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Tc_01
06-04-2016, 10:54 PM
Short version of the question:

When my needle clip was at the top slot, my 250r was bogging really really bad at acceleration, until the higher rpm's. Then i moved the clip to the bottom slot and it accelerates much better, but still has a significant bog..does this mean i need a richer needle? Or richer main jet?

Ive gotten confused with the different threads ive read.

Long version of the question:

I started trying to jet my mikuni tm38 on my 84 250r. Originally, when I tried to accelerate, it was really doggish at low to mid rpms. Once it hit higher rpms, it took off very strong. I originally had the E-clip on the center notch of the needle. I meant to go richer by putting the clip at the bottom notch, but because i was paying attention to two things at once, i accidentally put it at the top notch. Acceleration got significantly worse. I took it apart to move the clip the other direction. This is when i realized i moved it to a leaner position rather than richer. So this time i moved it to the richest position.. it accelerates much better than the previous two positions, but it still bogs significantly.

Ive read that the main jet only affects 3/4 to full throttle so based on that i would say i need a richer needle..but ive also read that if your clip is at the richest slot, it means you need the next size bigger main jet. I can kinda understand that because the fuel has to pass through the main jet in order to even get to the needle. And this is only my theory, which could be dead wrong: if the main jet is too restrictive, the needle would need to be in its richest setting to create enough vacuum force at the main jet to pull more fuel through it??

The_Steve_Man
06-05-2016, 10:37 AM
Here is a chart that shows what each part does. You can change the needle jet on those as well I believe. You can put a richer one in. http://uploads.tapatalk-cdn.com/20160605/f35b23c3701388c7ae37be6d93c45181.jpg

Sent from my MotoG3 using Tapatalk

cr480r
06-05-2016, 03:52 PM
The circuits overlap so work from the bottom up. I would put the needle back in the middle position and dial in the pilot circuit before putting my focus onto the needle. It's also important not confuse hi and low rpms with throttle positions.

Tc_01
06-05-2016, 06:53 PM
Thanks for the help guys. Ive gotten it to respond much better. The fact that the circuits overlap is key here, because I used your advice and started dialing in the pilot system. Got a richer pilot jet, because the sprk plug color was suggesting it, and dialed in the air screw correctly. This effectively made the bike pull much harder during early acceleration.

I suppose this is because, although many call it the idle jet, it not only controls idle function, but it plays a pretty big role in early acceleration because during throttle opening, even though youre starting to rely on fuel and air through the jet needle and needle jet, the fuel and air is still getting drawn through the pilot system and its still a high percentage of the mixture being drawn into the motor.

Then i played with the clip position. At middle position it was still lean when i was at half throttle so i adjusted the clip down one notch. After changing these two things, the bike pulled much harder during acceleration. But was still a bit lean at half throttle so i moved the clip down to the bottom slot and now its even stronger and the spark plug is a nice tan color. The acceleration isnt 100% perfect but its very close!

Thanks again for the help! Hopefully this will help other people having trouble as well