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Discussion Starter · #1 ·
hello guys im back bean away because of work and things but im back i have a race next week and iv only just rebilt my gp3 rep bike have no idear why i striped it befor xmas but i did anyway and just rebilt it yesterday now heres the thing befor xmas my last ride it boged down a bit but iv bean told by polini uk that its beacue i wos running a 90 jet in and that thats fine in the summer but winter cold i need a 94 or 97 so i got them but today i need to get the bike ready what one should i use and by the way i had a black circil on my piston head when i rebilt it so i cleand it could that be because of the 90 jet when it needed a bigger one? helllllp
 

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That black circle was a hotspot starting that was caused by either running too lean or having a squish thats too tight for the octane of fuel your running which would cause predetonation.....

Do you happen to know the squish setting?
 

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Discussion Starter · #3 ·
i dont know it off by hand but when the baral is on the piston dome sticks out at the top so i think the squosh will be very very tight do i need to strip it down agen and maby pute 2 gaskits on? and also im runing a 90 jet in it at the moment and iv bean told to have a 94 or 97 nin it ?
 

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i dont know it off by hand but when the baral is on the piston dome sticks out at the top so i think the squosh will be very very tight do i need to strip it down agen and maby pute 2 gaskits on? and also im runing a 90 jet in it at the moment and iv bean told to have a 94 or 97 nin it ?
You need to make sure of your squish ...You want an ideal squish of between .035" -.040"sae..That is the ideal squish for using premium 93 octane pump gas....any tighter than .035" will require higher than 96 octane fuel....any tighter than .030" will require 100+octane..I run my small 2 stroke chinese engines on the edge. .022"-.028" sae and thats super tight...I have to run them on no less than 112octane VP race fuel...

The jetting is important but knowing the squish and compression ratio to know the proper octane to control predetonation is critical because being properly jetted will still cause you high heat problems if theres predetonation muffing up the power.............
 

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Grab some 3/32" solder and a digital caliper and check your squish...

Orient the solder so its parallel with the piston pin to get a good accurate squish..Do both sides of the piston to get an average or do one side for a quick referrence......

Knowing this is important so you can add the right thickness cap gasket or basegasket to get the squish into proper performance range.....

This is being done on a cag head but it shows the steps...








 
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