9 replies to this topic

[blackbelt1990]

Morning all, I'm after some information before I start dissembling two carbs.

How does the choke actually work? I have had two different carbs on my mini recently and both seem to have a different approach.

On my original carb (HIF38), the choke quadrant was just the right size to touch the accelerator quadrant and hold it open slightly when you pulled on the cable. I thought that this must be how they work, just holding the accelerator open slightly.

However, I now have a HS6 on my mini and not only do neither of the quadrants touch, it's impossible to move the choke cable more than a few mm and as a result... I can't go out if it's cold lol.

So, basically, is the choke just a device to wedge the throttle open, or does it have it's own mechanicals inside a carb?

Cheers!

[Carlos W]

The choke also restricts the air flow to richen the mixture.

 

I've got a book called the fundamentals of motor vehicle technology which explains this really well, but We're moving and the wife has already packed it.

 

Not something I use for reference very often, but...

 

http://en.wikipedia....iki/Choke_valve

[nicklouse]

It should also richen the mixture by dropping the jet a few mm. Or some such. The throttle opening is only a part of it on particular carbs.

[blackbelt1990]

So if it can only move it a few mm on my HS6, something is wrong?

[Spider]

As per the thread title

 

 

or another approach

 

 

 

However, on the HS6, there is a Jet on the bottom of the carb, it can be seen on the outside, when the choke is operated, a lever pulls that jet downwards. It does sound like the linkage connecting this to the fast idle cam (that the cable actually puls on) is jamed.

 

<edit: the Jet is plastic (on the outside) and usually Black or Red in colour>

Edited by Moke Spider, 03 January 2015 - 10:45 AM.

[Berrybox]

[quote name="Moke Spider" post="3196712" timestamp="1420281812"]As per the thread title
 

 
or another approach
 

 
 
This may have made my day

[Dusky]

HS carbs ( dont know about hiff)  dont have a real choke. They just pull the down and open the throttle a bit to get a very rich mixture, they are not choking the car with a butterfly or similar.

Works about the same as the enrichening device of a weber dcoe.

Your mechanism is probably jammed ( as I had with an old carb too). Spray a good penetrating oil and afterwards some wd40 and pull the jet down manually to free it.

[blackbelt1990]

Thanks guys, if I can't fix it then I'm planning on changing back to a larger HIF instead!

[69k1100]

A larger single carb is less bother.

[Ethel]

SU's are different, the term "choke" comes from fixed jet carbs where a throttle before the jet restricts air to lower pressure and so make for a richer fuel/air ix.

 

HS carbs achieve the same "enrichment" by just lowering the jet so less (thinner) needle is in it - allowing more fuel.

 

Hif's are a bit more sophisticated and have a separate "choke" jet, controlled by a simple spindle valve. The jet can be seen as a little hole just below (after) the bridge (raised bit that makes the venturi with the piston with the main jet in the middle) the choke cam is mounted on the end of the spindle. 

 

On a HS the choke enrichment affects the mixture all the time because the jet is always lower than normal. The Hif choke doesn't affect the main jet and as the piston rises (with increased engine load) so does the air pressure above the choke jet, in its shadow, so less fuel is drawn through it (when it's not needed, helping your mpg).

 

The choke cam on all SU's also opens the throttle a little to give the fast idle setting. If you watch a HS choke linkage closely you'll see it's in 2 bits so it sets fast idle before it starts to move the jet.