if you love a mystery, read this one.

[klrbarnes]

Here is a good one: I have a 1990 KLR 650. I can start the engine without much problem. If I rev it right away, when the engine is absolutely cold, there is no problem. But if I try to rev it after it has warmed for 30 seconds (with or without the choke open) the engine hesitates, sputters, and dies. This happens during the first 1/8 of the throttle twist. I've investigated and cleared some of the usual suspects: 1. There is no water in the fuel. 2. The fuel filters (plastic screens) are properly seated on the petcock (although the one on the reserve port was off for some time. I know that some silt/crap got into the carb before I re-attached it. The carb has since been cleaned). 3. The vacuum tube from carb to petcock is working, and gas is flowing to the carb. 4. The choke lever and cable work properly. 5. Floats and float needle are keeping the resevoir filled properly. 6. I think bad gas is not an option, since it idles ok, and I did ride the bike home after the sudden onset of the problem one night. I've removed, cleaned and reinstalled my carb SIX times now. According to the chart in Redondo Rob's excellent website on the Keihin CVK40 carbureter, 1/8 throttle twist is the where only the pilot screw, the pilot jet, and the needle diameter come into play. The needle moves flawlessly every time I've checked. The pilot screw, spring, washer and rubber "o" ring are clean and adjusted to their pre-problem depth. The pilot jet is clean and clear. I've used small rubber tubes to blow air through the various jets ports and air intake holes. I've checked to make sure air escapes from the desired ports. The only one that seems to be hard to blow air through is the starter jet, which only functions when the choke is open. I've also used steel guitar strings to probe passage ways and found no obstructions on the straight ones. The starter jet passage is way to bendy to stick anything through. OK, that's your crime scene. Any suggestions on the guilty part(y)? Bonus Question: The famed "doohickey" is the timing chain tensioner spring, right? OK, if that breaks, what are the symptoms? A seized engine is the only one I've heard of so far. Many thanks-