Hi Guys

I was on here recently with a query regarding ty forks and you were so helpful I am back now with another query.

I have stripped down the cylinder on my recently bought TY175 and found the piston head stamped with 68. I am assuming this means it has been rebored to 68mm. If so what other alterations should I be looking for or carrying out to go with the rebore. ie; should the jets be bigger and if so what size and any other alterations?

You shouldn't need to alter the carb for a rebore unless you're doing a big bore 200-220cc upgrade.

Mine went from 67mm to 72.5mm bore and from memory the only thing I needed to change was to go a half-size richer on the pilot jet (25 to 27.5)

Thanks guy's I will leave well enough alone then. There is a short flat spot at the begining of a rev and thought it might be related.

'At the beginning of the revs' would be in the range where the slide cut-away plays an important part.   You could try sticking your thumb in the inlet, just before the slide, and seeing if the additional choke-restriction helps remove the flat spot.

We had a Honda that hesitated at this throttle opening, and as we couldn't get a slide with less cut-away, we ground 1mm off the engine side of the slide, to lower the cut-away down a bit...  it cured the problem, but you do that type of mod at your own peril..!

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On 10/23/2017 at 7:31 AM, sh59 said:

Thanks guy's I will leave well enough alone then. There is a short flat spot at the begining of a rev and thought it might be related.

How the air screw is set makes a big difference to the off-idle response

Yes the air screw does make a difference, the books say set for a peak of the revs, but setting a bit further rich may give a better response to a blip of the throttle.

Chain-saw manuals always say to set the fast running jet for maximum revs then richen to reduce the revs by about 1000rpm, to ensure the best mixture when under load.

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