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Petcock works on reserve, not so well in fuel setting?

  • Thread starter Thread starter Tola
  • Start date Start date
T

Tola

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Got my bike to a point where I can start it up to balance carbs, etc..

Gas from the petcock flows fine in the fuel position at first but as the bike runs you can see an air bubble forming in the fuel line and the bike starts to bog and stalls as if it's not getting fuel. If I turn it to reserve it runs perfectly? There are 3 gallons of gas in the tank and the petcock does not leak and internally everything looked good although it's not rebuilt??


'77 GS750
 
My first thought was a clogged fuel vent..on cap. But then i started to wonder why it would still work on reserve? You could try running with cap off to see if that is the case. I doubt will be watching this thread to get answer i am curious.
 
Might be time for new petcock- the main circuit ("on" spot) should flow fuel copiously when engine runs and 3 gallons of gas in tank- seeing air in line is bad news.
 
Whole design seems fairly simple, think a rebuild would solve the problem. I don't think it's sucking in air.
 
Are you sure you are turning the lever the correct direction? :-k

With the lever pointing DOWN, it is in the ON position, where it should normally be.

Rotate the lever clockwise, so the lever is pointing forward, you will be in the REServe position.

Rotate the lever anti-clockwise, so the lever is pointing backward, you will be in the PRIme position.

The PRIme position will allow fuel to flow freely. Either of the other two positions will require some vacuum from the engine to open the petcock.

.
 
So the petcock is oriented properly, what I did just realize while re-reading tutorials is that the vacuum line coming to the petcock is coming from #2 carb. and it should be from #3, is that correct? I was reading some stuff on bikecliff's site and didn't realize it was for the CV carbs. That's probably why the carb. syncing didn't go to well also.
 
Tola said:
So the petcock is oriented properly, what I did just realize while re-reading tutorials is that the vacuum line coming to the petcock is coming from #2 carb. and it should be from #3, is that correct?
Click to expand...
That is correct. The vacuum for your petcock comes from the #3 carb on your VM carbs.

Just to make sure you number your carbs correctly, #1 is under your clutch hand, #4 is under your throttle hand.


You mentioned carb sync. With your VM carbs, there is no "master" carb, so there is no specific order to adjusting your carbs, just adjust the ones that are farthest from the others, you will quickly get them all the same.

Having your vacuum port to the petcock mistakenly open as if it were a vent would certainly lead to some interesting vacuum readings. :oops:

Where are you in Vermont? I have nothing to do this weekend.
icon_shrug.gif


.
 
On your bike the petcock vacuum hose nipple should be on #3 carb (third from left as you sit on bike). Later models with CV carbs had vac hose comimg off #2 carb. You can test petcock for correct operation by putting some clear vinyl hose on petcock vac nipple and sucking to make sure it flows well in ON or REs spot with vacuum applied
 
So I'll switch the lines around and hopefully it will solve the problem. Carb sync was pretty interesting. I adjusted 1 & 2 then went to 3 and as I adjusted slide rpm's shot way up and all the sync fluid levels changed. Nothing like standing over a hot engine with gas fumes on a hot humid day!!
 
I'm really starting to appreciate my gravity fed petcocks and single carb. on my Triumph!
 
Tola said:
So I'll switch the lines around and hopefully it will solve the problem. Carb sync was pretty interesting. I adjusted 1 & 2 then went to 3 and as I adjusted slide rpm's shot way up and all the sync fluid levels changed. Nothing like standing over a hot engine with gas fumes on a hot humid day!!
Click to expand...

Don't let it run too long, that engine will heat up quickly and so will the gas in your tank. Put a fan on it at least! There was one post in here some time ago where the guy's header got cherry red!
 
sedelen said:
Don't let it run too long, that engine will heat up quickly and so will the gas in your tank. Put a fan on it at least!
There was one post in here some time ago where the guy's header got cherry red!
Click to expand...
Gee, I seem to remember that, but just can't place who that was. :-\\\

IMG_3824.jpg


But, in all fairness, that was NOT during a carb sync. That was on a bike that I had only had for a few hours.
Had not yet pulled the carbs for cleaning, had not adjusted the valves, I was able to get it to run, but only on full "choke",
and that took it to about 4000 RPM. I was in the process of trying to evaluate stator output when it happened. :o

.
 
Switched vacuum line to proper carb. and problem solved, petcock works perfectly, Rookie mistake! Thanks for everyones input.
 
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