Carb tuning tip

Carb tuning tip:

If your carb is backfiring heavy when you let off on the gas after being at wide open throttle, your pilot jet may be too large. If it's lightly popping when revving down, the pilot jet may be too small.

When properly jetted, your motor should rev smoothly up from idle to wide open, and back down.

If your motor is popping or bogging when you're on the throttle, it's usually an incorrect main jet size, dirty carburetor, poor fuel or fuel supply, or incorrectly adjusted valves.

The pilot jet controls the volume of fuel delivery in your carburetor from idle to approximately 1/2 throttle, when the main jet takes over.

If you've upgraded your carburetor, air intake, or exhaust, you may find you're running leaner when idling. A larger pilot jet can help restore the correct air/fuel ratio at low speeds.

Most stock 150cc carburetors come with a size 35 pilot jet; Pilot jets should be increased in size in single steps: 35 to a 38, 38 to a 40, etc until you reach the correct air/fuel ratio when idling and at full throttle. (Courtesy of buggypartsnw)

16 comments
  • A wideband will make it easy to tune. I run a koso. I've spent lots of time tuning and I can get them close but the wideband lets you get it dialed.

  • but people are cheap and dont want to spend money on a Wideband. O2 sensor and getting the Bunghole done LMAO

  • Thanks man, I screenshot this post lol

  • Eric YaboyEzy

  • What is a wideband?

  • Wideband is o2 sensor and bunghole that you put into your exhaust and hook up to you laptop. Like they do on cars when it's on a dyno.

  • Wideband let's you spank everybody.

  • You still scoot? Another day of riding with the top 3 bikes in KC and you are no where to be found...

  • I saw the pics from Sunday. 3 people with my number and nobody calls me. It's like you get tired of loosing

  • Walt Jiminez this post helped me understand a lot. I'm in the process of rejetting now. 150cc the carb I got from you, uni air filter, ngk spark plug, oml ignition coil, replaced all the air and fuel lines. I rejet the main from 105 to 108 still burning rich, 108 to 110 still burning rich. About to check the spark plug again. The issue I'm having is once I get to about 35 mph the throttle does nothing past the point. If I give it throttle it eventually chokes out and dies on me. Is this an issue related to rejetting or am I chasing my tail? Any help would be much appreciated!

  • if its dieing at 35 then its probably not getting the fuel needed to continue combustion. i would be at 108. make sure you are using quality fuel if possible Non ethanol fuel. Also check the fuel pump. They clog up. could be the issue as to why its not able to pump the fuel to the carb at higher speeds.

  • I can only get to 50 with mine I tried changing jets I am at 110 also right now before the jet though I was doing 54 with a 150cc still can't figure it out

  • Victor Bustillos why did you change the jet?

  • I could get nothing passed 53 my old bike did 58 then shortened my stock exhaust and cut the intake tube with open filter and it started popping now the popping is gone but I lost top end

  • Oh yeah new mikuni fuel pump also, i was thinking the same loss of fuel. The lines have fuel in them when it dies. I'll go back to 108, I think it ran better there. I'll check out the fuel pump as well. Thanks Walt

  • Ok so I have a stock 150 with upgraded exhaust and intake. Brand new carb, switched out the main jet to a 108 and the idle jet to a 38. I have tuned the carb to where it is not bogging down wot, but it is popping when I left off. My valve are set at .4/.5 and I have a new fuel pump on the way. So do I need to go bigger on the jets to stop the popping?