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someone please set me straight on vacuum connections

  • Thread starter Thread starter pasteurized
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pasteurized

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So I just cleaned my carbs and replaced the diaphragm in my petcock valve. After everything being apart for a few months (have been out of state for a while) when I put everything back together I don't think I did things correctly. First of all, on my second and third carbs, there are vents (?) on the outside of each carb. What should those be hooked up to? And in front of carb #1, there is a small metal tube sticking out of the intake (basically, right after the carb connects to the block). I have noticed that while riding it, at idle, at speed, and while revving it, I get a "backfire" out of this tube. It is very audible as a loud sudden hiss and I can feel the bike shake each time it backfires. When it "backfires" from that hole if there is no vacuum line hooked to it, I can feel a big shot of air shoot out each time. I currently have the petcock's vacuum line hooked to that, but I am sure that isn't right. What is supposed to go there? Where is the petcock supposed to get vacuum from? After having it hooked up to the current location, my bike decided to dump the contents of its gas tank on the ground last night and while I rode it to my girlfriend's house, where my tools are. I am sure the place it is plugged into currently provides way too much vacuum and this is probably why it dumps gas continuously out the bottom of one of the carbs, even while riding. Sorry for the long-winded post, but I am in search of answers and I haven't really come up with anything definitive on the forums.

BTW, this is for a '77 GS750.

Thanks!
 
sweet.....a problem i think i can actually help with...i've got a 78 gs750...had the same problem after the carb rebuild.
the line coming out of the petcock should go to the #2 carb(left to right)(mine would always pop and backfire hellaciously :evil: ) lines #1, 3, and 4 are atmospheric vents. the reason you are probably losing alot of fuel is b/c your petcock is set to prime instead of reserve or on...this constantly dumps fuel into the carbs, if you leave it sit for more than 5 min. you'll start seeing a puddle under the bike...I trully hope this helps you out :D
 
cool, i will hook everything up in this way. i actually tried the petcock in a bunch of positions and i know very well what each position does (or is supposed to do) but i think having the vacuum hose plugged into the wrong spot created way too much vacuum for the bike and caused some problems. now one more question: do you have a small metal tube sticking out between the #1 carb and the engine block on your bike? im wondering because i have one and have no clue what it's supposed to connect to.
 
vacuum

vacuum

You can't have too much vacuum. There's two ways that I can think of off the top of my head that the petcock can leak.
1: Internally, the diaphgram can be punctured and allow the vacuum of the engine to draw unmetered fuel directly into the engine, not a good thing.
2: The seat that actually does the sealing of the fuel to stop it flowing to the carbs can leak when there's no vacumm applied. This would cause the carbs to leak if the float valves were not sealing correctly. This could be bad as the fuel either leaks on the ground (expensive waste of fuel) or worse, fills the crankcase full of fuel. In a perfect carburetor there would be no fuel leaking from the float valves but we all know the worlds not perfect....don't we :-).
You said "After having it hooked up to the current location, my bike decided to dump the contents of its gas tank on the ground last night" .
It sounds as if the float valves are leaking...Mike
 
I don't want to cause trouble but the vacuum/vent connections previously posted are not correct. Your carbs, VM26, have two floatbowl vent line nipples, one on carb #2 and the other on carb #4. These should be run under the seat and be clear, no kinks. The vacuum line from the petcock goes to carb #3 nipple.
The tube sticking out (each carb manifold will have one) is supposed to have a rubber cap over it. This tube is used to attach a vacuum tool for carb synching. Get a cap for it.
 
hey keith, thanks for the reply and setting me straight. one last question... the floatboal vent line nipples are on the bottom of the carbs, right? or are you talking about the nipples sticking out about halfway up the carbs? same thing with the #3 nipple... not sure which nipple you are talking about. and THANKS for setting me straight on the synching tube.
 
pasteurized said:
hey keith, thanks for the reply and setting me straight. one last question... the floatboal vent line nipples are on the bottom of the carbs, right? or are you talking about the nipples sticking out about halfway up the carbs? same thing with the #3 nipple... not sure which nipple you are talking about. and THANKS for setting me straight on the synching tube.
Click to expand...
The 4 small 1/8" lines, 1 under each floatbowl, are overflow lines. These should be routed so any fuel overflows go on the ground. The brass nipples on carbs 2 and 4, about half way up the left side of the carb and pointing upward, are the floatbowl vent nipples. A 3/16" line connects to each of these and is routed under the seat. These must not kink. The other brass nipple on carb 3 is for the petcock vacuum line, also a 3/16" line.
Note: IF YOU RUN POD FILTERS, you must remove the two floatbowl vent lines and leave the nipples open or you'll get fuel starvation.
 
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