20 replies to this topic

[Sisco]

I am currently trying to fine tune my HIF44 carb and am a little puzzled.. hoping someone can advise.

I have recently fitted electronic ignition and set my timing at 10 BTDC however for the carb to run ok and spark plugs to be a nice light brown colour I need to have the mixture screw fully in which I can't remember doing on previous minis I have owned. (usually the screw has been somewhere in the middle with adjustment left CW and CCW).

The screw has always been fully in and passed the MOT like it a couple of weeks back so assuming that emmisions were fine like this. The carb has standard air filter box.

Is there something wrong with the jet height inside which means I have to have this all the way in to run right. I have been trying a few different combinations of backing of the mixture screw whilst adjusting the idle speed but haven't touched the fast idle. would the fast idle screw make any difference to this?

My knowledge of carbs is limited but I get the feeling that either there is something not right or that if I had further adjustment on this screw I could better set it up.

[ACDodd]

Is the jet stuck? This is quite common!

AC

[Sisco]

Thanks for the response.

how would I know if the jet was stuck? I am guessing it is moving as when I back it off it does weaken the mixture?

I am just suprised it has to be all the way in to run normally.

Does the fast idle adjust the jet height at all?

I am tempted to take the carb off and check the inside but wanted to check on here before incase it is normal or something just needs adjusting.

[Dan]

No but it does add extra fuel through a separate path so it will mess the setting up if it's not right. Although it should mean you have to set the jet leaner if it's adding un-needed fuel.

[Sisco]

Thanks Dan. Am I right in thinking with the choke all the way in, the fast idle doesn't do anything only with it is slightly pulled out.

It doesn't sound like it is related to the fast idle, I am fairly sure the screw is not touching the cam at idle.

Is there any way of easily checking whether the jet is seated properly? is the initial jet height adjustable to start with before fine tuning with the mixture screw?

I would like to adjust it richer however the screw runs out of thread so something doesn't seem right.

[KernowCooper]

I would think its time to see if the jet is stuck? and have a look inside and you can see exactly whet the mixture screw is doing

[Dan]

Yes the first part of the choke movement picks up the idle and nothing more, further through the range it starts to add fuel. If the cable is relaxed and the stop screw isn't touching then it's not doing anything.

As above it sounds wrong and you need to have a look inside to know more. It may be assembled wrong or the bi-metal lever could be bent or any number of small issues really.

[Dan]

Having said that, why do you want it richer? There isn't another issue making the car lean that isn't carb related like an air leak is there?

[KernowCooper]

Have you access to a co meter to see what the mixture is now? and what effect moving the mixture screw has ? If any.

[Sisco]

Thanks for your suggestions. What would I need to remove to be able to see the jet?

I want it richer as it still seems a tad on the lean side to me even with screw all the way in. I have breather pipe connected and no leaks that I know of. I have a colourtune on the way so am planning to use that to check how it is burning.

The emmisions were ok at MOT so I am assuming they are at an acceptable level. If I back off the screw it makes it leaner/lumpy and cut out however I fairly sure I should not have the screw fully screwed in to get it to be able to run and dont like the fact that I can't adjust it any further as I have ran out of screw.

Any guidance on how to check the jet would be a big help.

[KernowCooper]

Ok so the screw does something by backing it off, to get access you need to remove the carb from the manifold and turn the carb upside down and remove the bottom cover which will give access to the float and the jet assembly, now wind the screw and see if its raising and lowering the jet or is the small lever bent?

[Ethel]

There's all manner of things that can muck up the fuelling on an SU. It's basically a U tube barometer that overflows into the carb from one end, so if you add more liquid (float valve) or mess with the pressure at either end it can misbehave.

The hif choke is a very simple valve similar to a ball valve, it's not directly related to the fast idle as it should stay closed until it's turned further. Because of the location of its outlet it flows proportionally more when the throttle is nearer to its idle position. It does sound like a strip down & service could be in order

http://sucarb.co.uk/...tter-inspection

There are also some good vids from University Motors on Youtube

[Sisco]

Thank you guys for your advise. I will take it off as the weekend and remove the bottom cover to have a look see what's going on with the Jet.

Is there anything else to look out for while I have the bottom cover off is there anything adjustable that would be making it run lean?

[Sisco]

anyone know if the amount of fuel been stored in the chamber of the hif carb affect how far the mixture screw needs to be? Wondering if the float not being set right could cause the screw to need to be fully in?

[KernowCooper]

You set the float level which gives the correct fuel level its on the SU link Ethel posted and how to check it Sisco, have a read if you've not already.