2016 300RR Top end rebuild.

[Mezcal]

Hi gents, Long time since my last post
As I just clocked 104 hrs it was time for a new top end. After speaking with Beta Australia and Dave from 2stroke Performance It was suggested I go with the B size - an A was the original. I ended up getting one from Beta Aus, 2t Performace didn't have all the gaskets on hand.
The piston from Beta Aus is a Vertex and the porting(?) looked quite different to the original, my techo friend who has rebuilt plenty of 2 strokes noticed this and said "Well if thats what the Beta mob sent it should be fine".
And it is fine other than how it runs off Idle. Previously it was buttery smooth all through the rev range, but now up to 1/4 throttle its quite fluffy or spluttery?? I am not sure how best to describe it, but from 1/4 throttle up it cleans up and feels just like it did before.
Jetting maybe? I bought the bike new, and after I ran it in I changed the jetting as follows:
NECJ #3 clip position
162 main
and 38 pilot - which was standard.
I ride at sea level with temps in the 15 - 30 C (60 - 90 F) range.

I'm not questioning the work my buddy did, he owns quite a few 2 strokes including a TM and rebuilds them all, as well as rebuilding a number of 4st KTMs.
I believe its jetting related, I want that beautiful buttery smooth throttle back!
Any advice would be greatly appreciated.
Cheers
Scott

[aj_mariner]

Clip in #2 spot with the N2Z_ series feels the best. Other things to remember, carbon buildup will increase compression and require richer jetting, or clean up previously fat jetting...so the feel will be a bit different after top end.

On another note, did you measure anything before increasing to size B?

[Mezcal]

Thanks for the reply, There was a little carbon build up on the top of the piston but it came off fairly easy. Nothing was measured, I took the advice from the 2 places mentioned - as I was over 100 hours it was a no brainer to them although my buddy who helped with the top end thought I should have stayed with the A size.
I have a 35 pilot jet, so I will see if that helps anything.

[wwguy]

I rebuilt my 2016 Xtrainer 300 top end at 100 hours. According to the parts books mine uses the same piston as the 300RR and 300RR RE bikes. Mine had a Vertex piston in it from the factory and the replacement I got from my Beta dealer was identical.

[Mezcal]

That is identical to the new piston I put in.
After a quick ride with the 35 pilot jet it feels pretty good now, it is still a bit fluffy off idle but that may be due to adding 100ml of oil to the fuel as per the manual after the new top end. This is the piston that came out of it

[Goatse]

That is identical to the new piston I put in.
After a quick ride with the 35 pilot jet it feels pretty good now, it is still a bit fluffy off idle but that may be due to adding 100ml of oil to the fuel as per the manual after the new top end. This is the piston that came out of it

I just picked up a new 300 piston yesterday,and it looks like the one in your pic.

[Mezcal]

I just picked up a new 300 piston yesterday,and it looks like the one in your pic.

It would be interesting to see what piston comes out, and if it is a vertex, interesting to see if it changes the bike at all.

[Goatse]

I just picked up a new 300 piston yesterday,and it looks like the one in your pic.

It would be interesting to see what piston comes out, and if it is a vertex, interesting to see if it changes the bike at all.

I put the (used) 300 piston in not long ago, and it looked the same as this replacement piston.

But the 250 piston I pulled to put in the 300 kit, has the sides like your new piston does. At the time, I figured it was just a 250 vs 300 thing.

[Mezcal]

Ah ok.
Well a small update - going to the 35 pilot cleaned the bottom up quite a bit.
Last night I lowered the needle (NECJ) to 2nd clip and it cleaned it up a little more, but have I leaned it out too much? With the A/S turned all the way in the bike does stall, and idles fine at 1.5 turns out. I think i'll put the 38 pilot back in and leave the needle on the 2nd clip and see how it feels. I should mention I still have about 1/3 of a tank left with 1% premix while doing all this tinkering.

[aj_mariner]

If your air screw turned all the way in stalls the bike then you have a good idle fuel/air mix. I wouldn’t worry too much about being a touch rich until the break in is complete and your first tank is done.