3120XP runs lean after 272XP coil swap (problem resolved with a 1.18mm drill bit)

[John Stryker]

This saws carb has a fixed high jet, and after the coil swap, wide open throttle rpms reach over 13,000. Even with the low backed out to 3 1/4 turns, the rpms are scaring me at 13,100. Does the high jet need to be drilled out to a larger size? Or is there another solution to getting more fuel? Thanks.

 

[Evan]

Put the limited coil back in untill you can tune carb.

 

[John Stryker]

Anyone who's drilled out the high jet, could you please share your results? Thanks.

 

[RedneckChainsawRepair]

@huskihl you doing the H side installs yet?


I know long ago guys would put in a H side for 25. No clue what one would get today.

Yes you can also just open your fixed jet too.

This one had a H side added.

 

[huskihl]

@huskihl you doing the H side installs yet?


I know long ago guys would put in a H side for 25. No clue what one would get today.

Yes you can also just open your fixed jet too.

This one had a H side added.

View attachment 1047708

Yeah I’ve been doing it for a few years now for people

 

[huskihl]

This saws carb has a fixed high jet, and after the coil swap, wide open throttle rpms reach over 13,000. Even with the low backed out to 3 1/4 turns, the rpms are scaring me at 13,100. Does the high jet need to be drilled out to a larger size? Or is there another solution to getting more fuel? Thanks.

You can drill the H jet though so that it’s plenty rich in the summer and that should get you still pretty close in the winter. Or you can have someone add an adjustable H jet

 

[John Stryker]

You can drill the H jet though so that it’s plenty rich in the summer and that should get you still pretty close in the winter. Or you can have someone add an adjustable H jet

If I drilled it the next size up, do you think I would be able to use the Low screw to fine tune it? I imagine a H screw installed would make it a lot more tuneable.

 

[huskihl]

There is an overlap in what each screw does, which is why you can make a slight difference with the L speed screw on higher RPMs. Because of the way the circuitry is inside the carburetor, adding the H jet the way I do it only adds fuel to your existing jet. Meaning the new H screw turned all the way in will be exactly like you have now. It won’t turn off all the high speed fuel

 

[John Stryker]

There is an overlap in what each crew does, which is why you can make a slight difference with the L speed screw on higher RPMs. Because of the way the circuitry is inside the carburetor, adding the H jet the way I do it only adds fuel to your existing jet. Meaning the new H screw turned all the way in will be exactly like you have now. It won’t turn off all the high speed fuel

Thanks for sharing your wisdom. I may try to drill the H jet a wee tiny bit and see, and hope I don't wreck it.

 

[huskihl]

Thanks for sharing your wisdom. I may try to drill the H jet a wee tiny bit and see, and hope I don't wreck it.

There are larger and smaller replacement jets available if it’s one of the newer carbs

 

[John Stryker]

Hi everyone. An update that seems to have solved the lean running H fixed jet. I figured out that the H jet has a nozzle that is around 1.12mm inside diameter at the tightest part. I tried drilling a 1.14mm hole, which made no real difference. So my final drill size that I used was 1.18mm, and that seems to be just about right. WOT is around 12,500 rpm. That is with the L jet turned out about 1 1/2 turns. Problem solved!

 

[John Stryker]

The drill bit I used was a 1.24mm bit beltsanded down to reduce the diameter to around 1.18mm. I knew I'd never find the right drill bit, so I made my own, being the cheapskate I am, lol.