Mixture?

[mattyguk1]

Hi all, I'm currently setting my 1098 up after an overhaul. I've set the timing statically and the car starts fine so I've moved on to the mixture. The carb has been rebuilt so I'm trying to set the mixture, I've tightened the jet fully and undone again 2turns/12 flats and it started fine, I've needed to loosen the jet another 3 flats to allow me to lift up the piston by about 1mm without the engine faultering which was think is correct? One thing I'm not sure on is that the revs heighten when I remove the air filter Is this correct?

[philthehill]

If you lift the carb piston slightly the engine should speed up and then fall back. When that happens the mixture is not far out.

Removing the air cleaner should not increase the engine revs.

Phil

[mattyguk1]

Thanks Phil,I’ll have another look tomorrow. Have you got any idea why taking the filter off may cause the revs increase? So in theory I should be able to set the mixture with the filter removed? Would be easier to see how much I’m lifting the piston with it off

[liammonty]

While removing the air filter shouldn’t have a great effect on revs at tick over, it will of course affect the mixture, as it’s harder to draw air through a filter than through, well, air. So of course you should set the mixture with a (clean) filter in place. The response of the engine when you lift the piston, for example, would be affected if there’s no filter in place.

[mattyguk1]

Thanks chaps, reset the jet this morning to the 2 turns down and with air filter removed the rev speed stayed constant so that's good. It seemed to be running ok set like that so until I take it out for a run I leave as is. I think I may have been pushing the lift pin to far up originally and getting the wrong reading, I'm correct in thinking I only need to lift the piston a couple of mm am i ?

[alawrence10360]

I recently went through this process but struggled to get conclusive results
So following advice on this forum I used the colour of the plugs as a guide
After adjustment I ran it for say a couple of miles and adjusted accordingly
It’s not quite as scientific but it worked for me
I was surprised how small an adjustment effected the colour
I was told to aim for the colour of a digestive biscuit...

[philthehill]

The plugs should be the colour of a digestive biscuit if the fuel/air mixture is set correctly.

If the plugs are covered in dark chocolate the mixture is too rich.