PWM for car choke?

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hey guys, i have an automatic choke on my car (bimetallic spring) and if i set it so the choke works - it never kicks out completely (ie fast idle is always on)

so i got a generic choke to replace the old one. only problem is that the generic choke kicks back in shortly after i turn the car off - which makes the car rev like mad and makes me look like an idiot at the traffic lights!!!

so this is my plan - i want to hook up a 555 timer to the bimetallic spring so it keeps voltage on it for a while after i turn the car off, so then the choke should only come on when the motor gets cold again.

i am also thinking of pulsing 12V onto the spring (say 50% duty cycle to put the choke "half on"). How do you think the spring will react??
 
i cant see that as being an easier solution.

the problem is that the choke works on the ignition switch - when the ignition is on, 12volts is suplied to the choke and the coil unwinds (therefore slowly releasing the choke)

by the time i get to the shop, the choke has just released -> i turn the car off -> there's no power to the spring -> it coils up again just enough to kick the fast idle back in-> and the car revvs like mad 'cos the motor is still warm.

so i was thinking of using a timer that says on for about 10 mins after the car is turned off, so then when i do leave it for a few hours, the choke will kick in as normal. the resistance of the spring is pretty high so im sure i could just use a small transister to do the swithing. i would also need to make it so the timer switches on only when the ignition is off.

i just wonder if i could pulse 12v onto the spring to make it move back a little bit (if i need to)
 
The problem is that the bi metalic strip cools faster than the engine on very short trips so by using the coolant jacket heat is held around the choke unit thus keeping the mechanism warm. It may be worth checking the adjustment of the choke unit also. Loosen the choke unit and turn it slightly (only a few degrees) in the "off" direction to put a bit of tension in the mechanism.

I cant offer any suggestion about the 555 but if the bi metalic is in good order it shouldn't need any power on it after key off.

John
 
so i put the old (original) choke back and set it so its works nicely, and found out that it is only short trips that i have this problem..... but then yesterday it was an unusually warm day ( 'cos we're going into winter now) and the choke didn't kick out at all - the motor was idling at almost 3000rpm. this makes no sense to me cos the spring (when warm) should move way past the point where the choke kicks out (about 30 degrees more) so a few degrees in the ambient temperature should have no effect to the turning off of the choke.

i think when i get some time i am just gonna put the generic spring in and make a timer that keeps 12v on it for about 10mins after the car is turned off and not worry about pulsing it..... and pray it works cos its giving me tons of grief :smash:
 
What make of car is it you are working on?

Is it a twin choke carburettor (twin venturi/barrel) It's possible that the second throttle butterfly isn't closing fully to its stop. They are normally, but not always vacuum operated so if the linkage dries out and becomes stiff the return spring wont fully close it.

Opel cars were bad for it when using the Pierburg carb. The second throttle butterfly had a plastic ball joint connecting it to the operating vacuum unit which if left unlubricated by bad maintainance would become sticky when hot.

It would drive fine until you drove it hard and opened the second throttle then the butterfly wouldn't fully close again when the vacuum dropped.

The air passing through the second throttle draws fuel in with it giving a high idle speed even with the first butterfly at the idle position. It also gives an unadjustable CO% though I guess you dont have access to a CO meter.

Try lubricating all the linkages and working them back and forth a bit and retry it..

Also start the engine and lightly spray WD40 or similar around the bottom of the carb area to check for air leaks. If you have a leak anywhere the solvent will be pulled in and cause your engine RPM to increase.

Give it a go and let me know how you get on.

John
 
thanks John for the advice.

the other day i took the bi metallic spring off completely and ran the car. the butterfly drops down (fully open) very easily and i checked to see if it was sticking and it wasn't. the fast idle also disengages as soon as you tap the accelerator - so its not sticking either. i am pretty sure its the spring giving me grief.

right now i have the generic one hooked up to a power supply in my room. at 10v it draws 600mA and as it warms up the current drops a little (after about 10 mins now its reading 520mA) so if i use a small signal transistor (2n2222) as a switch it should be fine, right?

this spring has moved about 90 degrees in 10 mins, and seems to stop there. i know it only has to move about 45 degrees to turn the choke from fully on to fully off. the problem with this one is it moves back very quickly, i am going to test it now to see exactly how long it takes to return to its "cold" position. then i will build a timer to (kind of) compensate for this.


i drive a 96 nissan sentra, and spoke to 2 people with the same model and they have never had a problem with their choke. i guess i got a dud
:bawling:

oh yes there is one more thing - a vacuum on the carb pulls the choke butterfly open when i start the car (so even if i set the butterfly completely closed, as soon as i start the car it is half open) but the vacuum diaphram cant be broken or else it wouldn't pull - it just doesn't make sense that it opens the butterfly when the choke should be on.
 
Hi Dan2.

I've been to see my Japanese car guru and he says..........

Check around your carb area on the inlet manifold. You are looking for two narrow coolant pipes that take hot coolant to the manifold and back away. They are rubber hoses that go onto metal stubs and they block up with rust and sediment. The effect is that the area which should be at coolant temperature to keep the heat into the bimetal is cold so it gives the problem you describe of re applying the choke too soon.

If you check them and they are as clean as a whistle with crystal clear coolant flowing freely from them then all is not lost. In that case the car needs a new thermostat as it has stuck open giving the effect of over cooling the engine and once again lowering the temperature of the heat mask water jacket area. I wouldn't have thought over cooling too likely in your climate, not like in the UK we have 47 words for rain here.

Hope this helps matey, let us know how you get on with it.

John
 
wow thanks for going out of your way to help me out John!!

yesterday i decided to do some homework, so i took the generic spring that i have, and also took out the original spring and hooked them up in series to my PS. i left them on for about 10 mins ('cos the choke should be off by then) and found the generic spring moved twice as much as the original - and that the original is not moving enough while the generic one was right where its supposed to be.

then i turned them off and they both took about the same time to return to normal - which was about 20mins...........but then i saw what could have been the problem.

there is a metal "cover" between the carb and the spring unit with a small gap for the choke linkages to come through. i had it resting on one of the springs and after 10 mins the spring heated it up so much that i couldn't hold it. so i think this metal cover (which was't put on with the generic spring) holds heat from the spring, and probably transfers heat from the carb body to the spring aswell, to keep it hot.

so i put everything back with the generic spring and didn't have any trouble last night, but i will just have to wait and see.

As for those water pipes, i did see them, and the mechanic that disconnected them said we don't need them here 'cos it doesn't get cold enough (we never have snow). but i still tried to see where the pipe is supposed to go but i couldn't find it. but no water goes near the bi-metallic spring.


luckily today is a really warm day (especially for autumn) and this weather is normally when the choke wouldn't disengage - so if it passes the test for today then i might have it sorted out.

thanks again John ;)
 

GK

Disabled Account
Joined 2006
john blackburn said:
No worries, good luck with it.

John


G'day John

Being both an Englishman and a mechanic (?) I have an off-topic question for you, if you don't mind :)
I’m looking for a twin DCOE Weber manifold for my 2.25 litre Landover Engine.
I’ll make a mould and cast my own if I have to, but would you have any idea who may make/supply such a thing?
I’ve search and searched with no luck (including Webcon.com.uk, who now make and distribute Weber Carbs – they only do a 2 barrel downdraught manifold).

Cheers,
Glen
 
V8? It's a lot of work but you get a very special feeling every time you start it up!

I had a 2A lightweight with a 10.5 to 1 V8 in for a while, the only thing they can't pass is a petrol station.

I will have a search and see what I can turn up, it's a bit of a rarity, the sidedraughts didn't often find their way onto 2.25s

A mate has a pair of new 40DCOEs for sale that have only done 4 weeks on a Fiesta if you are after carbs as well.

John
 

GK

Disabled Account
Joined 2006
Ok, thanks John I appreciate it.

Although a V8 would definately be nice, I've already got a worked 383 stroker in my daily driver which costs me enough to run, so the 2.25 liter will stay in the Rover.

I've got a set of triple DCOE45's from my 260Z, which is now taking a different direction with the induction, so it's two of these are what I wan't to use on the Landrover.

Cheers,
Glen
 
Right you are, I bet that sounded wonderful with three of them snorting in the wildlife! I will ask around and see what I can turn up.

A mate built this a good few years ago, a 1056cc 6 cylinder Honda CBX with 3 40s chucking fuel in faster than gravity could keep up with. It ended up needing a big electric fuel pump and the carbs make more noise than the open pipes can.

John
 

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Hi Glen.

I've just noticed your link to the Land Rover project, you Sir are a star.

I was laid under an 80 inch series one removing a hideous 2.2 BMC diesel when I heard of the fall of the Berlin Wall. It didn't make the snapped crank seem any better mindst but at least we were spared David Hasselhof.

John
 

GK

Disabled Account
Joined 2006
Thanks John

PMSL at your mates three wheeler. I’ve never heard of DCOE’s on a motorbike before, let alone triples! That’s insane. As mad at the brother of my brother in law who has installed a ~180kW Hyabusa (spelling?) engine into his paddock bash Yamaha quad.

The triples in the Zed were on my all out L28, fitted with 38mm chokes and making 280bhp at about 7000rpm. The sound from the trumpets towards 8000rpm was incredible. Went really well. Before I changed the diff to a 4.625:1 unit for a better launch from standstill I could comfortably cruise at 150mph.
I once missed a big male red kangaroo by about a foot at that speed, so maybe it’s a good thing that it revs out at only 125mph now :bigeyes:

My original plan with the 2A was to do it up original, but since I already have the Webers I figured why not. The engine itself is bog stock, but I reckon getting rid of the horribly restrictive stock induction and exhaust should really free it up. Besides the intake manifold all it needs for the Weber upgrade is some slight sheet metal modification to the inner left hand guard to make some extra room. So, twin DCOE45’s, extractors and a 2.25-inch exhaust. It still isn’t going to go like a hot rod, but it should sure sound nice!

Cheers,
Glen
 
The same bloke who built the CBX also has a pre unit Triumph with a Shorrock supercharger with a single 40 on it. Sadly I have no photos of it, it starts with one kick, hot or cold and idles immediately. When it was originally built he fitted a small Amal carb into the inlet manifold as a "starter" carb but it proved needless so off it came,

The Reliant "trike" owner is no mate of mine! Seemingly it was a mint Reliant before he got his porky hands on it. He didn't do the work himself which makes it even worse, he payed good money for that making.

Do you think the 3 bearing crank will be up to the 40s? Sounds like Roo bars might be needed if you are going to be hurtling round in Roo teritory. If fuel is the only consideration I think that a 2.25 with 40s would probably use as much,if not more juice than a well set up V8 would and you have many more crank bearings in the Rover/Buick engine........

Heres a couple of Hyabusa based things that tickled me.........

http://www.youtube.com/watch?v=ddhrD3a2I1M

http://www.youtube.com/watch?v=oPQIizRp9ck&feature=related

John
 
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