12 replies to this topic

[Stu.]

Could those in the know give me some help with the following please.

 

I have an 850 1979 Austin Morris Mini completely standard with an HS4 carb.

 

I had an isolated problem starting one day so I checked the plugs which were sooty. I cleaned them and it then started and ran fine. I made a note to self to check the mixture with my colour tune later in the week which I did which showed the mixture to be very rich. No surprise as the plugs were black.

 

I turned back the mixture nut to give a blue colour on idle using the colour tune, but as soon as I took it out it there was no power on pick up indicating too lean fuel so I kept adjusting the nut till it ran ok. 

 

On checking the plugs they are once again sooty black, so now it seems I cannot get it to run normally without it being really rich. Using the spring needle to push up the dashpot on the carb to check mixture only gives correct results when the mixture is rich, so it seems a bit odd when the colour tune shows its way too rich.

 

I've checked the timing and that's correct.

Dashpot oil correct.

New plugs make no difference

 

As I can't seem to get the engine to run well unless the mixture is rich I'm thinking the carb and float bowl may need servicing, which would be my next thing to try, after I've 're checked the valve clearances, but wondered if I may be missing something obvious. 

 

Any advice would be most welcome. Thank you.

 

 

[HUBBA.HUBBA]

I may be wrong but is the hs4 carb a bit big for an 850. Is your chock cable on correctly?

[Stu.]

It's been on the car for years. The Haynes Manual shows cars post 1976 were fitted with HS4' s. The choke cable and idle are properly adjusted. No vacuum leaks either.

Edited by Stu., 02 March 2014 - 06:55 PM.

[carbon]

Stu,

 

Would suggest taking off the dashpot and check the needle for wear just around the idle position - this is about 3mm from the lower face of the piston.

 

Your HS4 has a 'swing needle' which rubs against the jet when in use, over time this causes the needle and jet to wear. Some wear can be taken up by adjustment but if the needle is significantly worn you can get the symptoms you describe.

 

If you find it is worn I would recommend replacing just the needle in the first instance.

[Stu.]

Very many thanks Carbon. Some stuff I've read elsewhere today suggest similar. I'll take a look tonight. Am I right in thinking a worn needle has a tiny flat or ridge on one side ?

[ACDodd]

You engine needs a proper tune up with a needle change being likely to get the best from the engine.


AC

[Stu.]

OK I've checked the needle and there is no wear anywhere. It looks like new. Would you still recommend I renew it ?

 

Any other thoughts ? Could it be the jet that's suffered the wear.

 

Please help I'm running out of ideas.

[KernowCooper]

Have you checked the float height, float for damage and the needle valve and seat, either could cause the engine to run rich

[Stu.]

Could you explain to me how the float issues and setting could cause rich running ? I thought the float and needle just controlled the level of fuel in the bowl. If the float bowl has fuel in it to feed the carb how could the float settings affect rich running ?

 

Please don't think I'm questioning your experience, far from it, I just wondered if you would take the time to explain this to me as I don't understand ?

 

Thank you for your help.

[Gremlin]

When all ok, the float stops anymore petrol coming into the float bowl at a certain level, because the air above the fuel in the float bowl and the jet are at atmospheric pressure, the level is equal, so depending on where the float shuts off accounts for where the fuel level sits in the jet.

Now I know that the air above the fuel in the jet is not at atmospheric pressure because the engine is sucking it in but this is the principal

[carbon]

Stu,

 

Other suggestion, if you don't already have an in-line petrol filter just before the carb, then get one fitted. The small plastic ones are just fine and only cost a couple of quid.

 

This will stop any dirt/rust getting through and causing the needle valve in the float chamber to not fully seal. When this happens the engine runs rich and your petrol consumption also goes up...

[Stu.]

Thanks Chaps. I was thinking about the float bowl & carb fuel levels today after Gremlins explination. It makes more sense to me now so thanks Gremlin.

 

So the way I see it in my mind (I think) is that the level of the fuel in the jet is determined by the fuel level in the float bowl, a bit like a water level, so if the float bowl level is too low for whatever reason then similarly the fuel at the jet will be too low too, which would explain why I can't get the car to run properly under load if I set the idle mixture correct using the colour tune. 

 

I did notice that it ran better when I pulled the choke, which lowers the jet level, which I guess may compensate for the low fuel level in the jet.

 

I also noticed that the fuel in the jet seemed too low when I set the mixture nut lean, and also noticed that the gap between the float bowl float and lid was huge ! 

 

I think you might be onto something because I have to  richen the mixture a lot to get the car to run well through the range, and by turning the mixture nut I'm actually lowering the jet to get to the fuel level which is too low in the jet hole. This then gives overly rich running because the needle is not in the optimum position at idle due to the lowered jet position.

 

I'm going to check the gap of the float bowl to lid tonight and see if correcting it to the proper gap rectifies the problem first.

 

One step at a time .........

[Stu.]

Have you checked the float height, float for damage and the needle valve and seat, either could cause the engine to run rich

So all I did this evening was replace the float lid and float with a different one I had spare. The replacement float to lid lever clearance was accurate so I thought I'd try it. It took me a bit to get the idle correct but once I'd got it near I stuck the colour tune in to check for idle mixture. I got that about right, possibly a bit lean, and took it for a short run. It ran really well without any significant loss of power or hogging down like before so I stayed out for a short while.

 

Once I got home I 're checked the idle mixture which showed good a nice blue combustion flame. The plugs were all a nice Gray colour too with no carbon present.

 

So I hope I've found the issue to be incorrect float bowl lever clearance. I measured the gap on the one I removed and it was 8mm. The max gap recommended in Haynes is 4.76mm so it seems the fuel level in the jet was way too low.

 

I'll see how it goes but just wanted you to know how I got on, and to say thank you very much to the guys above for helping me.