as petrol so the firing voltage required to fire the spark plug is considerably higher. Therefore HT leads, ignition coil, rotor and distributor cap need to be in excellent condition.

SPARK PLUGS.

Temperatures in the combustion chamber using LPG is higher therefore a reduction in heat range of the spark plug may be required. One heat range colder will usually do the trick. Also reduce the spark plug gap by 0.1mm.