My brother and I are trying to fix his 2002 GS500E.

We were on a bike ride a couple of weeks ago, when his bike suddenly died.  As soon as you twist the throttle, the bike would die.  We managed to get home as the bike still ran on choke - however any twist of the throttle would bog the engine down and die.

Here's a video: https://drive.google.com/open?id=1-i1NEbmlYhSVv89wqGgamk72q05ylXCF

We know the carbs are the issue, as I have an 08 GS500F and have removed my carbs and put them on his, and suddenly everything works perfectly.  We were still using his airbox and petcock.

So far we have tried absolutely everything we can think of to get the carbs running, we must have disassembled them about 10 times now!

The main jet and the bucket the needle valve sits in were a bit clogged - we cleaned these out but no change.  The diaphragms are in great shape as well.

So today after getting fed up of guessing, we decided to strip my carbs and move over all the parts to see if this made any difference.

We moved the following parts:  Diaphragm + Needle, Float, Needle Valve + Bucket + all 3 jets.  Then feeling fairly positive we put it all back together and guess what? Still the same issue...

So what am I missing here? I feel like we've been fairly comprehensive in our process but we can't solve it.

Interestingly I ran my carbs on my working GS500F without the airbox on, and the bike bogged down on throttle just like my brother's GS500E.  Could this mean we have a massive air leak? I don't know.

Things I feel we can rule out due to my carbs workings are the airbox, the petcock and the rubber boots the carbs go inside on the engine.

So does anyone have anything extra they can add to this? I am completely stumped!

Thanks in advance 

Air leak, vacuum leak, stuck float needle valve, low floats, clogged passage in carb, failing petcock, missing o ring somewhere. Hundred other things.