Petcock set for Prime to run reliably. Is this an issue?

[92GS500Lv]

Good day,

Hey , I am taking a break from troubleshooting , just getting out and riding.  I bike a rescued had a horrible fuel rust problem 1993e left with water and bad fuel for 6 years, tank rotted out and fuel lines contaminated for years.
I replaced the carbs with used ones and rebuilt them just in case.  Replaced the fuel lines and replaced the fuel take with F version.
Vacuum line is connected from carb to frame mounted petcock.
Frame mounted petcock and tank petcock were cleaned of a white calcium looking substance.  All fuel line mounting tubes are clear and flow liquid. 
So here is the issue.  The fuel does not flow properly in on or reserve settings.  If I squeeze the feeder hoses for main and reserve from tank when in the on or reserve setting I feel a pulsing. 
The symptom is that the bike does not rev above 4,000 rpm , idle is ok,  and fuel starves after about 3 min of running. Also gets stuck at high idle or does not immediately come down from a wot snap.

If I put it on Prime,  It runs fine and I go around down no problem.  When I stop I set if back to On.
I cleaned the diaphragm,  I might replace the frame mounted petcock if I find one at a reasonable price.

The big question is:  Will there be anything wrong with running it on prime all the time. and shutting off when stopped ?  I have had bikes for years that have the regular on or off petcock, and I religiously turn them off when I stop places.  Is this any different just letting this issue go ?

Just curious.   At the end of budget so not looking to pay more than $30 for a new one, especially if this will not correct the issue.  Just like getting on the road for some riding.

[Watcher]

Also gets stuck at high idle or does not immediately come down from a wot snap.

This is usually a sign of a vacuum leak or otherwise a very lean running condition.  If it isn't getting fuel in the required volume this definitely makes sense.

If I put it on Prime,  It runs fine and I go around down no problem.  When I stop I set if back to On.
I cleaned the diaphragm,  I might replace the frame mounted petcock if I find one at a reasonable price.

If the petcock itself isn't the issue, it may simply be the vacuum line that actuates it is clogged/kinked.  Easy enough to test by just disconnecting it and seeing what happens.

The big question is:  Will there be anything wrong with running it on prime all the time. and shutting off when stopped ?

Other than the fact that it's bypassing/ignoring the actual problem?  No.  It won't harm anything, unless you forget to turn the petcock away from prime and your carb float valve is worn or out of spec. 
In fact, on both of my GS's my solution to petock issues was to remove the factory petcock entirely and replace it with a simple in-line ball-valve.  Unless you feel like getting creative with fittings you'll lose your reserve capability, but unlike the factory petcock a ball-valve isn't going to just break on you.  Even for someone that doesn't have experience with simple petcocks, it's not hard to learn an extra step in the start up/shut down procedure.  In truth, the MSF still teaches to manipulate the fuel valve when applicable.

[nurms]

Hi there! Your GS has the old bad petcock, same as mine. That petcock were replaced in 1999 by suzuki because it is not reliable. The problem is that the membrane softens (that happened to me and I had to also run with pri) in a way that vacuum is not strong enough to open it. The vacuum line is infact restricted in the petcock by blocking one of the holes with some black rubber.

Other problem is that the membrane is so old, the piston/plate of the membrane will eventually wear/grind a hole to the rubber part of the membrane. The grind is due to the fact that flow comes in and leaves from the same side. It is just poor desing I think. My friends gs had this problem. The vacuum then sucks fuel straight in to the left cylinder and the spark plug will eventually break down. You can identify this because your bike will run poorly (with only one cylinder) and the left spark plug is all black and maybe even wet and the right will be ok. I have just replaced those petcock with on/off model. Tourmax sells new replicas of that original petcock for about fifty euros! I might buy that.

Cheers,

[Sporty]

 The passage for prime is small and has a sharp turn. It's a small drilled passage that bypasses the diaphragm.
Fuel volume / delivery is reduced.

You maybe experience a loss of power. It would be more noticeable during high RPM operation or climbing grades with Wide Open Throttle.