Oh where to begin...

So I keep having a surging (feels like it's lean) issue at wide open throttle, and most noticeable at highway speeds with a top speed of 90mph on the speedometer. When I purchased the bike it ran perfect and this surging originally started once I added a lunch box air filter and rejetted. I tried everything to get rid of the surge (different jettings/shims), so I ended up putting the original main jets and air filter back on. So my setup now is all stock with the exception of a petcock off a honda crf 250x. The surging is still there and here's what I've tried:

gs500f petcock, crf 250x petcock, original petcock
created a shroud to put around the carb vent tube, and moved the tube in all different places
cleaned carbs (many times)
adjusted float levels
replace spark plugs (ngk)
cleaned tank vent hole
checked for vacuum leaks
checked valve clearance and all are in spec
checked for pinched fuel lines
I've gone through several tanks with different gas
adjusting throttle cable
checking for pinholes in the carb diaphram, and both doughnut gaskets are in place

I checked the compression today right cyl dry:150psi left cyl dry:120psi wet:180psi

Could a difference in cylinder compressions be causing the surging? The compression is still in spec, but just barely.
The motorcycle is a 1995 with only 8k miles on it. So I'm surprised by the low compression.

I haven't done any plug chops to see the coloration, but I should/will.

Just seeing if anyone else has any other ideas. Thanks for your help.

perhaps a clogged fuel line perhaps?

Carb vent hose/crosswind stumble maybe?

I've switched the fuel lines around (mostly because of the routing on the 500f petcock) to no avail, so I don't think there's any blockage and it's the same fuel line that was on it originally when it was running fine.

I put a shroud made of plastic around the carb vent tube to block the wind and now have it routed back to the battery. When I purchased the bike the vent tube was actually pointing into the wind and it didn't give me problems.

:/

Thanks for the responses.

Has anyone experienced sticking needle slides before? I'm reading the gixxer board and that seems a possible issue since this surging began after I cleaned the carbs.

Have you synched the carbs?  Having that much difference in compression cylinder to cylinder may be highlighted by out of synch carbs.

I haven't tried syncing them. I'm not certain, but I don't think I messed with the adjustment screw. I will try syncing them though and see if that helps.

I forgot to mention that I've seen a decrease in fuel milage too. I was getting upwards of 70mpg now it's around 55-60. Reading the Haynes manual on synchronization, it says decreased fuel mileage as a symptom.

Solved! Vacuum leak from the throttle body shaft on the right carb (the e-clip). Thanks to Soloratov and this thread: http://gstwins.com/gsboard/index.php?topic=60170.msg692466#msg692466

Quote from: Slack on May 05, 2013, 10:08:41 AM
Have you synched the carbs?  Having that much difference in compression cylinder to cylinder may be highlighted by out of synch carbs.

New to the site and this is an old thread, but............
synching the carbs have NOTHING to do with compression testing. You check the compression with WOT so you don't restrict air into the cylinders. 

Ace, Slack was referring to the possibility of the combination of both issues (carbs out of sync & different compression on each cylinder) being the cause of stumbling at higher rpm. He wasn't saying the carbs needed to be sync'd in order to get proper compression tests.

OK, if you say so..............