1978 gs750 Potential Shim issue?

[Benjisus]

Hello everyone!

I am currently working on 1978 GS750 and it is sooo close to running properly. I just recently had the carbs completely rebuilt and tuned for pods, and the bike runs perfectly.. for a short while.. After about 20-25 minutes of riding, the bike starts to have issues, puttering, possible backfiring? and then it will slowly die on me.
Im hoping someone here might be familiar with the issue? Or could point me in the right direction? I have been told to check out the shims, or potentially the petcock. I checked all the clearances on my shims a few months back and they all seemed to be within the recommended clearance.. but maybe I did not look closely enough?

Thank you all!

 

[allojohn]

Sounds like fuel starvation/petcock/fuel filter or battery depletion. Not likely anything to do with the shims. Check your charging here

 

[Big T]

What method did you use for the shim measurement?
This is not a shim issue
Either fuel issue or,
Are you still running points ignition ?

 

[hannibal]

Did you sync the carbs after you had them rebuilt?

 

[Agemax]

Did you sync the carbs after you had them rebuilt?

How is that likely to cause issues after riding for 20 minutes?

 

[Steve]

Did you sync the carbs after you had them rebuilt?

How is that likely to cause issues after riding for 20 minutes?

Well, it's either an attempt to gather some (apparently non-relevant) information, or just another scatter-gun approach that is often taken here. Seems that just about any problem can be fixed by carb rebuild, carb sync, valve adjustment or a coil relay mod.

I agree, it's not a shim issue, nor a carb sync issue. Dying slowly sort of rules out electrical issue, so I am leaning toward a fuel issue, as well.

He is in Salt Lake City. It tends to be warm there. The phrase "tuned for pods" worries me just a bit. If it's still just a little too lean, and running in that heat, it might be seizing slightly. Have you done plug checks (in spite of the "tune")?

.

 

[Agemax]

Well, it's either an attempt to gather some (apparently non-relevant) information, or just another scatter-gun approach that is often taken here. Seems that just about any problem can be fixed by carb rebuild, carb sync, valve adjustment or a coil relay mod.

I agree, it's not a shim issue, nor a carb sync issue. Dying slowly sort of rules out electrical issue, so I am leaning toward a fuel issue, as well.

He is in Salt Lake City. It tends to be warm there. The phrase "tuned for pods" worries me just a bit. If it's still just a little too lean, and running in that heat, it might be seizing slightly. Have you done plug checks (in spite of the "tune")?

.

Never ceases to amaze me to this day the amount of people who think that synching the carbs can solve so many problems on their bikes, i have seen this diagnosis on issues ranging from bad misfiring to high speed vibration :-k

 

[Steve]

Agreed. The most obvious sign of the need for a sync job is at, and just above, idle speed. By the time you get above 1/4 throttle, the effects any mis-synchronized carbs will be negligible.

.

 

[TeamDar]

Make sure that the crankcase is not overfilled with gasoline. That causes running issues after the motor warms up and starts vaporizing the gas. Dar