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after rebuild bike needs more air?


kenDAWG
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after doing a topend rebuld on my little dt125r i fitted some carbon reeds and refitted the snorkel while i was at it took her out for a run and she wouldnt go past the 4,000rpm sounded like there wasnt enough air getting to her so i pulled the snorkel back off and she went fine until 6-7k rpm same sound (she has been well run in now)

i have never seen the point in all these holes drilled in air boxes but i couldnt think of any thing else so i drilled out 4 holes the size of 1 pence coins now she runs up to the redline just sounds soooo bloody loud!!

any ideas why with a change of topend she would need more air? and will this be ok?

cheers

ken

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Easy do the math.

4000rpm = 4000 x 125cc gives 500000 cc of air. Thats 500 litres of air a minute.

10000rpm = 10000 x 125cc gives 1.25 million cc of air or 1250 litres of air a minute.

You can get clever with velocities for given diameters and such but it does make you think.

Becomes obvious why the power difference between 2 and 4 strokes too. You can halve those figures for a 125 single.

As to the different top end. Think on this. If the bore is worn on a 4 stroke she burns oil and loses compression.

On a 2 stroke there will be less draw into the cases as charge from above the piston will be drawn down reducing the pull through the carb. Just imagine losing even 5 per cent of the volumes from earlier and the effect on mixture and performance.

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no no the filter was brand new pal, oiled it up lightly also...

im guessing like cynic says the topend was worn thefore not drawing as much air as a new topend!

just means the bike must have been setup with the worn topend as it was done in a garage before i bought it!

not to worry she goes well

result

ken

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