GS750e carb connections - vents or vacuum lines!?

[lieutenant dan]

Hey guys,

my first post here on the forum. I’m a Canadian guy building a ‘79 GS750 here in Switzerland - recently just laid off from my job (thank you Corona virus) so I’ve finally got some time to get my project finished!

My question is regarding the carbs on my bike. There are 3 hose nipples on the top side but when I got the bike they had not hoses attached - and so I have no idea what they are. I believe one of them is the vacuum line (#2 in the photo) that goes to the fuel valve.

Can someone help me figure out what I should do with the other ones - plug them or just leave them open??

[allojohn]

Sitting on the bike the the carbs are 1-4 from left to right. 2 & 4 are vents and 3 is the vacuum to the petcock. The hoses from 2 & 4 should be run over the airbox to calm air.

[Steve]

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[lieutenant dan]

Thanks! You guys are great.

So just for the purporse of trying to get the bike running I should be fine to plug the one on carb 3 and leave the ones on 2 and 4 open.

[Agemax]

Thanks! You guys are great.

So just for the purporse of trying to get the bike running I should be fine to plug the one on carb 3 and leave the ones on 2 and 4 open.

Yes...........

[lieutenant dan]

I had a go at getting the bike running but with no success. I sprayed some starter fluid into the air filter and it would run a bit but as soon as I stopped spraying it stopped running. Seems to me like it’s getting no fuel at all.

i thought the first place to check was the petcock. Pulled the fuel line off and sure enough Fuel flows when it’s in the Prime position. Then I switched it to Run and using lung power I tried to put some vacuum on it and see if I could get something to come out - but nope.

Does this mean there may be a problem with my petcock or does the valve just need more suction than my lungs can provide?

[Grimly]

The slightest lung pull on the vacuum pipe will open the diaphragm so it's not lack of vacuum. Prime will flow plenty to fill and run.
Make sure the fuel delivery pipe isn't blocked. If it's clear you'll probably have to strip and clean the carbs. By the time you've taken them off and apart enough to clean the needle valves you will probably find other blocked passages in them.

[allojohn]

These petcocks are great until they're not, repairing is problematic. Purchasing new/replacing with Pingle or some such is preferred. And as Mr Grimly says, once your past that the question is how clean are your carbs...

[lieutenant dan]

I found the problem!

The hose I was using on the vacuum side of the valve had a tear in it. so I Replaced that, put the valve in prime position (since I rebuilt the carbs and the bowls were empty) cranked it over and she started right up!

now the next issue I have is that the idle is too low. The engine stalls out unless I hold open the throttle a little bit. But I guess that’s no surprise because the idle speed knob was reset. Is there a factory setting for that knob or just screw it out until the engine idles at the proper speed?

[rphillips]

Good to see you're learning, & maybe I can also. Dealing with the lines to the carb. Weren't the lines size specific? Vacuum line small, at carb & petcock, fuel line middle at carb & petcock, & vent lines from carb large & wouldn't fit anything? Also just idle speed knob as needed, no one turn out or 3 turns in, just put it where it needs to be. Good luck.

[lieutenant dan]

Good to see you're learning, & maybe I can also. Dealing with the lines to the carb. Weren't the lines size specific? Vacuum line small, at carb & petcock, fuel line middle at carb & petcock, & vent lines from carb large & wouldn't fit anything? Also just idle speed knob as needed, no one turn out or 3 turns in, just put it where it needs to be. Good luck.

The fuel line from the petcock to carbs is obvious. but the nipples on the carbs, the two vents and the one vacuum line look identical.

[Big T]

But, if you turn the carbs and look at the side that goes to the motor, you can see the vacuum hole on #3, that goes to that nipple

The vacuum hose and the vent hoses are the same size