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Should my petcock be doing this (vacuum test)?

  • Thread starter Thread starter GS1000G Shopper
  • Start date Start date
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GS1000G Shopper

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In the process of my fuel injection project, I'm going through the fuel system, to include the petcock. I've been reading petcock threads for the past half hour and have not seen this answered.

My bike is the 1981 GS1000G. I pulled the petcock and decided to rebuild it with a K&L kit. After my thread reading, apparently that was a mistake. The question is acvtually moot, since with fuel injection I won't need to worry about carbs flooding, but it's the principle.

I put the unit back together following the tutorial on Basscliff's site. When it is together with the lever in "ON", I can blow on the fuel port and air w/ little restriction is heard flowing through the tank filter. To me, this means if it was in the tank, it would allow gas to flow almost unrestricted. As I understand the function, engine vacuum pulls the diaphragm open and fuel flows only in "ON" and "RES" positions.

The metal on the unit looked to be in good shape. I'm not going to shell out $80 for a new one due to the FI conversion, but I wanted to ask if this was proper.
 
maybe blowing through it is enough to overcome the spring that holds the diaphragm shut and allows it to open up slightly? try sucking on it instead, if your face turns blue i would say it is ok!
 
Agemax said:
maybe blowing through it is enough to overcome the spring that holds the diaphragm shut and allows it to open up slightly? try sucking on it instead, if your face turns blue i would say it is ok!
Click to expand...

No difference. If I take the cover off and push in on the diaphragm, it slows the airflow greatly. It seems like to me maybe the small o-ring on the diaphragm is too small to seal, but I don't fully understand how this is supposed to work.
 
The o-ring measures 4mm ID x 1.5mm thick. McMaster sells 2mm and 2.5mm (#9263K694 for Viton) versions.

EDIT
Mythbusters said:
Well there's your problem...
Click to expand...
I placed another larger o-ring behind it the small one and it sealed perfectly with little pressure. Relax the pressure, and I could draw air. Looks like I ended up with about 9.3mm OD. If my math skills are still intact, the OD of the OEM o-ring is 4mm + 3 (1.5x2) or 7mm. A 2mm o-ring would be 8mm OD, and a 2.5mm one would be 9mm. Looks like the 2.5mm o-ring is what is needed here.
 
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I would think that you may be applying more pressure than it's supposed to withstand. When the bike's not running, the petcock only has to withstand the force of the gravitational pull.
 
Shwartz said:
I would think that you may be applying more pressure than it's supposed to withstand. When the bike's not running, the petcock only has to withstand the force of the gravitational pull of the height of the fuel in the tank.
Click to expand...
Just finishing the sentence for you.
 
It was the small o-ring. Problem will be solved with a larger OD one (see post #4). I suspect this is why most rebuilds fail.
 
GS1000G Shopper said:
It was the small o-ring. Problem will be solved with a larger OD one (see post #4). I suspect this is why most rebuilds fail.
Click to expand...
And I would agree with you.

I replaced the o-ring in my 1000's petcock with one that was slightly larger in diameter.

http://thegsresources.com/_forum/showpost.php?p=1476500&postcount=24
PICT3079.jpg


finished my $1.98 petcock overhaul a few days ago. I think it came out well considering it was completely plugged up and corroded so badly I had to drill and scrape out all the passages, polish the sealing surfaces with 1K paper and reconditioned the tapered seat which was pitted (where the o-ring seals fuel flow), using a wooden dowel and valve lapping compound.

3~4 hours of work and it looks like new again.
Click to expand...
 
tkent02 said:
In the rebuild kits, the metal pin that the o ring sits on its too short.
Click to expand...
I measured the old one and it was 18.6mm OA (includes the part by the spring). The new one was 18.3mm OA.

When I excluded the boss for the spring, the difference went from .3mm to .5mm, so my friend tkent02 is correct. It does not appear this part can be reused.
 
In your F.I. application, the vacuum valve is unnecessary, you only need the petcock to have a place to attach a hose, and maybe to select reserve or not. I would block off the vacuum valve part completely and save a few bucks.
 
That was my thought (see post #1), but if it is there, it should work. If I can fix it for $10, I'll do so. After I test the repair, I'll have a pack of 24 of the o-rings. If anyone else needs one, I'll sell them for $1.10 mailed in the US.
 
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