RB30, potential auto choke issue?

[Throbbinhood]

Hey guys, have a bit of a weird one that I haven't been able to quite sus out...

Every now and then (3x in the last year), when I'm driving in very cold conditions (late last night, in the snow the time before that), I'll be driving and while partial throttle will be ok, anything past half throttle results in the car bogging down and losing power. If I clutch in and try to give it a bootful, she'll bog down and wont get past 3k rpm. If I clutch in without revving it, she sits around 1500rpm, higher than her usual 800-900rpm idle. By the time I pull over to whip the top of the airbox off to have a look, the issue seems to resolve itself. Last night, I just pulled over for a minute and let it idle, and it was all good.

Would I be right in thinking it could be improper operation of the automatic choke? That basically, on the freeway last night, intake temps were so low it caused the choke to kick back in, and so when I'd put the foot down she'd run rich because she wasn't getting enough air? Would also explain the high idle?

Is there an adjustment for when the automatic choke kicks in, or can I disable it somehow?

[mudnut]

It sounds like the
second stage of
the carby sticking.

[Throbbinhood]

It sounds like the
second stage of
the carby sticking.

Nah, I checked that, it all moves quite free, vac operation is all fine.
I spoke to a carby shop. They said that because I've plugged the air heater thing (the heatshield over exhaust into airbox section) that it could be causing the whole thing to be too cold.

I can't surely be the only one that's plugged this? Otherwise we'd all have water in engine at any crossing...

Does anyone know how the automatic choke is activated? Is there a thermostat somewhere for it?

[mudnut]

Check that the choke break
diaphragm ( top rear of carby)
isn't holed
or split. Its job is to
pull the choke off and
keep it there.
And/or unblock the
pre-heat flap for the cold
weather.

[Throbbinhood]

Check that the choke break
diaphragm ( top rear of carby)
isn't holed
or split. Its job is to
pull the choke off and
keep it there.
And/or unblock the
pre-heat flap for the cold
weather.

I'll check the choke break.. The preheat flap was all stuffed, and it's not a sealed thing.. Can carbies not operate cold at all? What difference does the cold air make once the car is running and at temperature?

[Patrol'n]

Nah, I checked that, it all moves quite free, vac operation is all fine.
I spoke to a carby shop. They said that because I've plugged the air heater thing (the heatshield over exhaust into airbox section) that it could be causing the whole thing to be too cold.

I can't surely be the only one that's plugged this? Otherwise we'd all have water in engine at any crossing...

Does anyone know how the automatic choke is activated? Is there a thermostat somewhere for it?

You're definitely not the only one with the stove/ heater pipe plugged mate.

I've had mine plugged for ages without issue. (Had a different issue recently with idle but I believe completely unrelated), likely to be dirty fuel in the idle circuit. I have not found any adverse affects from plugging the pipe many months ago, no affect on choke operation at all.

[mudnut]

I re-read your post
and it really does
sound like the second stage
binding while cold.
As the revs rise to 1500.
It is almost a carbon
copy scenario of what
happened to mine.
Also check the screws
inside the carby.

[dom14]

Nah, I checked that, it all moves quite free, vac operation is all fine.
I spoke to a carby shop. They said that because I've plugged the air heater thing (the heatshield over exhaust into airbox section) that it could be causing the whole thing to be too cold.

I can't surely be the only one that's plugged this? Otherwise we'd all have water in engine at any crossing...

Does anyone know how the automatic choke is activated? Is there a thermostat somewhere for it?

I removed the exhaust manifold shroud ages ago. The foil hose that connects the exhaust hot air to the air cleaner was never connected. So, plugging the hose shouldn't cause any dramas. Mine's going fine without any of it.
choke thermostat is a bimetal spring inside the auto choke housing. As the engine warms up the spring absorbs the heat and expands, which effectively open the choke flaps to make the mixture leaner.

Your issue is not likely caused by the auto choke closing the flaps by cold air intake when the engine is already warm.
It's more likely WOT circuit issue, rather than the idle circuit issue.

If you look at the picture, there are three screws that you can loosen and then turn the auto choke clockwise or anticlockwise to make it lean or rich. There's a mark on the plastic and the carby body to align it. Normally you don't need to touch this at all. In my experience, the bimetal spring rarely wears out or fails. But, whether the flaps or the shaft get stuck inside from gunk is another issue, and it is a possibility, but not common. Even though if that's the case, running bit too rich shouldn't rev limit & bog down the way you described it. Having said that, make sure the electrical connections to the auto choke is not loose or disconnrcted somewhere. I don't think the auto choke is the issue, but it's worth checking it anyway.

If the carby has never been rebuilt, then it's time for a one.
In the meantime, get a carby cleaner and spray generous amounts of it while revving the engine(warm engine).
Spray into the throttle as well as choke flaps. Hopefully that will clear up any blockage and friction and solve the problem.

Not sure how the depressing of the clutch makes the revs jump from 800 to 1500(I'm guessing on neutral). Somebody might be able to explain it.

[dom14]

It sounds like the
second stage of
the carby sticking.

Yeah, I think that's the case.

[Throbbinhood]

The rev's aren't jumping from 800 to 1500... My normal idle is 800rpm when the car is warm... So, driving down the freeway normally, car fully warm, idle's at 1500.. Until I'd been driving down the freeway for a fair whack of time and then I experience the 'bog down' issue, and I press the clutch in, my idle is 1500 - same as when the choke is operating and the engine is cold.. Hence my train of thought of a choke issue..

WOT with car warm/day weather and car hauls ass. It's only after a long freeway run late at night, and a snow drive that I've had these issues.

I double checked the secondary butterflies, moving freely, no sticky spots on the carby, it's operating correctly.

Spoke to another carb specialist about the stove pipe, he said it shouldn't make a difference in vic temps.