Thread: Tuning the carburettor 3.5 V8 for gas and fuel

unless you want to fork out stupid money on lpg injection....

old school engines running dual fuel either get setup to run well on petrol or well on lpg but not both due to the wildly different combustion characteristics of LPG and unleaded.

LPG handles more advance than petrol so it sounds like they've set it up to be economic on LPG without causing excessive problems on petrol. But as youve found wind it hard enough and it pings.

I used to add 5-7 deg of advance in the distributor for running on LPG way back when.
I wonder if the mechanical advance in the distributor is not working as it should.
Does it have vachume advance also? If so check the lines have been conected right.

If you are running on gas 90% of the time I would suggest tuning enough advance for optimum performance on gas, and run premium petrol when you need to. The higher grades seem to run well on LPG advance. The exact advance will depend on your compression ratio, low compression engines run well at around 10 degrees advance at idle. Check that the mechanical advance works smoothly with increasing revs, and not all in a bunch by 1500rpm (this would indicate a worn advance mechanism).

LP Gas on a carby type motor usually wants more ignition advance earlier than when running on petrol and less overall advance than petrol.
Lpg is usually a conversion of a petrol motor.
What this means is the distributor is set up for the wrong ignition advance curve to get the best out of LP gas.
Base ignition settings have little value on LP gas because of the wrong ignition curve.
To dual fuel ignition timing wise is even more of a problem as you really need two different ignition curves, one for each fuel.
Base ignition settings are OK for petrol with a distributor with a ignition curve designed for petrol and that particular motor.
Each gas mixer and convertor set tends to be different from motor to motor as they are installed usually as one offs as after market and this tends to effect motors differently.
In a nut shell to set base distributor timing on a motor designed for petrol running on LPgas is a big compromise which is different for each motor.
A timing light will be a guide, but nothing more.

I found the best way is to give the motor what it wants and the best way is to do this is to road time the motor under load.
This can be done safely on a dyno, or if you a luckly enough own a forward control vehicle drive up a moderate hill at medium revs and adjust distributor base timing by hand until the motor pulls best.
It can be done by trial and error driving up the same moderate hill and checking the performance after each run.
LP gas mixture settings and convertor type will have big results on performance too.
Most people buy LP gas conversions on price and usually get the poorest performing type systems that cause problems over the vehicles life.

Certainly my 3.5 runs well on gas or lpg - still runs out of puff on hills though.

I am using 9 degrees btdc - no pinging, no obvious difference in power on petrol or gas. Cruising along 14.5mpg on petrol and 11.5mpg on gas so this is where the difference shows. Electronic ignition improves fuel consumption a lot.

my LPG system as an old system and seems to suit the carbies.

garry

Originally Posted by Blknight.aus

unless you want to fork out stupid money on lpg injection....

old school engines running dual fuel either get setup to run well on petrol or well on lpg but not both due to the wildly different combustion characteristics of LPG and unleaded.

LPG handles more advance than petrol so it sounds like they've set it up to be economic on LPG without causing excessive problems on petrol. But as youve found wind it hard enough and it pings.

I would love to get a set of thise dual LPG/ULP injectors for the Commodore.

The important concept I am trying to put across is that it is running equally poorly on gas and petrol and that it has run better in the past on both gas and petrol.

Originally Posted by alien

I used to add 5-7 deg of advance in the distributor for running on LPG way back when.
I wonder if the mechanical advance in the distributor is not working as it should.
Does it have vachume advance also? If so check the lines have been conected right.

I have imposed on you a lot in recent times and I think I will be imposing on you again on this one. Your assistance and advice is much appreciated.

Originally Posted by bee utey

If you are running on gas 90% of the time I would suggest tuning enough advance for optimum performance on gas, and run premium petrol when you need to. The higher grades seem to run well on LPG advance. The exact advance will depend on your compression ratio, low compression engines run well at around 10 degrees advance at idle. Check that the mechanical advance works smoothly with increasing revs, and not all in a bunch by 1500rpm (this would indicate a worn advance mechanism).

Thanks for that. What RPM should I be checking the timing at and would that be with the vacuum advance connected or disconnected?

Originally Posted by Mick_Marsh

Thanks for that. What RPM should I be checking the timing at and would that be with the vacuum advance connected or disconnected?

You check idle timing at idle, typically around 600rpm. You check mechanical advance with the vacuum hose off, then you connect the vacuum hose to see how much more advance you are (or aren't) getting with the vacuum connected, with the engine running at at least 1500rpm.

Ok.
Many thanks to Alien who had a fiddle around with the engine today.
His gut feel is there is something not quite right with the distributor.
Although, he did suggest the cams might not be right. How are the cams driven? Can they be incorrectly timed?