TY175 Carb jetting question

[Takhli44]

I recently bought 2 175s, the old beater that runs really well has stock carb and 180 main jet as well as Boyesen reeds. Pulls well from very low RPM on steep hill in 2nd gear. The beauty has Boyesen reeds, BJ racing carb (small bore) runs well and very smooth but will die under the conditions above. Will make it in 1st. Bikes have very similar gearing. Thought I would try stock carb that came with beauty, it has more snap down low and idles well but is so rich on top that it will load up and hardly run under full throttle.

Tried in order, 180...170...160...150...140...100 main jets (came with 240) all were too rich but 100 is getting pretty good. Ran full throttle for a couple hundred yards in 5th gear, pulled in clutch, hit kill switch, pulled plug and it looked normal except for a little black on one side. The jet needle is one groove lean from center and pilot is #25.

I have several more smaller jets in increments of 10 but it seems that I have moved a long way and may have something else going on. Any ideas or suggestions or just keep going smaller? It runs pretty good with the 100 but still 4 strokes a bit on top. Perhaps I should run the BJ carb and avoid very low RPM in 2nd?

I should add that I live at 4000 foot elevation and ride most trials between 5500 and 9000 ft. both have identical stock timing of .070 inch BTDC, plugs are.028 inch. Is there a timing that works better than stock?

I switched to BJ carb for now and may switch beater and beauty carbs to see what happens.

[TriCub]

Is the stock carb the type with the jet in the side. If so the "O" ring seal on the jet holder may not be sealing properly.

[Takhli44]

The jet holder serves as the float bowl drain. The "O" ring appears to be in good order. For the 180 jet I used the jet and holder from the other bike and was still too rich which should eliminate the holder and "O" ring as the problem. Thanks for the reply TriCub.

One other difference is beater has a dent in the exhaust just past center of sharpest bend that likely reduces the diameter 25% but this is the strong runner.

[TriCub]

The jet holder serves as the float bowl drain. The "O" ring appears to be in good order. For the 180 jet I used the jet and holder from the other bike and was still too rich which should eliminate the holder and "O" ring as the problem. Thanks for the reply TriCub.

Isn't there an oring at the bottom of the needle jet, to seal between it and the float bowl?

[Takhli44]

Yes there is, I'll look into that. That makes sense that a leak there would cause a rich mixture. Thanks again TriCub.

[Takhli44]

Well TriCub, you called it...pulled the float bowl and the O ring on the jet needle surround looked flat so took the surround out and it would drop into its seat with zero resistance and would even rattle a bit. Now fixed up with a new O ring I will put it back together, go back to a larger jet and go from there.

Trials has a great community and this forum has been particularly good in sharing information.... Thanks to all who considered this.

Went to 240 main jet, runs good.