zippy

Check your rear wheel bearings for play. I was in an event and had a horrible clicking noise, so much I thought something was wrong in the clutch / transmission. I went so far as to pull the clutch cover off in the paddock to investigate. I found nothing, DNF the event. later discovered the rear wheel bearings were sloppy as all hell and was causing the rear sprocket to move out of alignment enough that the chain would "click" off the edge of the sprocket. I couldn't really hear the whine, so I would say it's probably normal. I can't remember if you state you changed gearbox oil or not, watch for tremendous amount of metal shavings or chunks on the drain magnet. Some small gook type build up is normal.

use the long end of the Allen key to access the bolt. then use a wrench (5mm I think) on the Allen key to loosen. It is easier with "ball-end" Allen keys.

The Hell Team has quite a bit of information for GasGas https://www.thehellteam.com/technical-support/gas-gas-parts-and-service-manuals 1998 Parts book: https://drive.google.com/drive/folders/0B0Mfile5AhnmcjY5NXFOaF9ZaVU?resourcekey=0-pVgEGHcAoWYLR7cvC2r7fA

What I typically aim for on chain tension on my 2015 GasGas: Bike on stand, wheel off ground. Able to place 1 finger between top of chain tensioner block and swingarm. From the back, look at tire and chain, give the tire a spin and make sure the chain runs straight and the rear sprocket is not crooked.

I had forgotten about the difference in reed valve intakes

the Keihin carb should attach directly to the airbox rubber, no adapter. But it can be a pain in the ass to do. the adapter was for the smaller diameter Dellorto. GasGas Pro models 03 - 08 frame attached parts should fit GasGas Pro models 03 - 15 Engine parts should be interchangeable (Possibly as late as 2021, but not positive) Good resource for information at The Hell Team website https://www.thehellteam.com/technical-support/gas-gas-parts-and-service-manuals

Are you saying if you are in 4th gear and give the bike a lot of throttle (gas) the bike will shift on its own into 5th and if in 5th gear it will shift on its own into 4th? There is a known "clunk" and a little hard shift between 4th and 5th and back down from 5th to 4th, that is just a characteristic of the GasGas Pro transmission.

Probably a "certificate" is out of date.

Sorry, best I can do is Australian https://www.thehellteam.com/technical-support/gas-gas-parts-and-service-manuals

You have the TXT model before the Pro model came out. Frame matches the frame in the first photo I posted, look closely at the footpeg area. Also The exhaust header where it enters the midbox is horizontal, the Pro in angled at that point. The radiator on yours is "inside" the frame, the Pro the radiator sides are visible. 250cc vs 280cc, The Pro model has a number in the casting of the cylinder. Shifter side, 25 = 250cc; 28 = 280cc I don't know if the TXT model also has that number in the cylinder casting.

Throw up a photo and the hive mind will be able to tell you. Quick reference - there are other differences but this is quick: TXT model 2002 and before has a longer shift lever 2002 and later Pro model has a shorter shift lever Also frame differences

What is the finger height measurement? If it is too low there will not be enough leverage to disengage the clutch. this is from the 2015 users manual, finger height is 17.5mm +1mm -0mm, other years may have slightly different measurement specs. Credit goes to the Hell Team https://www.thehellteam.com/technical-support/gas-gas-parts-and-service-manuals

You just recently had the top end done. Was the bike running fine before that? definition of fine for this case: continuous running, only shutting off when rider wished to do so. was the engine removed from the frame or only the barrel, head, and piston removed? Start looking at what items were disturbed, disconnected, etc. Any sharp bends in the wiring that could have broken wire inside the insulation.

I think @Andy may have moved on to other endeavors. It has been a long time since I have seen any activity by him. I am not sure how often this site is monitored by him anymore.

and it doesn't take much to shorten the bars to where it is comfortable. I think I cut 1/2 inch off each side. Start small, you can always cut more off, a bit harder to add it back on. Risers won't mess with your ability to learn, being uncomfortable and in pain will mess with your ability to learn. Down side of using risers with low bend bars, it brings the center of the bar up so there is more possibility of hitting your body with the bars on big ups. Bars with a taller bend will give height but keep center of bars out of the way of the body.