How important are Vacuum hoses on the carbs?

[bob654]

I have been rebuilding a GS750 78, ( USA import) here in Scotland, for a few years, most recently, a head skim, valve timing, ignition timing.

Carbs have been, apart, a few times, cleaned, a few times, on and off, a few times, jets swapped , a few times, etc.
Latest issue was, not firing on 1 without choke.
I found an air leak, and haven't tried it again since ( to scared).

What I wanted to ask today is, HOW IMPORTANT IS IT TO HAVE ALL THE VACUUM HOSES AND PETCOCK HOSE ( running off a axillary tank at the moment) PLUGGED IN? To the running of the bike.

I have been reading loads on carbs, ( first build) and have seem very little on vacuums in that time.

Edited March 26, 2022 by bob654

[Gixer1460]

Normally if vacuum isn't required then line's should be plugged otherwise you have a huge air leak

[bob654]

Just got this reply on an other forum ( excuse me) do they contradict?

 

When using an aux fuel supply with direct fuel feed you just need to plug the fuel tap vac hose (#3 carb) so it isn't drawing air and weakening the mix on that cylinder.
A 6mm bolt or similar stuck in the end of the hose does the trick.
The rest of the hoses apart from the fuel pipe/s will be vents ( carbs # 2 and 4 ) and must be left open to atmosphere

[Jaydee]

As your axillary tank is gravity fed, it makes the vacuum pipe redundant so block it off. It's not needed as its vacuum is not pulling against the diaphragm in the tap thus letting the fuel free fall, ie gravity fed when diaphragm engaged.

Anyone who ever changed a stock diaphragm tap for a Pingle gravity fed tap (very common practice on this site) will have either blocked the vacuum pipe or taken it off and replaced it with a bung to match the other 3 carbs in a rack of 4.

Leaving the vac pipe open is a major air leak as mentioned.

[bob654]

Thank you.

[Gixer1460]

16 hours ago, Gixer1460 said:

Normally if vacuum isn't required then line's should be plugged otherwise you have a huge air leak

 

15 hours ago, bob654 said:

Just got this reply on an other forum ( excuse me) do they contradict?

When using an aux fuel supply with direct fuel feed you just need to plug the fuel tap vac hose (#3 carb) so it isn't drawing air and weakening the mix on that cylinder.
A 6mm bolt or similar stuck in the end of the hose does the trick.
The rest of the hoses apart from the fuel pipe/s will be vents ( carbs # 2 and 4 ) and must be left open to atmosphere

Sorry - I see no contradiction? Exactly the same advice! And same as repeated above.