2 replies to this topic

[whawes]

I've got an early 1275 A+ engine which has been transplanted from a Metro. It has a HIF 44 carb, electronic ignition, cone air filter and large bore twin exhaust. Spark plugs, leads, mechanical fuel pump, distributor cap and rotor arm have all been changed within the past two months.

The engine was running OK, but overfuelling badly judging by the sooty black tailpipe and plugs and atrocious fuel consumption (15-20mpg!). I wasn't able to sort this by leaning off the mixture (turning the adjustment screw on the base of the carb on the drivers side clockwise/inwards) and was fed up with the noisy operation of the electronic fuel pump, so changed it for a mechanical one in an effort to solve both problems.

Fuel pump is now silent, fuel consumption is now about 50% of what it was and after the first drive yesterday the tailpipe was almost clear of soot. Job done, I thought, but Mini has other ideas.

During a second drive last night, the car developed a slight misfire on the overrun. Checked all vacuum hoses, plug leads etc, can't find anything obviously wrong. Drove the car again this morning and found that the misfire is much worse when the engine is cold. Once the choke is fully in it still misfires ever so slightly on overrun, but it's much less pronounced. There doesn't seem to be any misfire under load (unless it's so slight I'm not noticing), only on the overrun.

What am I seeing here? Based on the adjustments made I'd have said mixture too lean / fuel starvation, but the tailpipe is now as sooty as before which seems to contradict that. If running too lean I'm told you'd also expect the engine to run hot, but water temperature at least has shown no change - it still sits on the halfway mark at all times.

I wondered if the dashpot oil might be low, but it seems OK.

Stumped again - any ideas please?

Edited by whawes, 23 April 2007 - 09:04 AM.

[Purple Tom]

It will pop slightly more when on choke, as when on choke it is effectively overfuelling...

It sounds to me like the carburretor needle is completely incorrect for the set-up you're running. I assume the needle hasn't been changed and the car hasn't been set-up correctly on a rolling road, as you don't mention it.

Ideally (as is almost always the case with these type of running/fuelling issues) you need to get it onto a rolling road and get it set up by someone who really knows what they're doing. Been as you're in Maidenhead, so not too far from ML Motorsports I would get it set up there by AC Dodd (a personal recommendation as he did my Mini and it was perfect).

If you can't do that, then I would suggest finding out what needle it has, then downloading WinSU and using it to find a needle thats a bit leaner through the rev range, and then driving the car with the new needle, identifying where it is rich or lean, and doing it that way. Its expensive as needles are about £10 posted from Burlen, but it can be a decent way of doing it, and has worked for me in the past. You need to also make sure the timing is correct and all other components are working correctly too.

Hope that helps.

[Grayedout]

Best way to sort it will be a rolling road session !

That is the only way to ensure the correct fueling at all running conditions !