Lpg 100%

[commy vk]

I am running a carb 3.3 on 100% gas since removing the fuel tank.
I am keying the engine over twice as long as I did to start her up when cold.
Is this normal?Is gas harder to ignite than petrol?
She starts up instantly when warm though.

[torch]

Yes gas is harder to ignite than petrol

Many lpg/petrol systems are designed to start on petrol when cold and switch over as soon as the motor is going, dont know if that is how yours was setup prior to removing the petrol tank.

whether you have problems or not depends on how long twice as long is? If its more than 10secs you probably want to get the matter seen to or you may not even be able to start it at all at very low temps or with a battery that is on the way out.

Fixes typically involve use of a primer.

[commy vk]

Thanks for replying and I'm still at the experimental stage.I've unhooked the auto choke and see what happens in the morning.I know that VKs were always a hard starter when cold.I'm thinking that when the choke butterfly is closed there would be a gas restriction until it is warm enough to retract?

[75hjprem]

mate have a talk to a gas installer and see what he recons, having a better ignition system and higher compression will help gas ignite too.

[torch]

Thanks for replying and I'm still at the experimental stage.I've unhooked the auto choke and see what happens in the morning.I know that VKs were always a hard starter when cold.I'm thinking that when the choke butterfly is closed there would be a gas restriction until it is warm enough to retract?

that could be the problem, sounds like it was set up to start on petrol, yep pull it out.

mate have a talk to a gas installer and see what he recons, having a better ignition system and higher compression will help gas ignite too.

Sure better ignition can help, but they can run fine on systems set up for petrol, there isnt much that can be done about compression other than shaving the head and it does help performance, not so sure it makes them easier to start though. It does bring up a good point though, the plugs might be set with the maximum gap for petrol in mind, which from memory is about 1mm? for a 6cyl? vk. Reducing the plug gap by 20% from the petrol spec is typically what is required to get a satisfactory amount of spark for lpg only use and easy starting.

[ari666]

but they can run fine on systems set up for petrol,

rubbish

lpg needs colder plugs and f'k load more timing to run properley. this "dual fuel" business is just a joke. either you get a tune for gas or get a tune for petrol, there are no in betweens.

and as far as you starting issues goes, your probably finding that your mixer is empty and its taking a while to fill. sucks to be on gas. but thats what you get for using a cheap crappy fuel that isnt designed for your engine.

[75hjprem]

[QUOTE=torch;1754821]
there isnt much that can be done about compression other than shaving the head

actually there is a lot more you can do to increase compression; thicker gaskets, flat tops.... turbo :P

[torch]

rubbish

lpg needs colder plugs and f'k load more timing to run properley. this "dual fuel" business is just a joke. either you get a tune for gas or get a tune for petrol, there are no in betweens.

and as far as you starting issues goes, your probably finding that your mixer is empty and its taking a while to fill. sucks to be on gas. but thats what you get for using a cheap crappy fuel that isnt designed for your engine.

Sorry dont like your tone: for cold starting problems(which this thread is about) colder or indeed hotter plugs will make absolutely no difference, as for timing yes the initial timing needs to be advanced for best performance and the total amount of timing at higher rpm actually reduced, however, this wont make it any easier to start, nor can it be achieved without getting the dizzy recurved, the timing is best left as is if retaining the existing dizzy, I dont anticipate our enquirer wants to do much other than get it to start on lpg for the moment . Given that the car has been running on lpg with the current plugs, the current plugs should be fine with the gap adjusted, though I agree that since petrol is no longer being used and plug fouling is basically no longer a concern, then running a colder plug is a sensible precaution to prevent pre detonation.
And yes i agree, the length of the the gas supply tube from the convertor is a factor in how quickly the gas takes to enter the engine.

[torch]

[QUOTE=75hjprem;1754959]

there isnt much that can be done about compression other than shaving the head

actually there is a lot more you can do to increase compression; thicker gaskets, flat tops.... turbo :P

Perhaps I should have said "done easily or cheaply", but adding thicker gaskets will lower the compression .
Also, yes turbos raise the pressure of the mix before being fed into the cylinder but do not affect the compression ratio, this is strictly the ratio of the volume of cylinder at bottom of the stroke/ volume at tdc.

[db_notso]

think of pure lpg like a diesel motor
when dead cold start it needs to warm up and cant thanks to no combustion chanber preheating (like a diesel has in form of glow plugs)

in brisbane its illegal to have soley lpg as it fails to meet emissions regulations on cold start...... hence you cant start it easy since
removing petrol side of system
every taxi still has petrol part gets used once a month but still has it ( in brisbane anyway, interstate maybe you get away without emission to extent)

[ari666]

Sorry dont like your tone:

thats ok, you dont have to like it.

[commy vk]

As the car came off the factory floor on petrol (the original super with lead) the carby would be set up so to suit.I figure by switching over to lpg only the carb would take on a whole new role and become just a square thing that the gas can enter to feed the engine.Maybe I'm being a bit too fussy and expecting a bit too much.I always thought that the new cars on factory gas was to protect the top end from startup wear.I reckon someone is making bucket loads on all this anti pollution crap. Thanks all for the info.