two stroke porting

[Rota Ash]

Hey lads i have an 02 Rm 250 with 25 hours on it, a Gnarly pipe and a cut down stocko muffler, the bike has decent bottom end and a bucket load of midrange which dies off like its hit a brick wall in the top end, im gonna throw a new ring in it in another 10 hours and am thinking of touching up the port's while it's apart, thing is , is that this is my first 2 stoke and i would like to know how porting affects these things, so do u guys use a degree wheel to measure port timing, do u usually just widen the port an smooth things out, if i smoothe things out and lift the exhaust and inlet ports by a couple of mm will this give me a bit more top end without sacrificing the bottom end it already has, i love the power the bike has just wish it wouldn't hit the brick wall as hard,

any advice would be greatly appreciated

[MXLord327]

Before you get into porting, try a different pipe.  The Gnarly is meant for bottom end and midrange only and flattens out a lot on top.  I have one on my YZ250 for the woods, and a Fatty for the track.  I also still have the stocker, it is a compromise between the two.

[JohnN]

Rota Ash, porting is a mixture of science and art.... personally I would not suggest that as a beginner you attempt to change port timing, which you note is suggesting.

I would ensure that your power valve is working correctly and is not all "gummed" up... this could easily cause the problem your experiencing...

And as MXLord327 is suggesting it can also be the pipe... but for my money I'd begin with the power valve...

Let us know how you make out..

[dogger315]

I agree with the other two responses and would add that your "cut down"
silencer will also boost low end at the expense of top end power.  If you
look at SX specific silencers, they are all shortys to help provide that big
hit at the bottom.  Conversly, an "outdoors" style silencer is normal length
for better top end.  If you like FMF, I recommend a Fatty or SST pipe and
a Powercore 2 silencer.  The right pipe and silencer combination along with
a good functioning power valve will wake up the already stout RM power
band.

dogger
 

[Rota Ash]

what's the best reed valve to get, should i get a new reed valve assembly like a v force, they seem expensive, are they worth it, anyone know any links to test comparisons on reed valves?

[JETZcorp]

Reed valves, power valves, silencers, who needs them?  I don't get why the good old rotary valve didn't catch on.  They provided so much power, and the throttle response was better than a reed valve engine (which has a bit of hesitation compared to piston-port or rotary valve).  And, it was a spinning piece of metal instead of some bits of fiberglass bending back and forth.  Unbreakable!  Ever wonder how Kawasaki could make a 100cc flat-tracker that would pull massive wheelies with full-grown men onboard?  Rotary valve!

That's my rant for the day.

[dogger315]

what's the best reed valve to get

It comes down to what part of the powerband you want to
enhance.  For better low to mid and improved throttle response,
The VForce 3 is the ticket.  For better top end, the Rad Valve
is the choice.  I have used both over the years and now use
the VForce exclusively for 250 cc and up and the Rad Valve
on 125s. 

dogger

[G-MONEY]

Rad Valve   pinner to winner    don't send your money to Italy keep it here!Boysen

[Out of Order]

I copied this article from TSR. It's not on their website anymore, so dont bother looking for it. This is for a 92-95 RM250, it might work for newer RM's if the cylinders are similar.

TWO-STROKE TUNING WITH COMPUTERS

BY ERIC GORR

The design evolution of two-stroke engines was greatly accelerated by the

proliferation of computers. Tom Turner is one of the most experienced two-stroke

tuners in America. He markets his own line of design software. Some of the programs

are based on Dr. Blair's work (but "Improved Greatly") but most are TSR original designs. Turner

receives feedback from other tuners who use his programs for applications ranging from mx,

road racing, watercraft, ATVs, and snowmobile racing. He uses this feedback to

continually fine tune the programming code for better engine performance.

I use Turner's Programs everyday in my tuning business. Here is a case study of how I used

computer design software to improve the performance of a Suzuki RM250 (1992-95)

ANALYZING THE STOCK ENGINE

The design of the cylinder head, port timing, and pipe must be coordinated to

peak simultaneously in the RPM band. There is a proper order to the design

process. First I start by measuring the engine specifications and inputting

the dimensions into the TSR Computer Programs, to look for shortcomings in design.

The cylinder's exhaust port time area peaks at a relatively low 7,200 rpm. The

transfer port time area is just the opposite. The ports are very wide and have

high time area and a high rpm peak. However they don't start to flow properly

until about 5,000 rpm. The problem with the RM250 is that the exhaust port is

too small and the transfers are too big. That is why the stock engine hits hard

in the mid-range, because the exhaust and transfer ports come into sync right

when the exhaust valves open. However the transfers are too large to flow

adequately at low rpm to give the engine tractable power and the exhaust port

is too small to give the engine any over rev. This is an easy problem to fix.

REDESIGNING FROM THE INSIDE OUT

This is the proper order for redesigning the engine specs; exhaust port time-

area, blow down time-area, transfer port time-area, cylinder head, reed valve,

and pipe.

EXHAUST PORT

The chordal widths of the exhaust and transfer ports, and

the port opening timing were input into Turner's PORT2000 program to determine

the rpm band width of the stock ports. Time-area is a calculation

based on the width of the exhaust port, the point when the port opens, and the

total port open duration in relation to RPM. It is a very difficult

calculation to do on a hand-held calculator but the computer performs it fast,

accurate, and easy. The program enables the tuner to adjust the exhaust time-

area for the power band we are tuning for. On the RM250 the exhaust port's

timearea was too low and the exhaust opening timing and width had to be increased

to allow the engine to peak at 8,800 rpm.

BLOW-DOWN TIMING

With the exhaust port in the correct position, we

need to set the opening time of the transfer ports and time-area so the cylinder has

adequate blow-down time area. The blow-down phase occurs from when the

exhaust port opens to when the transfer ports open. Too little blow-down will

cause burnt mixture gasses to flow into the crankcase when the transfers open and

pollute the un burnt gasses. Turner's PORT2000 program has an

automatic "FINDIT" feature that is used to determine the optimum blow-time-area

of the blow-down phase. FINDIT tells you the distance from TDC where the transfer

ports should open. Now that you have the opening timing of the transfers, you

can use the PORT2000 program to find the optimum width dimensions of the

transfer ports. The RM250 transfers were way too wide and the original

transfers didn't peak until 10,000 rpm. I narrowed the widths of the rear

transfer ports with epoxy in order to reduce the transfer port time-area to

peak around 8,800 rpm.

CYLINDER HEAD

Once you have determined the optimum opening timing for the

exhaust port, you will have the effective stroke length. That is the distance

from TDC to when the exhaust port opens. That is the key to designing the

combustion chamber because it is linked to the compression ratio. Turner's

COMPRESS program enables you to design the combustion chamber with variables

http://tsrsoftware.com/compress.htm

Sorry for being so long. The article as you can see has a lot to do with TSR software, but I hope it helps. :D

[MXLord327]

G-Money, I don't think you have your facts right about the V-Force, Moto Tassinari is not from Italy, their shop is in Lebanon, NH, right on the Vermont line, and from what I have heard, they are great people!!  I love my V-Force 3's in my YZ250!!!

[2T Institute]

V force 2 works best for high rpm all thoseGP 125's can't be wrong.

TSR software is OK but it doesn't look at the engine dynamicaly, Port 2000 is a good place to start, but by no means has all the answer.For instance it doesn't take into account the ignition and what changes in the curve do.
To answer the question, there is no short answer, no quick fix and no short cut.Everything has to work in harmony. There are a few things you coud try first would be to retard the ignition slightly and then chop 10mm from the header of the pipe.

Yes the mighty disc valve is the go although reed technology has caught up with discs lately

[graham472]

What I've done to my 01,02 and 05 RM's was machine 0.5 mm off the head and use 2 base gaskets which changes the port timing and compression ratio for better top end. Best thing is it's cheap and easy to do.