another jetting question

about 2 months ago i post on here about my jetting all ****ed up. its pretty much right now. i have a 170 main, 42 pilot. when i had the problem before, it was popping and back firing on the spot when i was reving it. that problem is fixed now.

but when i am riding and i let the throttle off it does all these little pops, is the bike still a tad too lean ? should i put in a 45 pilot jet ?

only does it under engine braking.

What is the elevation and temp where you ride.

sea level, around 20-30 degrees celcius. not faranheit

although, at this other mx track which would have to big quite high up. maybe 50 metres about sea level, i havent noticed it. not that i can remember but.

i have the perfect carb setting on my bike, and so my brothers, we have the same pilots and everything...but mine is making those "popping" noice you said, and he´s does not! Mine also do it when i´m standing still and reving the motor up, then when the rpm goes down, those popping noices are comming again!

But not my brothers bike...i don´t know what the **** that is!

Whould also be habby if someone could tell what it is :naughty:

If your head pipe is not sealing to your head good it can suck in air when you close the throttle and create a poping noise as the cooler air sucked in exspands. Pull your head pipe off and reinstall it using high temp. silicone to make a good seal where it slips on the head. I use permatex ultra copper on my ktm pipe. Good luck, hope this fixes it for you. :naughty:

Popping on decel is generally to rich. raw fuel is still in the exhaust port/head pipe and the heat from the pipe ignites it and 'pops'. usually you can solve this by leaning out your air/fuel screw. if the bike was too lean it would hesitate on acceleration or be flat.

please explain your theory , this i have got to hear., also if you don't mind let me in on the seceret of where you learnt this


In my opinion, for whatever it's worth, I would check your fuel screw. Stock setting is 1 1/2 turns out, I would go another 1/2 turn and see if that cures your problem. To me what you have is a lean backfire. What makes adjusting the fuel screw really easy is a Scott's fuel screw. It has a "T" handle, so adjusting it wherever can be done sitting on the bike and using the handle position for reference small adjustments are easy and if they are wrong you can quickly go back to the previous setting.


im with you merlinn


the theory is what i stated. if the bike was lean it would accelerate poorly, when fuel was being delivered. when you are on overrun or decel, the raw fuel is igniting when it is in the hot exhaust. usually your fuel screw can cause/fix this (barring the fact that if any tuning was done prior it wasnt totally fackered up in the low speed jet/needle area). and, in reading 'motorcycle fuel systems techbook' is states for rich mixtures "there may be popping and banging in the exhaust on the overrun as unburnt fuel lights on the hot exhaust". lean mixture would "poor acceleration and tendency to spit back in the carb" (or airbox).

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