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, 201015 yr by Hammer

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.

tried that also, even removed the tube and still the same

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.

blocked banjo filter? its volume of gas you don't have

I would follow copemechs advice first thought was HT problem

I would follow copemechs advice first thought was HT problem

ht doesn't know what gear its in though?

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.

Checked the gap on the old plug it was .025 thinking it might be a weak coil

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.

The Spark Plug was the problem

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, 201015 yr by copemech