YZ250 top end. Two stroke experts please reply

**DVS ME** · 21st July 2011, 19:12

Hi i just had my top end fail on my 05 YZ250 the piston melted on the exhaust side and damaged the bore so its a perfect time to upgrade.
The bike has a FMF fatty expansion pipe and power core 2 muffler, v Force reed block It goes very well but im after more power.
There are a few options ive been looking at either of which i will still rebuild the bottom end with new gear box bearings, new rod, crank bearings and seals so i know it will be good for the new top end.

Heres the options ive come up with.

1. Brand new barrel forged piston either wisco or wossoner gasket set light port job and maybe up the compression.

2. Sleeve the barrel and do the above.

3. Sleeve the barrel to 265cc and fit a wisco piston and do the above.

4. Send the barrel and head to eric gorr in the states and get him to do the 265cc kit he offers bored oversized, re nikasil, port job with options on where you want the power band, comression to suit pump or race fuel.

Heres my wish list.
It needs to run on pump 98 fuel.
Not to alter the way the power comes on too much.
pertty much be like the same bike but more power from bottom to top.
I need i to be reliable.

I know theres always trade offs between power responce and reliability so i would like to know other peoples responces and expericences.

I will be re jetting to make sure this doesnt happen again.

Also and people who have been down this road or know of people who do good work in relation to this please add to this.

Thanks look forward to your replys

**cheese** · 21st July 2011, 19:34

Sleeves are not the best idea. They are ok, but the heat transfer is always a bit shit. Can you get an athena big bore kit?

Someone just posted yesterday that they are doing alot with porting 2 strokes.

**scott411** · 21st July 2011, 20:05

i am not a fan of sleeves at all, there is a company in nz that does nikisel now, its in ashburton and i have had a number of good jobs done by them

do not up the compression if you want to run on 98,

the 265 kit would require mods to the power valve normally, if you want to go that route either send it to the states, or by a athena kit, i have had a bit of experince with big bores mainly on kx125's, Eric Gorr does a good job, but you will have to get the jetting sorted back here (fuel and atomsphere differences), and it will be a bit of trail and error to get it right, even if you are going to run av gas back here get him to do it to pump fuel spec, what they call race fuel over there is alot higher octane than what we call race gas

personally, i do not see the big advantage in fordged pistons in anything but full race apps, and i run cast pistons on preference, vertex or pro x are both good, vertex being my pick of the aftermarket ones, personally i prefer stock pistons but do not have any idea what hte yamaha ones are worth and may not be worth it,

**ktm84mxc** · 22nd July 2011, 15:21

As you've done most off the normal hot ups besides a big bore kit & itz not enough how about a CR500 mill in a CRF chassis easy to get 70hp, talk to Dave Leary as he had 1 built a couple of years ago, you can contact him through Cyclespot Honda.

**Mungatoke Mad** · 23rd July 2011, 10:57

Originally Posted by DVS ME

Hi i just had my top end fail on my 05 YZ250 the piston melted on the exhaust side and damaged the bore so its a perfect time to upgrade.
The bike has a FMF fatty expansion pipe and power core 2 muffler, v Force reed block It goes very well but im after more power.
There are a few options ive been looking at either of which i will still rebuild the bottom end with new gear box bearings, new rod, crank bearings and seals so i know it will be good for the new top end.

Heres the options ive come up with.

1. Brand new barrel forged piston either wisco or wossoner gasket set light port job and maybe up the compression.

2. Sleeve the barrel and do the above.

3. Sleeve the barrel to 265cc and fit a wisco piston and do the above.

4. Send the barrel and head to eric gorr in the states and get him to do the 265cc kit he offers bored oversized, re nikasil, port job with options on where you want the power band, comression to suit pump or race fuel.

Heres my wish list.
It needs to run on pump 98 fuel.
Not to alter the way the power comes on too much.
pertty much be like the same bike but more power from bottom to top.
I need i to be reliable.

I know theres always trade offs between power responce and reliability so i would like to know other peoples responces and expericences.

I will be re jetting to make sure this doesnt happen again.

Also and people who have been down this road or know of people who do good work in relation to this please add to this.

Thanks look forward to your replys

Get a 4 Stroke !

**browny** · 23rd July 2011, 12:04

just buy a bigger bike if you want more power

**DVS ME** · 23rd July 2011, 18:01

yeah thanks for the obvious replys "get a four stoke" "buy a bigger bike" that just makes me want to make mine faster. I love my bike and two stokes this is my last effort before I step over to the dark side.

**noobi** · 23rd July 2011, 18:17

Buy a 300

**CHOPPA** · 23rd July 2011, 22:49

Standard piston and standard barrel. VHM head, some porting and tuning and setting up the squish would be as much as you wanna do. Ignition mods like vortex might be something to look at as well

**CRF119** · 24th July 2011, 18:14

I have heard some bad stories about Wisco so i wouldn't run that. Mate is using a wossoner he has just rebuilt his CR250 with oversize barrel after melting his piston on a exhust side from riding it to work because the car wouldn't start

.

When i had my CR125 it was by far one of the fastest and it wasn't over size. I asked to run wisco because i thought they were the best i was told no. Some bike shops are not even fitting these to bikes now due to issues. I ran Pro X who i am told make a lot of OEM pistons.

I used http://www.electrosil.com.au/cylinder-coating.htm this shit is good my CR125 had many years of hard riding and it never had an issue.

Other mods where.

Pro Cicuit Platinum 2 Pipe
304 Silencer (Not the shorty it lacked top end)
V force carbon reed block + I modded the cage to flow more air
Bored and polished carby Gained 2-3mm
Bored exhust housing
No Toil air filter

Would have been nice to have a ignition with adjustable timing.

Before you go and throw a lot of money at the engine. Take a look at the suspension. Horse power gets you from A-B faster, good suspension makes going from A-B Effortless.

Next bike i buy before i go nuts on the HP mods and bling stuff i will be getting the suspension all re valved with new Smart valves or similar from RG3

**v8safety** · 25th July 2011, 12:16

remember that if you do all you want to the topend how old is the bottom ? will it handle the stress of the big bore on an old bottom end , you'll be spending close to 3 g for all the work consider upgrading to a larger two stroke , ktm 300 , gas gas 300 , or go kx 500 or cr 500 , unless you love to maintain bikes then think 450 4 strokes . nutters in the states shoe horn a cr 500 motor into crf chassis.

**DVS ME** · 26th July 2011, 18:34

Thanks for the replys ive decided a path to go down.
I have stripped the bottom end got all the bearings out will go to auckland bearings in the next few days to match them up.
Craig at Motor x parts has the crank and is puting new bearings and a rod on for me
he has been a great help easy to talk to and has some good parts highly recomend.
I have also bought a brand new genuine yamaha cylinder off ebay cost $660 landed alot cheaper than your local yamaha dealer and should be here in a week or two.
Im going to buy a wossoner forged piston and bottom end seal kit (I have most gaskets from the spares kit that came with the bike)
I have looked into porting or cleaning up the ports but im not sure yet if i want to touch them as the factory look to have it right the first time.
I will look into the squish or compression ratio and might slightly raise it.
So all going well should be back togeather in about 3 weeks.

thanks

**CRF119** · 27th July 2011, 00:04

I have stripped the bottom end got all the bearings out will go to Auckland bearings in the next few days to match them up.

This is very important that they match them not only for size but tolerances. Wrong tolerance for the bearings RPM range will mean bearing fail. Id be spending the coin on genuine.

Ill put it this way im rebuilding a Jet ski which has bits of a 650 engine and bits of a 750 engine the bearings are the same size but one set is designed for a few more revs then the other.

**B0000M** · 27th July 2011, 15:12

i have a CR250 for sale that has no trouble outrunning 450's under acceleration

you could buy it, as a known good product (some of these clowns might back it up

)

or you could buy some Moto Tassinari reeds for it, which will also give you some good gains.

how much compression is your engine currently making?

if you want the reeds message me, i can do them cheap (new & legit)