Cold start - Choke?

[Moreton Landy]

Slightly theoretical question, as I usually start with petrol and switch when warm. On the odd occasion when I start in LPG and it the engine's cold, I've been using the choke, but switching to petrol ASAP until warm. So here's the question. Is a choke necessary if starting in LPG? I know the answer is probably 'try it', but it's never the first thing on my mind when setting off, and I wondered if there is some 'science' on this?

[zoltan]

You've not said which engine

On my V8, equipped with the popular Stromberg carbs :D The choke cable opens the throttle conveniently which allows a slightly higher idle speed when cold on gas. This stops her stalling when I get to the end of the lane and turn on to the main road and means I don't have to knock her out of gear and left foot brake whilst keeping up the idle

[SimonHobson]

Slightly theoretical question, as I usually start with petrol and switch when warm. On the odd occasion when I start in LPG and it the engine's cold, I've been using the choke, but switching to petrol ASAP until warm. So here's the question. Is a choke necessary if starting in LPG? I know the answer is probably 'try it', but it's never the first thing on my mind when setting off, and I wondered if there is some 'science' on this?
There's nothing wrong with starting on gas, even in this weather - provided you have enough antifreeze and coolant flow through the reducer to avoid the reducer freezing before the coolant warms up. When running on gas, the choke will do nothing for the mixture (because there's no petrol in the carbs), but on most carbs it will open the throttle a bit to give you a fast idle. As zoltan says, a fast idle avoids those awkward stalls when it's still very cold, the oil's still a bit thick and draggy, and the mixture is probably a bit out.

I've had the 110 for 4 years now, though the last two have been with EFI, and the Disco before it for 8 years. So that's 10 years of mostly starting on gas - in fact at times I've been unable to use petrol for various reasons (such as failed pump). The main reason I've started on petrol in the past has been if the battery wasn't tip-top (4 mile commute didn't help) - if it gets a bit low then I recall on the Disco that cranking was a lot slower and there could be times when there wasn't enough voltage to make a hot spark. In those situations, I'd start on petrol (easier to light up) and immediately switch to gas (no warming up on the choke - V8s are thirsty enough without that :eek:)

[Moreton Landy]

You've not said which engine



Sorry, standard 3-bearing S3 2286 petrol, Romano single-point system.

Simon, I read somewhere that starting on gas can reduce fuel economy, as the gas may not be vapourising properly until things warm up a bit. I'm pretty sure that LPG will vapourise well enough at low temps (as witnessed by the vapourisation when uncoupling the fuel refill nozzle!), so is this another urban myth? Is there any downside to running on LPG only? Is running on choke at all likely to impede vaporisation? Cheers!

[SimonHobson]

Simon, I read somewhere that starting on gas can reduce fuel economy, as the gas may not be vapourising properly until things warm up a bit. I'm pretty sure that LPG will vapourise well enough at low temps (as witnessed by the vapourisation when uncoupling the fuel refill nozzle!), so is this another urban myth?
Yes !

Is there any downside to running on LPG only? Is running on choke at all likely to impede vaporisation? Cheers!
I'm not aware of any - other than the possible icing issues already mentioned if your antifreeze is weak. Using the choke will not affect vapourisation in any way.

It could be argued that starting on LPG is better for the engine - starting on patrol and full choke means throwing lots of petrol in which can wash oil off the bores.