Auto choke electrical heating - why?

[CJH]

I'm stripping down a carb that I bought for experimental purposes. I opened the choke housing and these bits dropped out.



As far as I can make out from my Haynes big book of carbs, the spring is the electric heating coil and the broken white ring must be one half of the plastic support for the heating coil - another identical half is still in there in one piece.

Now my understanding of the choke mechanism is rudimentary at best, and I can't understand why there's an electric heating coil in there.

According to Haynes:

The choke strangler flap is controlled by a combined coolant- and electrically-heated bi-metal coil. On VW engines (but not Vauxhall), the electrical supply to the choke is made through a coolant-heated thermal switch. The electrical supply initially heats the choke coil after the first start from cold. As the coolant passing through the bi-metal water housing warms up, it adds to the heating action applied to the choke spring. The choke flap will thus remain open while the coolant (and engine) remain warm. When the coolant reaches a preset temperature, the thermal switch cuts out the electrical supply (VW only), and the coolant flow remains the only source of choke heating.

I thought the choke operated when everything was cold, and stopped operating once the water temperature rose - the water temperature being the indicator that the engine is warming up. So what's the point of artificially warming the choke housing via that electrical heating coil - isn't that just fooling the choke into thinking the engine's warmer than it is?

[silverbullet]

Haynes Book of Lies strikes again

[itchyfeet]

Good question
I don't know but I wonder if it is because the choke needs to start to come off quite quicly and the water temperature does not react quick enough.
my late van has the heater permanently connected so it's heating even on a hot summers day when fully hot, Haynes shows this on the heater relay circuit but the connections are definately origional and I have seen other late vans the same.

I also wonder if when started luke warm the leccy heater is more important as choke needs to come off quickly.

[CJH]

I think it must be your first point - the water doesn't react quickly enough.

The Haynes wording is quite specific - "The electrical supply initially heats the choke coil after the first start from cold". I don't know what the preset coolant temperature is that turns off the electrical heater, but I suspect when starting luke warm it may already be turned off.

I suppose my implied question is, do I have to fix the broken electrical heater or not. If I don't, I think the choke will not start to come off as quickly, but it will do once the water temperature starts to rise.

I wonder why Vauxhall decided to do without the electrical element but VW added it.

[MidLifeCrisis]

In theory the electrical coil heating up when you start the engine is what heats the choke bi-metallic strip which in turn opens the choke over a few minutes (5ish) - then as the coolant heats up that takes over the role of keeping the bi-metallic strip heated and the electric coil is turned off by the temp switch that measures the coolant temp.

Without the electric coil working the choke will take much longer to open as the bi-metallic strip will only be heated by the coolant which takes much longer to heat up!

[CJH]

Agreed. So without the electric heater the choke stays on longer than it needs to and wastes petrol until the coolant temperature starts to rise. Is that about right?

[MidLifeCrisis]

Agreed. So without the electric heater the choke stays on longer than it needs to and wastes petrol until the coolant temperature starts to rise. Is that about right?

Yep, correct - so not the worst thing in the world, but you might want to secure the positive lead going to the coil if it's not going to be there so there's no danger of it shorting somehow

[CJH]

Well I think I'd still make the electrical connection on this choke unit (if I was going to use it), because internally the connections to the heater coil are just as safe with or without the coil in place.

But I doubt I'll use this choke unit. It seems virtually impossible to repair the heater coil, so my choices are to swap over the choke from my existing carb when I fit this one, or to try and obtain a replacement choke unit. I note that Gower and Lee list it as a complete part in this diagram (part no. 28), so I might find out what they'd charge for a new one.

[CJH]

Well the choke unit is available new, but since it would cost five times as much as I paid for the entire carburettor I think I'll pass.

[itchyfeet]

you could glue that holder back together and make a new coil from resistance wire?
you can buy resistance wire on ebay, and if you have a good unit you can measure resistance.
you can measure diameter and number of turns and coil the new wire on something of the right diameter.

or it's possible that coil is just broken at the ends and can be reused.

https://club8090.co.uk/wiki/Pe ... burg_Choke" onclick="window.open(this.href);return false;

[marlinowner]

I ran for several years with the choke butterfly wedged open because of problems with the choke. Pump the throttle a few times to get some petrol in, had to be nursed for a minute or two in freezing weather but no real difficulty.

[ajsimmo]

Brickwerks,
http://www.brickwerks.co.uk/index.php/a ... l?___SID=U

[CJH]

Brickwerks,
http://www.brickwerks.co.uk/index.php/a ... l?___SID=U

Ah, that's a better price, thanks Andrew. But I'm tempted by Itchy's idea to repair it. It does look like the the existing coil has just snapped at both ends - maybe I can solder it back together. The grub screw seems pretty seized so I don't know how far I'll get. I'll get some plusgas on it.

[kevtherev]

My manifold carb and choke heaters are manually switched on.
There's an audible and physical change in the idle speed when the choke receives heat if I turn them on after starting.

[CovKid]

That makes a lot of sense since otherwise the coil is being heated all the while the ignition is on which always struck me as a complete waste if not shortening the life of the coil. Once the engine is warm, its not needed.