Is the vacuum line to the petcock necessary to get it to run?

[Guest]

So im working on getting my 1977 GS750 to run, it seems like I have everything. I have spark (I took the plug out and saw spark). There is gas in the floats. And it seems there is compression although I havent gotten around to testing that yet (my friend has a compression tester but we haven't gotten to it yet). So first thing I did was check the carbs, well they where completely spotless. Its clear the PO went through this thing. then I checked valve timing, and I think this is correct although ive never done that before so im not %100 sure. then I realized the petcock is vacuum operated, but mine seems to be completely off when its in prime (WTF?). ON on ON. and Reserve on Reserve. This seems weird to me. Also I dont seem to have this vacuum line people talk about. Is this completely necessary just to get the thing to idle or do I have bigger problems?

[bonanzadave]

PRI should allow fuel to flow straight through. No vacuum required.

ON & RES must have vacuum on the vac port. Think of it as a switch. No vac - no fuel.

[chuck hahn]

Yes its needed to operate the petcock..except in PRIme as noted. And it must be there and plugged..or the nipple on the carb plugged..if your running an auxillary tank or a manual petcock like a Pingle.

[keglupton]

Does that vacuum port of carb 3 need to be plugged in order to run? Im trying to figure out why she wont start haha.

[chuck hahn]

If there isnt a vacuum line attatched to the nipple it must be plugged. Either stick a bolt or golf tee in the vacuum line or use a small piece of line with one end plugged to make a cap for the nipple. If the nipple isnt plugged its a HUGE vac leak and it wont start.

[keglupton]

Oh wow i bet thats why she wasnt starting even with ether hahahha. Ill update when i get the time to fix this and test. Thanks for helping out a newb guys!

[chuck hahn]

Either should have at least made it kick and sputter a little I would think. I see that you say the carbs looked spotless, but that doesnt mean squat if the tiny intrnal passages are full of crap and plugged. Sadly, the only way you can be certain of what a previous owner did is to go back and redo EVERYTHING yourself. POs can say whatever and its like a fart in a windstorm to my ears.

Carb rebuild guides on the right margin here..

http://members.dslextreme.com/users/bikecliff/

Oring kit for the carbs here. 1 kit does the entire rack of 4. See page 3 for info on the intake manifold orings. Get the maitenance done and then your in business.

http://cycleorings.com/

And the service manual here.

http://zeus.mtsac.edu/~cliff/storage/gs/GS750_Service_Manual_early8valve.pdf

[keglupton]

Thought Id make an update. I got it running. I hooked up the vacuum line to carb 3 like suggested but I also had to reshim the valves as they where all way to tight. After all that she ran great! It felt amazing to here her growl for the first time!

[Grimly]

Thought Id make an update. I got it running. I hooked up the vacuum line to carb 3 like suggested but I also had to reshim the valves as they where all way to tight. After all that she ran great! It felt amazing to here her growl for the first time!

Excellent, and thanks for the update. It's nice to hear of success.