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Only $10 for all 2026 SX, MX, and SMX series.
I will also try to measure the porting and compare with stock and my YZs.
Best pipe we found was the '03 KX125 pipe. Cheap, easy to fit and makes more rideable power than anything else we tried..................
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This webpage give an overview of all Mugen Power kits made: http://mugen125.tripod.com/kits.htm
Look what i found!
I suggest you keep those Mugen pistons and use other pistons instead! Those pistons are "unobtainium". I personally use Vertex 22190 pistons which also come without the window on the intake side just as the Mugen pistons. I just add 2 small holes to the exhaust side for lubrication of the exhaust bridge.
Vertex 22190
You want to use 98/99 HPP valves as those are the best HPP valves for all those engines. Don't use the stock 90-92 valves as those definitely hinder best performance (lack of top-end)!
98/99 HPP valves with that unique L-shape which makes for better seal when closed. The stock 90-92 ones are too restrictive for top-end and still have blow-by when closed (in the area where the red arrow points to). You'll get more revs and top-end with newer HPP valves and as mentioned best low-end as well by using the 98/99 ones.
And keep also an eye on the o-rings of the head as those go bad with time. Esapecially the inner one. I found some modern replacements (and also kept the original ones untouched)
Are these Mugen pistons with windows ?
I wouldn't use such pistons.
There is a reason why Mugen pistons don't come with the window. There is no need for any experiments. Just follow my suggestions and you'll be rewarded with a fast engine mith awesome power.
The windows have come and gone through the years on the YZs so my guess is that it does not make a significant difference. In theory they will allow flow from the boost port into the crankcase but at that time the piston should be close to top dead center so the intake and crankcase pressure would be very similar and hence the flow will be low.
Got no marks.
Pit Row
Varlo Verona did those in 1990 for the CR250 which already had the larger plastic airbox. It fits 1990-1992.
I had it in 1990 on my VRP already where you usually still had the smaller airbox that was covered by the numberplates. But to be even more ahead i not only changed chassis but also the plastics and mounted those of the 250 to have a completely different CR 125 than my competition
Here's the spacier looking '89-'90 VRP airbox with integrated airbox which already had the bigger volume of the plastic airbox offered in years to come. VRP was really ahead of the time.
Is it correct that pistons for Mugen is flat top?
On my -90 CR125 piston has a dome, flat top came stock in -92 if I recall correctly?
Regards Marco
On round top heads the squish has already an angle and the overall design is "rounder" while on flat-top heads the squish seems flat with more of an angle to the dome.
Flat-top Mugen head:
Domed Mugen head:
Mugens have their own head design with 6 bolts instead of 5-bolts that come on stock heads. So you are forced to use those heads. And obviously over time also the heads are becoming rare. And doing head work on such precious items "hurts". When i bought one of my Mugens on Ebay it got delivered with the "wrong" head: a older Mugen head for round top pistons so useless for the '90 Kit i had in hand. Rather then waiting weeks to get hold of the correct flat-top head i decided to have that head modified. I now have a replaceable dome in that Mugen head which allows me to change shapes/compressions....it's good to know old Tuners with vast knowledge on twostroke engines
Pre HPP?
Regards Marco
'92 on we're flat-topped.
Surprised all the mugen kits weren't for domed Pistons, as they're supposed to be more efficient at high revs than a flattie.
They obviously do two variants, a flat top and a domed top. I would expect if you changed just the piston and head alone, whilst keeping the other variables constant, the flat top piston would make slightly better mid range, and the domed one more up top.
The differences are small, but looking at the attention you pay to the details, you know as well as I do, that a lot of small improvements add up.
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