hammer Posted September 26, 2010 Report Share Posted September 26, 2010 (edited) 2008 2.9 Thought I had carb issuse but have swaped carbs of same brand and size, what is happening is it revs fine 1st thru 3rd but in 4th and 5th under a load the thing just falls on its face like its run out of fuel or has lost spark. I can pull clutch in and it will rev fine with no load. It does this from 1/4 to W/O any idea what the problem is? 118 main 38 pilot Edited September 26, 2010 by Hammer Quote Link to comment Share on other sites More sharing options...
ishy Posted September 27, 2010 Report Share Posted September 27, 2010 2008 2.9 Thought I had carb issuse but have swaped carbs of same brand and size, what is happening is it revs fine 1st thru 3rd but in 4th and 5th under a load the thing just falls on its face like its run out of fuel or has lost spark. I can pull clutch in and it will rev fine with no load. It does this from 1/4 to W/O any idea what the problem is? 118 main 38 pilot I would check it isn't getting starved for fuel before the carb, take the fuel tap off and take the thin tube from under the fuel tap filter and cut it in half, then stick one half back in. Quote Link to comment Share on other sites More sharing options...
hammer Posted September 27, 2010 Author Report Share Posted September 27, 2010 tried that also, even removed the tube and still the same Quote Link to comment Share on other sites More sharing options...
copemech Posted September 27, 2010 Report Share Posted September 27, 2010 tried that also, even removed the tube and still the same Well, if you are indeed sure of your supply side fuel, tank venting and carb, I would indeed be looking toward the ignition side of things. Sparkie should be NGK BP5ES, at a gap of .020-.025 max, Non resistor plug. A weak coil, poor grounding, or poor connections to the plug cap could also cause this. Might also disconnect kill button. Put simply, the high engine load conditions also cause the highest load on the ignition system. The little magneto type thingie has no real excess ability to overcome excess resistance of any type. So if one happened to throw in a sparkie at a wide gap, the system will not support it. Quote Link to comment Share on other sites More sharing options...
nigel dabster Posted September 27, 2010 Report Share Posted September 27, 2010 blocked banjo filter? its volume of gas you don't have Quote Link to comment Share on other sites More sharing options...
chewy Posted September 27, 2010 Report Share Posted September 27, 2010 I would follow copemechs advice first thought was HT problem Quote Link to comment Share on other sites More sharing options...
nigel dabster Posted September 27, 2010 Report Share Posted September 27, 2010 I would follow copemechs advice first thought was HT problem ht doesn't know what gear its in though? Quote Link to comment Share on other sites More sharing options...
1oldbanjo Posted September 27, 2010 Report Share Posted September 27, 2010 Find a long smooth hill and put the bike in 4th or 5th gear at low revs and low throttle. Then accelerate under full throttle and see what happens. If the bike accelerates smoothly and then just sort of hits a wall and won't go any faster.....I suspect fuel flow. If the engine instead abrubtly coughs, sputters, backfires......I would suspect ignition. Fuel flow problems generally have a softer onset then ignition troubles. Also a fuel flow problem will go away with just a few seconds off the throttle - while ignition trouble will probably remain as soon as you get back on the throttle. Since changing carbs did not solve the problem.....I would suspect you have a fuel flow problem in the tank assembly. Try using the tank/fuel valve/cap from another bike and see if the problem has gone away. Quote Link to comment Share on other sites More sharing options...
hammer Posted September 28, 2010 Author Report Share Posted September 28, 2010 Checked the gap on the old plug it was .025 thinking it might be a weak coil Quote Link to comment Share on other sites More sharing options...
copemech Posted September 29, 2010 Report Share Posted September 29, 2010 Checked the gap on the old plug it was .025 thinking it might be a weak coil There should be a little ground terminal wire on your CDI unit that goes to the mount. Be sure it is clean and tight. Many complain that spark is hard to see on a trials bike, yet generally this is because the plug gap is small, and the room is not dark. In reality, they will throw a long spark if one uses a special little tester. Should cover a good 6-8mm gap if i figure correctly. Have not directly tried it on the bike, but have done it many times on other magneto based systems. They must be able to overcome the added resistance of the air density under compression, which in this case is at least 10 times that of normal atmospheric pressure, thus let's say .250in, out of the motor and 1 atmosphere of pressure. Seems they are rated at about 20k volt as i recall. Quote Link to comment Share on other sites More sharing options...
hammer Posted October 3, 2010 Author Report Share Posted October 3, 2010 The Spark Plug was the problem Quote Link to comment Share on other sites More sharing options...
copemech Posted October 3, 2010 Report Share Posted October 3, 2010 (edited) The Spark Plug was the problem Well, there you go! No, plugs do not know what gear they are in, but they(and the rest of it)can react adversly to higher engine loads(in higher gears) as the internal pressures increase and must be overcome it creates more demand upon the ignition. In a jicky plug, the electrical path will divert to least resistance. Thanks for the feedback! Edited October 3, 2010 by copemech Quote Link to comment Share on other sites More sharing options...
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