Can't get mixture lean enough???

[AllThumbs]

Hi,

Having just rebuilt my carbs and re-fitted, I've found that one of my cylinders is receiving too rich a mix (as evidenced by a sooty spark plug).

I've adjusted the mix screw out to it's full extent to make the mix as lean as possible, but the plug is still sooty.

Anyone got any ideas as to what I should be looking at now?

Cheers.

[dunlop0_1]

Possible your float heights are are incorrect?

[steve wright]

look at float height and check it's not punctured by sticking it in a cup of fuel for a while.
Are you sure it's fuelling and not burning oil? Black exhaust fumes=fuel blue exhaust fumes=oil.

[AllThumbs]

Thanks guys.

You'll have to excuse my ignorance here, but what part does float-height play in determining the mix?
I thought it just determined how much petrol was held in the carb at any one time?

Cheers.

[gremlin]

if the fuel level is low, basically the fuel needs more suck to get it up the pilot jet. hence lowering the float heigts leans the mixture off.
check you have the main jet screwed in properly with the relavent washer (if it needs one) and if the washer is on the right way round. also check the mixture screw has the spring,washer and oring (in that order) fitted.

[steve wright]

And if the float is set incorrectly such that it doesn't shut off the fuel valve the pilot system will flood all the time. Should start well though!

[AllThumbs]

So...

... took the air filter off so I could see down into the carb intakes and fired the bike up. You can see the fuel being drawn into the carbs.

The too-rich cylinder looks as if it's getting more fuel being delivered to it than the other cylinder.

Point taken about the float level - but could it also be down to the jet needle being set too high?

I'm making some assumptions here, principly that even if the carbs are out of sync (i.e. more fuel being delivered to one cylinder than the other) if the mix is good I shouldn't see a sooty plug.

[gremlin]

not until high enough revs when the needle is passed the parallel section and on to the taper.
also check your air jets arent blocked. is it running rich just at tickover ? or at full throttle ?
try doing a plug chop at 1/4, 1/2 ,3/4 and full throttle to give a better indication of which of the metering systems is out

[AllThumbs]

Cheers Grem.

Happen to know what the correct float height is, and can someone point me to (or tell me) how to set it?

Haynes manual a bit thin on detail (i.e. no info at all).

Cheers.

[gremlin]

have you checked the washer under the main jet ? it will have one outside edge slightly rounded, this must be the surface you see when the jet is fitted, other wise fuel will leak past.
sorry, cant help with exact fuel height, I normally set them by eye and gas analyser reading. it should be the same visually as the carb that works . :) if not you bend the offending tang until it is.. height should be checked with the carbs held at an angle so that the float just compresses the small sprung centre of the needle if you are doing by eye