jetting problems on a yamaha dtr50 2007 model. what too do?

Question

i have de restricted the bike by cutting out the cone in the exhaust and cutting a larger hole in the airbox. i then put a 105 moain ejt int eh bike. but it melted a plug. too richen it up a bit, we put a bit of air filter foam over the airbox inlet. seemed too help. anyway, today we got a 120 jet, removed the foam and went down the road. got upto about 70kmh and it just went brrrhhh then died. feels much like its seizing, except it doesnt. i held on. then i let off, and the revs drop and it runs mint again. the plug is white. and now i ahve put the foam back on the airbox, it ticks over slightly lower.and afetr about 1/2 throttle going along it hesitates and misses.
helllpp!

right, went back too a 105 main jet. and foam over the air box. and now it runs but very veyr badly. it stutters and jumps. misses etc. how can this be when i managed too ride 60 miles on this setting the other day?
it cant JUST be jetting, can it?
the speedo has 4 settings, they aren ttalked about in the manual. could these affect ignition timing at all?
its the same bike as the malagutti 50 if that helps. minarelli am6 engine. mainly malagutti parts.
some one, please help.

Answer 1

The first thing you need to do is get the right main jet. Keep in mind the main jet alone determines the fuel-air mixture at wide open throttle only. After it's warmed up good, put in a new spark plug and run it wide open under load for maybe a half mile. Don't put around for a while before the wide open thing because you want to be running wide open immediately with a clean plug. Too lean (too small a jet) will cause a loss of power and if very much too lean, the engine will appear to miss or stumble. When it does, the will tend to jerk sharply. If too rich, you may smell exhaust and it too will miss and stumble, but in a more sluggish manner and the misses won't feel sharp like when too lean. Run it wide open for a bit, then shut it down and coast to a stop. Remove the spark plug, look closely and if too lean, the plug will be absolutely soot free and the white porcelain part of the plug will probably have very tiny white blisters. If too rich, the soot will be evident. No soot but dark colored means a bit on the rich side and grayish white but no blisters will mean a bit lean. After you make a change in jetting, use a new plug and repeat. The reason for using a new plug is because they nice and white, clean and all the same color. A used or discolored plug can throw your determination.

After you get the main jet right, you can then adjust the jet needle to correct the mid range mixture. I don't know if your bike had constant velocity carbs or throttle slide carbs. The former will have a big jar lid like affair screwed on the top while the latter will have a much smaller top and probably be more rectangular in shape. If running too lean, you can shim under the needle to raise the needle and thus richen the mixture. If you're mechanically adept, you can figure out how to remove the throttle slide but if not, you better get a manual or someone who knows. In the good old days the needles had grooves and a circlip, when you changed the position of the clip you raised or lowered the needle. If yours has only one position, you can make shims out of a soda can to put under the needle to raise it slightly. If you need to lower it, you can sometimes place the shim on top of the circlip to force it down, otherwise 'll have to get a different or aftermarket needle. The needle is the major controller of the 1/4-3/4 throttle position.

Make sure you're using the correct heat range plug....melting a plug was WAY too lean and you're lucky you didn't melt the piston. Also make sure the fuel level in the float bowl is correct, lower than normal will lean out a mixture and higher will richen it.

I doubt your bike has an accellerator pump, so twisting the throttle is going to make very little if any difference because gravity and the float valve are the main controller of getting fuel into the bowl. Also, a carb runs under the principle of vacuum and not pressure. Unless you have a turbocharger or supercharger that is, however I seriously doubt that. Don't worry about the air box, with enough time and effort, you can jet the carb to run under any condition...very restrictive air box, no air box, restrictive exhaust, straight pipe and any combination of those. It's time consuming and kind of a pain, but by not making too many major changes to the stock setup, you increase the odds of being able to use the stock idle jet and needle with the main change being the main jet. Carb parts are too expensive to be buying them just to experiment with.

Answer 2

Well in this case i would suggest you to change the air box. The jetting problem will continue due to the lapse of air pressure inside the carburettor. If u find the engine seizing then try to roll ur accelarator in quick movements to get the fuel level back in carburettor.. if still the problem persists then search out for a mechanic for sure... otherwise u will end up wasting ur fuel for no good reason..