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Discussion Starter #1
hi all just wondering if i did the right mod by fitting 38mm pj to a barell ported at 67.5mm or would i have been better of putting a stock 34mm on it,just wondering wether it would of improved my torque with the 34mm,as exhaust port is pretty big on this barrel so low end power is suffering abit.thanks paul
 

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Discussion Starter #3
hi slither yes there is that option but what i want to know is really is was there any gain by using the 38mm over the 34mm in performance on a 67.5mm ported cylinder, but thanks for your reply i appreciate it,cheers paul.
 

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Discussion Starter #5
ok thanks i will see how it runs when i get this etop end back together and when i next race see how it is out of the corners im altering a few things to try get some low end back we shall see thanks all.
 

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hi all just wondering if i did the right mod by fitting 38mm pj to a barell ported at 67.5mm or would i have been better of putting a stock 34mm on it,just wondering wether it would of improved my torque with the 34mm,as exhaust port is pretty big on this barrel so low end power is suffering abit.thanks paul
ok thanks i will see how it runs when i get this etop end back together and when i next race see how it is out of the corners im altering a few things to try get some low end back we shall see thanks all.
No TrQ improvement or Low to Mid range real increase, however; 3/4-WOT will be a significant improvement. If your cylinder was Ported & Head-Squish Modified properly, going to the 38MM IMO was the correct choice. You should be pleased...:)

Carlos
 

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Discussion Starter #7
ok carlos i will stick with th 38mm,i have the head off at the mo cos want it skimming also coulid you give me some idea on what tha squish should be like,at the moment the squish doesnt start to taper/angle up till its about 1mm inside the bore diameter im sure this is not correct and should start from the bore diameter which in my case is 67,5mm the dome has been altered to about 21ccwhats you take cheers paul
 

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Read the Squish write up carefully. Squish is measured form the outer edges of the piston & cylinder wall across the squish band. Typically the squish ranges from 1MM (.040"-.100") to 2.5MM, dependent on setup...mostly in the .060" range.

Carlos
 

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Discussion Starter #9
hi carlos dont think im explaining my question well enough!my bore is 67.5mm if i measure the outer circumference of my squish across the head i get 66mm so my squish angle doesnt start till its1.25mm inside the bore does this matter sorry if i havent understood your answer,im thinking the squish and dome was altered when this head was on standard bore,so should the squish band be altered to suit over bores,thanks paul
 

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OK, Understand now.

The Dome major squish diameter should match the bore diameter, that is ideally. And even be .25MM bigger to help compensate for miss-alignment of cylinder to head. Yet, from what I have seen most builders don't bother cutting the dome diameter to match the bore.

It will run as you described, however; it will run better as I detailed above. If it was my build; I would match the dome diameter to 67.50MM + .25MM, and set the proper squish.

Carlos
 

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American Opel and Carlos are 100% right.
I have only dune bikes and on 3 of them I have 38mm PJ carbs but have added a bit more compression and 2 having ported and 1 stock porting with compression added as well. Let me tell you all top end and perfect for where I ride my 250R's, only advice is can give you is 38mm best for top end and if you go that route have bike or bikes dyno tuned by someone who knows 250R, good luck.
 

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Discussion Starter #14
ok thanks well i race at grass track events which is like your flattrack i guess but what i have seen of videos the tracks here seem to be abit bigger and so yes definatley need the top end rpm i am currently trying to raise my compression abit upto 185 psi as only got 155 at mo due to head volume,thats a interesting user name you have sounds abit welsh! thanks paul.
 
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