Making your Polini/Malossi 130cc kit fast...!!

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Making your Polini/Malossi 130cc kit fast...!!

Postby spiderwebb » Wed Dec 28, 2011 8:38 pm

from the Paisans forum:

"Making your Polini/Malossi 130cc kit fast...!!

At the request of some of you dudes i will try and explain how to make your smallframe Malossi or Polini kit fly.

A bit of warning: You can really fuck up your barrel/cases and ultimately your wallet if you do any of this stuff incorrectly. Never ported before? Then the following is probably going to be way too advanced for you and best left to someone with experience. I assume no responsibility if you ruin your shit.

To make the Polini fly i recommend the following: Get the proper head (the malossi comes with one), intake, carb, gearing and pipe setup. In the case of my buddy's Polini PK we went with the MMW head, new 24 mm polini reed intake, TM 28, standard Primi primary with short 4th and Stoffi's Race pipe.

Note: Before and After I alter any barrel I fit it to the cases and check port timings. The way malossi and polini set these barrels up out of the box is a not suitable for expansion chamber exhausts. This is a brief explanation of how you optimize a barrel to work in conjunction with the other items like exhausts and carbs.

Note regarding the polini reed valve: I port the shit out of the cases and also remove the sealing pad when using a reed intake. Thus With this type of intake you do not have to take the crank timing into account. Reed valve intakes are my preferred way to modify these engines. If you want to keep the rotary valve intact then i would strongly urge you to use the Malossi with direct reed induction or brush up on your maths as you will need to factor in the duration of the crank opening on top of the transfer and exhaust timings.

Now, To get the timings to work with a true expansion chamber exhaust it is well documented that you need to run higher port timings to allow the pipe to work as it was designed too. A good rule of thumb is anything above a minimum of 120* transfer and 180* exhaust timings. I also like 30* of blowdown. Anyway use a packer (For the polini i machine a packer out of 2.5mm aluminum sheet and make a couple of gaskets in various thicknesses) and or gaskets to raise the barrel to get the transfers to 122*(+/- 2*). I then measure the exhaust which should be around 170* give or take 5*. I then advance the exhaust timing to 183* (+/- 2*) and mark it with a sharpie. I check and verify that mark at least three times to ensure it's where i want it. A STRONG WORD OF CAUTION as with most things like this... If you go too far with porting ONCE IT'S GONE IT'S GONE. Once i have that mark setup i scribe a line and optimize the exhaust port shape. I like to open the exhaust port up to achieve a safe yet good performing 67% or 68% of piston diameter. What's an optimized shape you ask? well lets just say i learned the hard way and by trial and error. I cant give all my secrets away. Just think of a nice radius with a flat style top.

Looking at the bottom of the polini barrel you will notice that the casting has two indentations that do not allow the transfers to be opened up to their full potential. I machine 3 to 3.5mm off the bottom of the barrel thus removing the bullshit indentations allowing the transfers to be open up 1/3 larger than stock. The malossi barrel is ready to go here so port away. NOTE: I rarely have to modify the direction of the polini transfers since they are nicely cast in the correct direction. The malossi's need a little work here but that will be for a later class

There's no point in doing the above if your intake cant flow enough fuel. I like to port the case intake open approximately 1/3 bigger than stock completely removing the sealing pad. In every occasion that i have done this i have blown through the lower wall of the cases close to the entrance of the crankcase. There's just no getting around it... you will blow through if you port it correctly. Anyway I've had the hole welded up and used JB weld. and after some odd warping to the gasket sealing surfaces from welding i exclusively use JB in this area.

Then i match the transfers to that case... typically you need to add some weld to the primary side of the cases to keep from blowing through but i have also gotten by without welding. A word of caution. I have opened the transfers too much on the barrel causing a lack of sealing surface from barrel to case. if you make this mistake A touch of JB works well in this situation as well.

OK so you have your all of your porting and port timings set... you're ready to go right? NOPE! You need to set the squish. On a 125cc setup like this i like to run 1mm to 1.3mm of squish. In order to get a polini/malossi barrel to do this you will have to deck the top of it. I measure the squish with solder or plastigauge and then remove the amount from the top of the barrel to provide the correct distance. example: Measured distance minus desired squish equals amount needed to me removed from top of barrel. Remember that if you try to alter the squish by altering the base gasket you will screw up your port timings so this is why I remove it from the top. If you fuck up here and take too much material you can salvage it by adding a head gasket

Anyway if you do all this correctly and your pipe is of good design (EG: Hammerzombie, PM40, Stoffis ROAD, RACE or CURLY or FRANZ) you will have a fire breathing smally. These mods however will not do much of anything for the kidney or the polini EVO type pipes.

Another note of caution: If done properly this formula will produce a very powerful little engine. IT WILL REVEAL all the weak parts in your engine like cruciforms, transmissions and clutches. it's better to start with all new bits from the get go.

Finally I probably forgot some things or if you need further explanation feel free to ask questions...

8) 8) 8) 8)
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Re: Making your Polini/Malossi 130cc kit fast...!!

Postby J2 » Mon Jan 02, 2012 12:15 am

And if you understand what is written here you can basically make ANY two stroke engine run to it's optimum potential.
tuning is not just slapping a kit on, or a pipe on. You have to choose components that work together, then do the math, and set them up correctly to produce power where you need it.
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