I have posted a few threads about the problem I am having getting my bike (2007 WR450) running right, and now that I have implemented more modifications and readjusted the settings I thought it would be best to summarise my problem in one thread with all information and work I have done as I still can't solve the problem I am having and I don't know what else to try at this stage. The Kran and others been very helpful on the threads and Guy Graham has provided extensive help in messages. All of which I am grateful for. I just thought though that I would put this out there one more time to see if there is anything I have missed or could try.
The problem is when twisting the throttle off idle the bike sometimes instantly stalls with a small backfire that kicks back on the starter motor. It often but not always happens when I am blipping the throttle with the clutch in or when I am slipping the clutch. It is almost as if when you rev the throttle just as the revs drop and hit idle you rev it again, that is when the bike stalls and backfires before any suggestion of a bog. It is almost like the sudden drop in revs followed by the sudden rise in revs again is too much or adds too much fuel or too much air to the equation before the idle can balance itself.
The problem becomes clearest when I want stop or do a tight turn or slip the clutch to try and move slowly. I pull in the clutch, but because the clutch drags slightly it puts a small amount of strain/drag on the engine which then stalls the bike unless I blip the throttle or give off idle revs. But blipping and off idle revs causes it to stall in this mysterious way. It makes it dangerous as it can stall at exactly the wrong moment, like at a road junction or difficult slow crossing/obstacle.
The problem has been there since I bought the bike, after cleaning the carb and after the modifications I have done. The latest carb modifications have helped slightly but the problem is still there. This is the spec and work I have done to the bike. I should point out that the only thing I have just done but not run the bike since, is clearing the pilot and fuel screw passages with wire to confirm not restricted (the passage was not visibly blocked and this was done to make sure there was no blockage narrowing the passages).
Airbox lid removed, baffle out of exhaust, shorter throttle stop, grey wire
Carb thoroughly cleaned with both carb and brake cleaner and all passages and jets blown out
New jets, 168 main, 45 pilot, 50 leak, 110 air, 70 air, 65 starter, JD red needle
Adjustable fuel screw fitted
GYTR AIS removal kit installed
2008+ AP diaphragm, rod, and cover (with original float bowl check valve removed)
Stiffer AP spring fitted and timing on AP set with strong, early, good squirt visible
Float height checked and within spec
Throttle valve plate seal is present and appears fine
The fuel screw seems to run best at half a turn out but is slightly vague and the change is very subtle. When warmed up, I tried opening the choke and the revs increased almost as if someone twisted the throttle. It didn't make the bike stall. If I pull the hot start instead of the choke, the bike does not stall but does run a little rougher. It can also be tricky to start, often wanting a slight bit of throttle twist.
The previous owner had 170 main and 50 pilot jet and had not fully uncorked the bike. I don't know if he knew something I don't. However, I am completely out of ideas of what to try now and I am getting tired of taking the bike apart and trying to sort this. I have no confidence in the bike with it as it is because it can stall at the most dangerous wrong moments.
I made a couple of short videos which show the problem after all new carb parts. The first is with the idle screw set where I initially thought it should be. The second video is with the idle raised slightly by about a sixth of a turn. In the first video the stall is very much what I'm talking about, although I'm blipping the throttle the point at which it stalls does not seem that harsh when actually doing it. The second video, the bike only stalled because I purposely whacked the throttle open at the end to try and get it to bog going to wide open, the bike bogs though before it stalls like you would expect. I also made a video of the AP squirt.