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It only runs on 'Prime'

  • Thread starter Thread starter razooki
  • Start date Start date
R

razooki

Guest
Well I finally got the bike started but it will only run if the petcock is on the Prime position. If I switch it to On or Reserve it bogs down and dies. Why? Also, my petcock has two nipples and I know the larger one goes to the T between # 2 and #3 carbs but I do not know where the smaller nipple on the petcock is supposed to connect. On another thread someone indicated that it should go on the #3 carb but I thought that was just a vent? BTW I know the sig says 1980 GS850 but I am positive I have VM carbs.

GS850008.jpg


I'm soooo close........

Thanks,

Raz
 
Those are VM carbs, the nipple on #3 is the vacuum port for the petcock, it should connect to the smaller of the two nipples on the petcock. The identical looking ports on #2 and #4 are float bowl vents and should remain open, or have a hose which is open on the other end.
 
They are definitely VM carbs.

The reason your fuel only flows when you have the tap set to prime is that you need to hook up a hose from carb No. 3 to the small brass outlet on the back of your fuel tap - that vacuum is what operates the thing.
 
OK thanks I'll do that and try it again but I only see a nipple on #1 carb there is nothing on #4.
 
razooki said:
GS850008.jpg
Click to expand...

In this pic, the closest carb is #1, the port is blanked off.
The next one in is #2, the tube you can see is the vent.
The next carb over is #3, that pipe goes to the petcock.
And #4 is on the far end, it is also a vent.

Wait a minute, I see a problem, the #3 is also blanked off. What bike did these carbs come from?
See if one of the other two ports has a vacuum when the bike is running, if not you will need to do something different, like get a mechanical petcock or find another vacuum source. For now just use prime and DON'T FORGET to turn it to on every time you shut off the engine.
 
Last edited:
In the photo you can see the brass nipples on 2 and 4. The nipple on 3 is slightly more forward and a tad trickier to get the pipe on - you'll probably have to pull the tank and connect the pipe to the carb first.
 
hampshirehog said:
In the photo you can see the brass nipples on 2 and 4. The nipple on 3 is slightly more forward and a tad trickier to get the pipe on - you'll probably have to pull the tank and connect the pipe to the carb first.
Click to expand...

OK, I'll hunt around for it and hope I find it.
 
I just took off the tank and looked closer but I swear I can't see a nipple on #3 at all!! tkent I don't know what bike they are from I thought they were stock. The engine and frame #s correspond to 1980 GS850 and my understanding was that it was possible for early '80s to have VM carbs. BTW it's a canadian bike if it makes a difference.
 
as tkent02 said here is where the lines go
#2 & #4 are for the float bowl vents either have an open ended hose on them or no hose
#3 is the vacuum feed for the petcock it goes to the small port on the petcock
the fuel line T goes to the big line on the petcock
here's a pic for you
and they are numbered from left to right when sitting on the bike and looking down #1 is on the left and #4 is on the right

carb%20lines1.jpg
 
dam son you have a set of 29mm smoothbores on there and they don't have a vacuum port on them so you have to run it on prime and when you aren't running the bike then turn the pet cock to on or reserve to stop the fuel flow


give me a few minutes and i can post a pic or 2 of what you can do to be sure that you don't get fuel flow when you aren't running the bike it's a mod that i did for mine i have 29mm smoothbores as well
 
Spyderman said:
dam son you have a set of 29mm smoothbores on there and they don't have a vacuum port on them so you have to run it on prime and when you aren't running the bike then turn the pet cock to on or reserve to stop the fuel flow


give me a few minutes and i can post a pic or 2 of what you can do to be sure that you don't get fuel flow when you aren't running the bike it's a mod that i did for mine i have 29mm smoothbores as well
Click to expand...

Spyderman when I ran it on Prime gas came out of the air pods after a while (maybe 10 -15 minutes).
 
razooki said:
Spyderman when I ran it on Prime gas came out of the air pods after a while (maybe 10 -15 minutes).
Click to expand...
was the bike running when the gas flowed out of the air filters?
PM me your phone # and i'll call you shortly and we can discuss it
 
razooki said:
Yep, but I'm afraid I'll have to change the float gaskets as the one on #1 was leaking.
Click to expand...
ok your gonna have to take the carbs off and reset the float height
pm me you phone number and i'll call you i just redid my carbs so i can tell you what you need to do
 
It's not just the gaskets you need to replace (if at all) but the float needles (if your float heights aren't set correctly). The gaskets are only really there to stop petrol slopping over the top of the float bowl - the fuel level should be a few mm below the top of the bowl and it's the job of the float needles to hold it back.

And I should have looked closer at the photos - they are indeed VM smoothbores - the nut on the bottom of the floatbowl is the giveaway. Spyderman has better eyes and me and spotted it dead right. I don't know what your bike is like but if you wanted to make a bob or 2 smoothbores are making big $ in the US at the moment; you could sell them and replace with standard and pocket the cash.
 
here's what i did to make sure i had no fuel flow when not running

i used 3/8 fuel line and a high flow honda fuel filter
mod1.jpg


mod2.jpg


mod3.jpg
 
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