How to build a bottom - mid HPP 125 motor?

Anyone got any tips?

I would buy 99 power valves but only one side is available.

Turn down cylinder ?

Machine head ?

Port work?

I have various cdis and flywheels 93, 95 and 97 if there is any difference?

Ask Nino.

Anyone got any tips?

I would buy 99 power valves but only one side is available.

Turn down cylinder ?

Machine head ?

Port work?

I have various cdis and flywheels 93, 95 and 97 if there is any difference?


You are probably going to need to buy the powervalves used.  You can get either 98 or 99 powervalves. 

Ask Nino.

No - i'd have no help as i'm a MXer and not really looking for low-end on my 125.


Definitely get the 98/99 HPP valves&guides though! Me too i bought 2 sets on Ebay used. They seal better when closed thus enhancing low-end.


Back in the day when i first started using my '98 i made big gains by installing a 3mm pipe spacer (between pipe collector and pipe flange), lowered the head by 0,1mm, used the stock '93 silencer (much shorter and 1mm smaller bore). You definitely want to keep as small a carb as you can get! Remind that older CRs even had 34mm carbs. But i'd say a 36 should be fine. Usually a pipe is helping tremendously to shift the power around but i'm not aware of any "enduro"-style, low-end only pipes. Maybe Dynoport??


Maybe keeping the HPP shut closed a little longer helps too? I had good success doing so to fight a slight bog i got when upgrading to a bigger 38mm carb. You can get adjustable HPP covers by


With reeds you might look into the softest reeds you can find. Maybe Boyesen aren't all that bad to start with but there's also other brands that offer reeds in different stiffnesses. Soft reeds make for better low-end response.


You could also try to lower the cylinder by installing just some slimmer base gaskets. Athena for example makes them in different thicknesses. This obviously raises compression at the same time which would come in handy in your need for better low-end. Stock base gaskets measure 0,5mm. So a 0,4mm could easily be installed without overdoing compression.


DON't polish any ports! Polishing is the worst you can do anyway but it definitely hurts low-end power.

Edited by nino

You could call Eric Gorr and see what he has to say about low/mid porting.  Not sure anyone on this site is going to have porting specs for that.  You are going to need to talk to a tuner.

Thanks for the replies, looks like first thing will be to find those power valves. Anyone got any ideas? None on ebay at the moment..

Do I need just the guides or the blades and guides ?

The plugs are for shifter karts or people building a pure rev motor. The motor will have a very narrow powerband.

The use of these will absolutely destroy the low end power.

I would have thought with all the shifter guys running 99 cylinders with plugs there should be a ton of power valves floating around

You will need the 98/99 ones which differ in that they have that L-shape. So you need both - the guides and valves themselves. Where the arrow points is where these actually seal while on older ones you can see through.

These HPP valves fit in every HPP engine.



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