ignition timing on LPG conversion

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hi, i have a 1974 holden ( aussie GM ) with a 253 V8 which i have just had converted to dual fuel with an IMPCO gas system

how can i optimise the ignition timing without a dyno. i know lpg can stand more advance and should ideally have a dual curve ignition, but i don't have one

any other tips on maintainence for this type of conversion? i have also fitted an upper cylinder lubricator

many thanks, johnny
 
More timing advance will give you more power and better fuel economy... to a point. That point is when the engine starts pinging. You can make the procedure as complicated as you want, but here's a simple method that I've used many times with good results:

If you have a reasonably quiet exhaust system, you can advance the timing a little at a time- drive it for a day or two after each advance. Once the timing is advanced far enough that the engine pings under certain conditions (probably with the engine under a load at low RPM), then you've gone too far. Back the timing off just a bit (until you hear very little or no pinging under any conditions), and you're good to go.

When you mention a "dual curve ignition", do you mean a setup with a vacuum advance and centrifugal advance? What sort of setup does your vehicle have? I'm not familiar with that Aussie V8. Could you score a vacuum advance distributor at a junkyard? I think it'd be worthwhile.
 
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Sorry for the late reply. I have lots of experience with propane powered vehicles, having worked on them at a shop and I also converted my Mazda pickup to run on propane. My last truck was a Ford pickup and it was a propane powered truck.

I expect you used a #300 carburetor. The dual-curve ignition would be ideal in your situation and if you plan on driving this vehicle a lot you should get one. In the absense of the dual-curve module, all you can do is set the ignition midway between the propane and gas settings. Propane needs more initial advance(Impco calls for 10 deg advance, I was running my trucks at 14 deg advance) but the total advance, under load, should not exceed 30 deg. You can get limit plates for the centrifugal advance to limit total advance. Spark plug gap should be set at 28 thou in.
 
johnny,
get a 308 block ASAP. You'll have the same economy, and enjoy it more. (the 253 has 3.625" bores, with a head designed for a 4" bore...it makes the gas flow terrible, and every 253 I've owned used the same fuel as a 308).

Try and dig up the works of John Bennet (used to post articles in one of the Classic Car mags, and had some really good info).

I had a WB ute with a 253 on gas. I drilled the EGR so that I had EGR on full time. Restricted one exhaust to keep the crossover heating the intake charge, and changed the vacuum advance so that it was full on at idle (constant vacuum, not ported). Had a copper coil wrapped around an exhaust manifold (wrapped in ceramic fibre blanket), and had the EGR run through that. Put the thermostat in the bottom hose (controlling coolant inlet temperature rather than outlet).

Worked really well...on gas.

Tried to rig a hot air induction (by modifying a Holden V-8 emission controlled air cleaner housing around the remote air cleaner/gas carb), and was partially successful on that...could lean the idle way back, and at WOT, the heating was nil.

Anyway, it was good fun, and I cut my LPG consumption way back.

Problem was that it wasn't that good on petrol, which I didn't want to run anyway.

When running on petrol, I'd add 104+ octane booster to the best fuel I could get. I was only running petrol for 1 tank (70 litres) in 4 tanks of gas (130 litre each), and I was mixing the petrol useage to a day a fortnight to keep the petrol through the system, and not going bad.

I added an upper cylinder lube dispenser (flashlube, but used Lucas in it) to the point that the heated EGR entered under the quadrajet to keep the valves intact.
 
A couple of references have been made to a 'upper cylinder lubricator'. It is not something that is used in North America for LPG vehicles, the Impco catalogue does not list it, I have never seen one and I do not even know what it is. My trucks ran well enough without an upper cylinder lubricator. Lpg is harder on the valves and, on the Mazda which did not have hydraulic lifters, the valve clearances would tighten up sooner on LPG than it did when the truck was running on petrol.
 
thank you everyone, especially shannow

funny how lpg conversions have taken off since the rebate scheme

and the shonky practices. my installer wants another $100 to install an enriching button and solenoid to overcome a cold hard starting problem. i would have thought that it was part of the job. oh well ...
 
bonnie john

Check your vaporiser, there should be a spring loaded "button/plunger" on the side of it. When they mount a cold start solenoid, it operates this.

To see if it's what you need, try depressing the plunger manually and see if it starts your engine.

Sometimes an older 253 won't have the vacuum to get a reliable start on gas.

When my gas engines were down on vacuum (valve wear), I'd start on petrol, then switch at the end of the street. Coming home from work, there was no problem starting.
 
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