The technical term is NPSH (net positive suction head) The definition of NPSHA is simple: Static head + surface pressure head - the vapor pressure of gasoline - the friction losses in the inlet strainer.
But to really understand it, you first have to understand a couple of other concepts: * Cavitation is what net positive suction head (NPSH) is all about. The gasoline boils in the pump inlet. * Vapor Pressure of gasoline varies with the fluid's temperature and elevation. As I was leaving southern British Columbia, it was about 2pm in the afternoon and the temperature was reaching the low nineties. The elevations were in the 5 to 7 thousand foot range and the bike started stalling out again. It was like some one would turn a switch off and on to the fuel supply every 10 seconds. Also the problem became worse if I would sit still and allow the bike to idle while doing something or stuck in traffic. The gasoline was heated up by the engine heat and it would start to idle rough and kill. Off to my right was an old farm house surrounded my trees. I pulled the bike in and knocked on the door. Explained I was having mechanical problems and asked if they had a daughter......sorry that is another story...:>) The gentleman coming to the door said I was welcome to work on the bike in his front yard and asked if I would like a cold glass of water. Nice people. What I found throughout the trip. Actually the farm house was being rented by a railroad engineer and his family. It was a 200 year old log cabin that was once a stage coach stop. Interesting history. I started pulling off luggage and removing the plastic panels around the gas tank. Not a minor job. I felt the problem had to be inside the fuel tank. Why would it start happening on the way back from the ride and not be a problem on the way in? Something had to have changed since I wasn't having the problem on the way up. I removed the fuel pump plate and assembly from the gas tank. Let me add, my problem may be slightly unique in that I have a modified gas tank with an additional nearly 2 gallon capacity. I hold 7.4 gallons. The tank has been bumped out in all areas and voids under the plastic cover for long distance riding. I may be picking up some additional engine heat because of these mods. Some of the problem may have been caused by this fact, but everything regarding the problem still applies to a stock set up. One thing I soon found was that my modified pump assembly was extended down a few more inches into the deepest section of the tank. The rubber gas hose used to extend it was the wrong type of hose. It was gas line hose but its outside cover was not made to be submerged in gasoline. It had virtually peeled off and lay at the bottom of the tank under the suction strainer. The people living at the farm house took me into town to get a new piece of fuel line. I removed the pieces of hose from the bottom of the tank. I pulled off the pump suction strainer, which is a very fine nylon mesh material covering a plastic frame. I noticed it was visibly showing signs of being pulled, by the pump suction, in against the plastic support frame. When I put the strainer to my mouth and tried to blow air through it, it appeared to be nearly totally blinded. Even though it didn't visually look dirty. The nylon weave material probably absorbed water and expanded. Nylon absorbs water. The pump was working against a nearly closed off suction. I imagine all GTS owners will find this is the same condition of their gas line strainer on 7 and 8 year old bikes. With the pump submerged in the gas tank, NPSH, doesn't become a problem until the pumps NPSH required is exceeded. This was being caused by the elevations I was riding at and the high ambient temperatures, casing the gasoline to become much hotter than normal. Both of which were now allow the gasoline to boil off between the strainer and pump....vapor lock. What had probably happened was the poor quality gasoline I was using in BC and AK, caused further blinding of the strainer. It is also possible the fuel water absorbing additive caused the nylon mess to tighten up its weave. It got just to that point were the slightest elevation and temperature change during the ride was now exceeding the minimum NPSH required by the pump. I tried a temporary fix, not having a replacement strainer. I used a needle and punched a bunch of holes in the strainer in hopes of allowing more gasoline to flow to the pump. It only helped marginally. The pump pumps a rather high flow rate to the injectors and most of the gasoline returns to the tank unused. I still found I had to stop and top off the tank to over come the vapor locking while at higher elevations. When it cooled off in the evening and as I got back down to lower elevations the problem went away. I have a new suction strainer on order. I recommended all GTS owners replace theirs if they have any plans to ride at higher elevations in the summer. The gas tank on the GTS is completely covered with plastic and doesn't cool down as effectively as an exposed metal tank would. Just put your hand on the gas cap after you have sat in traffic to see what I mean. hawke
