My first ESR motor was a 350 big bore stock stroke, ESR 5 pipe, 39mm PWK, race gas motor fully assembled by ESR and that was the first motor to scare me on a quad! This was around 2002 and that motor was bonkers and very reliable it had well over 50 hard hours on it and that motor only met its fate when my brother accidentally put straight 87 octane lawnmower gas in with no pre-mix! I still have the piston on my work bench! My motor prior to that was the original oem cylinder/motor with a mild port job by a local Kawasaki mechanic and I thought that was fast, until I went into the big bore territory! What a difference!
I had a twist throttle on the R when I put that 350 big bore in my bike and that's when I had 2 friends on separate occasions completely destroy my OEM plastics because they both flipped over because of the power that motor had and that twist throttle made for a violent ride if you weren't a seasoned rider.
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