Disscussion, Importance of, Vacuum, (tubes) in carbs

[bob808]

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.

 

[zed1015]

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.

 

[bob808]

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

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

 

[zed1015]

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

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

No!.
What they are saying is that the fuel tap vac hose must be plugged if not in use otherwise you will get a weak mixture on that cylinder.
Pretty much the same advice as mine but with no detail.
Leaving the vac hose open will only affect number 3 carb though so won't be the problem you have on number 1..
Not firing on 1 without choke indicates either a blocked fuel circuit to that carb or a substantial air leak at the intake rubber which is preventing the venturi obtaining lower than atmospheric pressure and subsequently it's ability to draw fuel.
The choke on these carbs is actually an enrichener which suppies raw fuel so lack of fuel under normal running conditions appears to be the problem.

 

[bob808]

Thank you, very useful.

The choke on these carbs is actually an enrichener which suppies raw fuel so lack of fuel under normal running conditions appears to be the problem. .................................(Or too much air! Which would make a lot of sense I think.).................Thanks.