TR4/4A TR4A mixture problem - need advice please

[malbaby]

Adding to the above advice and assuming you have fitted new correct matching mixture needles and jets and float needles and seats
IMHO the problem is xs fuel through the carbs.
Therefore ....do you have a new fuel filter fitted close to the carbs to eliminate fuel contamination...if so must be a float, needle and seat problem.
I would fit new floats and set the float levels a little higher.

 

[glemon]

There should be a vent on all H and HS carbs, though some of the early ones don't have the tube but instead a hole through the cap or the side of the dome.

I'm not certain exactly what would happen if the vent were blocked or missing;

On my 100 the carbs somehow ended up with different float bowl vents. One had a tube vent on top, the other a simple castellated washer or spacer. When I rebuilt the engine and the carbs I didn't realize this and mixed and matched the parts incorrectly. I ended up with one float bowl was not vented. The results were interesting, the needle valve wouldn't shut, flooding the carb with gas. I literally watched it squirter up from the jet.

I scratched my head (figureatively) and tried to think why that would happen, here is what I came up with. Air is trapped in the top of the float bowl. As gas goes in it can't rise in the float bowl, or more precisely there is a resistance to rising in the bowl because air is trapped in the top. So, instead of the gas rising and closing valve the gas keeps coming in and seeks a way out, through the needle metering valve in the throat of the carb.

Don't know if that helps the problem at hand, but shows the importance of the vent.