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Vacuum Tube From VM Carb

  • Thread starter Thread starter MT.thumper
  • Start date Start date
M

MT.thumper

Guest
First, my apologies. I did search this topic and have read every thread I can find trying to figure out which is the vacuum tube from the carb to the petcock.

I thought I had it nailed when I found a post with a picture showing the nipple of the left side of carb #3. So I took a print out to my bike to compare and my carb is slightly different.

OK, so my bike is a '78 GS550E. Throttle spring/mechanism is between carbs #3 and #4. There are tubes on the left side of #2, #3, and #4. the tube on the left side of #3 is slightly more forward (toward the engine) than the tubes on #2 and #4. Attaching a picture. Is that the vacuum tube that should be connected to the petcock?
 
Yes, that appears to be the correct tube. It even appears to have a tube headed off in the general direction of the petcock.

You can verify that is the proper tube by starting up the bike and checking the end of the tube for vacuum. If you feel vacuum, that's it for sure.
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Steve - Thanks! It's headed the right direction because I pulled it out from being stuffed down in front of the airbox. Was also checking to see if it looked long enough to reach the petcock.

I'd test for vacuum as you suggest, but can't get the bike started. New battery turns the starter. There's gas in the carbs. Plugs are wet (and spark). Now looking at the points as the possible culprit. One sparks inconsistently, and the other doesn't seem to spark at all.
 
Smaller inner diameter hose

Smaller inner diameter hose

You could pull both hoses off the petcock and look at both nipples, the fuel nipple might have a bigger diameter than the vacuum nipple, so you could also identify the vacuum hose as being the one that has a smaller inner diameter. This method won't work if the PO put a bigger than normal vacuum hose on it though.
 
From that carb shot the bike looks like a low mileage creampuff.:D Steve's advice is spot on; that tube is the vacuum line.

This is most likely not what you want to hear, but you would be ahead of the game to just pull those carbs off and go though them properly. The O-rings are no doubt hard and brittle, particularly on the intake boots. Updating these parts is just normal maintenance for 34 year old motorcycles. For more info please check the Newbie Mistake thread linked in my signature. Best to be aware of the pitfalls before you get too far along in your bikes revival.

Good luck.
 
Bryan - The only hose connected to the petcock when I got it was the fuel line; nothing attached to the vacuum nipple of the petcock.

Ed - Thanks to you too. I've got nothing but old bikes ('76 XT500, '78 TT500, '66 CB160), so am aware that carb cleaning is just part of the deal. Am planning to get after it this winter. Wanted to sort things out enough to get it running at least once before pulling the carbs.
 
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