Rebuilt Carbs. Overflowing N°3

[Guest]

Hi,

I stripped, cleaned and rebuilt my 78 GS750 Carbs recently. Everything seems good. Bike ticks over well. But it's overflowing on N°3. I was going to check the float height however I'm now hearing it could be a problem with the vacuum fuel tap.

Can anyone explain how this could be causing the problem? Is it simply seeping from not shutting off properly? I'd rather strip and clean the tap than take the carbs off

[hampshirehog]

It's carbs off I'm afraid. The float valve is all that matters - not the petrol tap. Like you suggested, check float height (and double check before bending the tab - unless someone has done that before the height shouldn't have changed. And check the float needle with a strong magnifying glass - it needs to be perfect, as does the seat. Chances are you'll need a new set.

[joe79]

It's carbs off I'm afraid. The float valve is all that matters - not the petrol tap. Like you suggested, check float height (and double check before bending the tab - unless someone has done that before the height shouldn't have changed. And check the float needle with a strong magnifying glass - it needs to be perfect, as does the seat. Chances are you'll need a new set.

I suspect it's the float H, the needle and seat are new. I must have set it a little over. It's not a big leak. But needs doing.

I'm curious about the tap after reading a thread somewhere about a overflowing N°3 can be a symptom of a dodgy vacuum tap

[hampshirehog]

If the diaphragm on the tap is leaking, which is the only way fuel can get down there via the vacuum pipe, then no 3 will run as rich as anything and you'll foul the plug in no time. If you want to check that then plug the vacuum nipple on the carb and set the tap to prime (or use an auxiliary petrol tank).

oh, and new float needle and seat means check / adjust the float height; that's the only time the things should ever need altering.

[joe79]

If the diaphragm on the tap is leaking, which is the only way fuel can get down there via the vacuum pipe, then no 3 will run as rich as anything and you'll foul the plug in no time. If you want to check that then plug the vacuum nipple on the carb and set the tap to prime (or use an auxiliary petrol tank).

oh, and new float needle and seat means check / adjust the float height; that's the only time the things should ever need altering.

I suspect it's that already. I've not checked the plugs recently but I know N°3 is running rich.

What am I looking for with the test? Clean running no overflow problem is tap. Continued overflow suggesting problem is float?

[Big T]

I suspect it's that already. I've not checked the plugs recently but I know N°3 is running rich.

What am I looking for with the test? Clean running no overflow problem is tap. Continued overflow suggesting problem is float?

you are correct.

[60ratrod]

A way to check to see if fuel is flowing in the run position is to disconnect the fuel line and put the line or another piece of line on (depending on if you have the ability to pull it from the carbs without pulling the carbs) and have the line run to another container for a bit. If there is a lot of accumulationn like more than what would be in the line after shutting the bike off, then the petcock might be sticking open. Another way to check is to pull the vacuum line and suck on it. If you get fuel through the vacuum line, the diaphragm in the petcock is shot

[joe79]

Thanks guys.

I'll check over the fuel tap first.