listerdiesel said:

No, loads of carburated engines have this fitted, Series III's even

Peter

Click to expand...

I've only come across the mechanical systems with mixer rings and evaporators on carb systems.

There are essentially four types of LPG system ( broadly speaking ) which are we talking about?

Old mechanical - with mixer ring(s) on the intakes, and larger, vacuum sensitive evaporators - old world, but generally very effective, and easy to tweak. These are often augmented with ignition advance modules, to advance the ignition a few degrees on LPG, and some installs ( like mine) having additional engine vacuum driven control valves to lean the mix off slightly under low load. On really large engines, sometimes a mechanical metering device is fitted into the intake - a bit like a carburettor for LPG - I've only seen these on really big ( 6-litre plus) installs, although there are a few for smaller engines creeping in from Poland that I haven't seen yet.

Single point mechanical - often fitted to earlier EFI cars and catted cars. These are basically the same as above, with an ECU-controlled valve that modifies the LPG flow to the engine to keep cats etc happy. these will allow better leaning of the mixture during longer runs - a bit of an improvement on the purely mechanical systems, and use the lambda sensor in the existing vehicle.

Single point electronic - basically the same as the multi point, but using one central LPG "Injector" - only saw one of these, fitted to a small petrol engine, before the manufacturers went to multipoint below.

Multi-point - Very different in principal to mechanical, which rely on a venturi-vacuum to "pull" LPG into the engine. With multipoint, the LPG is evaporated, and fed to a "fuel rail" at constant pressure (with relation to the intake pressure). These have more complex ecus, and better LPG control, as it has one LPG solenoid valve per cylinder. This allows higher outputs,and better engine characterisation.

There is a good guide here:-
(no - I am not plugging the company - just found a good description).

http://www.go-lpg.co.uk/which_conversion.html


I've installed both the simple, mechanical only ( to a 101, and my 4.6 hotwire range rover) and multipoint (to a Merc V280, Lexus RX400h, and several Saab 4-cylinder turbos) and tuned a few more.

The mechanical systems, if correctly fitted and set up, result in very little power loss on LPG, and often run more smoothly, and deliver better low-down torque (and less turbo lag on my saabs). But they do need a bit of a "tuners ear" when setting up, which is why many LPG fitters dont use them now.
If your carb engine is down on power on LPG, then there are a few things to check.

Firstly, what condition is your ignition in?
LPG is harder to ignite, so any weakness in the ignition system will be amplified when on LPG.
So, check your plugs, leads, coil, dizzy, rotor - are they all looking good?

Secondly, is the LPG kit working correctly?
Hoses split, and the solenoid valves can jam - check that these are operating properly.
If you have an electronically enhanced mechanical system, try by-passing the LPG control valve, turning it into a mechanical only system - see if the electronics are messing you about.

thirdly, when was the LPG kit last serviced?
The LPG kit has filters etc, in it, these do need occasional maintenance, and the mechanical bit of the system does need occasional tweaking.
Also - the diaphragms and springs do bed in a bit - and the system usually needs re-tuning after a few months running.

A note on setting up - some tuners set the LPG up to run "on the lean side" - but this can result on low power and increased fuel consumption.
best practice is to set it bang on - in the middle of the mix.

If you cant set it up yourself (and with any electronic system, there will be a tuning procedure), then take it to a specialist to set it up.

Note that mechanical LPG systems need a little "drag" on the intake - if you've got a snorkel that faces forward (or sits in high pressure air - which is usually good practice on carb setups and diesels), then the LPG mixture will weaken at high road speeds.

Yes, going multipoint sequential is a possibility, abd it would improve the breathing of any V8 versus carbs, but it's quite a palarva.

OK, a progress report.
I did a check on the condition of all my ignition components, which are all in very good condition, and checked the timing, which turned out to be 2 degrees at idle!
I took the advice given here to advance the timing until it pinked on petrol and then back it off a bit, and its like a different car. Much more responsive, starts really well, no pinking under load uphill, temp stays constant.
I had to alter the airflow through the carbs at idle to get them running nice and even at tick over again but that wasn't difficult.
I have taken note of where the dizzy needs to be set to if i need to run it on petrol, and i will carry a 15mm spanner with me just for this job, just in case!
These questions about set up on forums can cast up many different solutions, some of which i'm sure are better than the one i've tried, but i was after a simple solution for the here and now this time. I'll investigate multi-point as the year goes on.
Thanks for the replies!
John

and it starts from cold on Gas now too!