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Random Questions

  • Thread starter Thread starter surveywaters
  • Start date Start date
Can someone explain how prime works on this petcock? I opened it up and there was no spring, so I fashioned one, replaced the o-rings and sealed it back up. It works fine with a vacuum and is sealing splendidly but prime want open it. I disassembled and inspected and don't see how it ever would work. Should there be a part between the plastic barrel of the selector and the plunger tip? I see nothing that would manual puch back the plunger and I see no bypass hole.
 
Maybe someone has been at petcock before you! But believe or not, prime does work on a decent petcock (I tested mine today before heading out in 48 degrees). Stop playing with it and get a genuine Jap made replacement and find other hobbies!
 
I can't do it. I will make this work, or at least understand why it won't; this is how we better ourselves. I have it functioning properly, but there is no flow on prime. I want to understand how switching to prime opens the plunger. I see no mechanical device which would break the seal.
 
surveywaters said:
I can't do it. I will make this work, or at least understand why it won't; this is how we better ourselves. I have it functioning properly, but there is no flow on prime. I want to understand how switching to prime opens the plunger. I see no mechanical device which would break the seal.
Click to expand...

When you move the lever to PRIME, it opens a BYPASS port inside the petcock body so the gas can flow out without any vacuum. I'll bet that port is clogged. Take it apart and make sure all the ports are clean. Good luck, Mark
 
Thanks hoppy, I must have done a poor job looking, I didn't see any ports
 
surveywaters said:
Nope, no passage, still a mystery how prime works.
Click to expand...

The passage is in the petcock handle.

There's a rubber whatsit with four holes under the handle.

When you move the handle, a passage in the handle connects two of the holes. Which two holes it connects determines what happens.
 
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not on this 1982 GS850gl. The handle turns a plastic bung. This is not the four hole style.
 
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surveywaters said:
not on this 1982 GS850gl. The handle turns a plastic bung. This is not the four hole style.
Click to expand...

When you look at the plastic bung,is there anything that looks broken? Also, when you have the petcock apart, when you move the lever does the bung move also? There has to be something not moving correctly in there. Post some pics if you can!!
 
Bung moves smoothly, nothing is broken, the knob turns it nicely. Goes from 'reserve' to 'on' properly, prime just does nothing. It can function with a vacuum in prime. I have looked at this thing for hours, there is no bypass hole, the only way for fuel to get through the plastic bung and out of the brass hose barb is by flowing through the orifice which is regulated by vacuum. There must be some mechanical means by which the diaphram's plunger is pushed back when the unit is put ino prime.
 
surveywaters said:
Bung moves smoothly, nothing is broken, the knob turns it nicely. Goes from 'reserve' to 'on' properly, prime just does nothing. It can function with a vacuum in prime. I have looked at this thing for hours, there is no bypass hole, the only way for fuel to get through the plastic bung and out of the brass hose barb is by flowing through the orifice which is regulated by vacuum. There must be some mechanical means by which the diaphram's plunger is pushed back when the unit is put ino prime.
Click to expand...

There has to be at least 2 holes in the body to allow fuel to flow to the handle/bung part. Remove the diaphram and remove the lever from the body. See if there are holes in the body. Remove the filter screen and spray some carb cleaner or air from the top of the body to see if the passages are open. Also look at the lever and plastic bung to see if there are holes in them. By the way, you said there was a spring missing, where did you put that spring?? Hang in there, we'll figure this out!
 
The fuel flows fine. There are two holes in the main casting of the petcock, one from the main pickup (on) and one from the reserve pickup. Both flow fine and align, when selected, with corresponding holes in the bung. At which point the fuel flows through the bung and passes the orifice which is regulated via the vacuum/spring. I placed a spring on the vacuum side of the plunger between the diaphram and the diaphram cover. Keep in mind the 1980 petcock is very different than the '81-'83
 
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I just read a thread by someone with a similar problem, they mentioned a spring like piece bent near a 90 that goes between the plastic bung and the diaphram plunger. The small tab inside the bung actuates this part pushing on the plunger. If this is correct, I need an image or better yet, a fiche.
 
surveywaters said:
I just read a thread by someone with a similar problem, they mentioned a spring like piece bent near a 90 that goes between the plastic bung and the diaphram plunger. The small tab inside the bung actuates this part pushing on the plunger. If this is correct, I need an image or better yet, a fiche.
Click to expand...

That makes sense, you would need a lever or cam of some sort to push open the plunger. I haven't found an exploded pic of your petcock yet. I have a friend who has an 82 GS650L that has that same petcock, but his is working fine. Too bad we couldn't take it apart to see whats up.
 
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