Yz 250 big bore two stroke

humidity
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vanwert, OH US
Edited Date/Time 3/30/2024 7:11pm

 I thought I could post about this and ask for help and advice . Thanks.

I rebuilt my YZ 250 engine with wiseco crank and a 293cc big bore kit.

I changed to 182 main and 55 pilot in carb.

I think my carb. floats valve stuck because it had a flutter at mid to high rpm and fuel draining after test run.                                             I opened up float bowl and found a large chunk of aluminum casting material from carb. Casting stuck in the float valve hole. 

I set timing to stock. .18mm. but I might try .10mm

I use the Hitlis power valves. pro circuit pipe. 

I still like my Rekluse clutch full auto. It has effortless lever pull and helps everywhere.

I matched the cases no ledges impeeding.

I used the V- Force 4 reeds.

I will test the bike again after I check floats and valve. But it seems like a completely different bike with a lot more power maybe 30% more feeling not sure. I just wanted to try out a big bore kit. I really thought my stock bore was perfect excellent running as 250.

Make sure the center power valve is moving free and full range first then dial in smooth movement on side valves. Needs attention during build.

I found a 3/16" ledge between cyl. and case so I blended that together, removing ledge. both stock and new cyl. are same, only cases need blended.

I used tools and videos from RMATV to help project and get my skills back.

I also use a super tacky marine type grease chassis bearings, other moving parts. I think this is best to avoid moisture in moving parts and bearings. I also use fully coated fastener lube ARP is best but anti seize is good, coat washer and head flange and threads. This will keep all fasteners better with proper torque values. I cringe how horrible fasteners are treated. Most are one time use nowadays with production, but we reuse.

Ring gap .018" - .022"

 

1
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kijen
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3/23/2024 1:08pm

Maybe go leaner on the main, bigbore create more vaccum aand suck more gas, on my esr i went to a 175 or 176?

 

2
humidity
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3/23/2024 5:20pm
kijen wrote:

Maybe go leaner on the main, bigbore create more vaccum aand suck more gas, on my esr i went to a 175 or 176?

 

I found a large chunk of aluminum casting material in float valve hole.

1
FWYT
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3/24/2024 8:37am

Right on, man. I have a TMR YZ-295 as well and I just love it. 

1
humidity
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3/24/2024 11:26am

I ordered a float gauge but I made one for now I believe the float was set lean previously but ran great as 250.  I reset float level. Tested 293cc bike again and still flutter at mid to high rpm. I will try the jet change to a 175 next test.  Any advice on the mid to high rpm flutter?

The Shop

humidity
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3/24/2024 1:06pm
kijen wrote:

Maybe go leaner on the main, bigbore create more vaccum aand suck more gas, on my esr i went to a 175 or 176?

 

All I have for now is a 175 main and 45 pilot to try. I did and it ran good with the 293cc same JD kit jetting as I had in my 250cc. I will try to buy jetting in-between these to test next. I removed the 182 and 55 combo to rich. No idle adjustment reaction.

humidity
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3/24/2024 2:59pm Edited Date/Time 3/26/2024 6:33am

I tested .10mm timing no rpm. So I tried .20mm and that seems like a usable combo now. I think it can get better. I will keep testing options. I have a serious flat turn track with million options to rip three gears on. I use my studded tires and I can rip extreme lean angle turns, tons of fun. If you're three hours away from anything better.

digger
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Upstate, NY US
Fantasy
3931st
3/24/2024 3:27pm

The last big bore YZ250 I did, I wound with an NECJ in #3 clip, 40 pilot, 168 main, and a #8 slide. This was with the stock 52 power jet and stock timing. The YZ’s 250’s really like the #8 slide. 

1
humidity
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3/26/2024 6:45am

Best result with parts I had was 175 main, 45 pilot, .20 timing, premium old pump gas, and it was ripping and revving again like previouse 250cc bore, now with a 293cc bore  good increase in power torque. I need to focus on riding not tuning before I can think about if I like it over stock bore. Seems good as so far just more torque.

1
TDC
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3/26/2024 3:44pm

Isn't running a 40 power jet the hot set up? 

I believe that's a Mr. Hitlis tuning tip.

2
cwtoyota
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Tacoma, WA US
3/26/2024 8:42pm
humidity wrote:
Best result with parts I had was 175 main, 45 pilot, .20 timing, premium old pump gas, and it was ripping and revving again like previouse...

Best result with parts I had was 175 main, 45 pilot, .20 timing, premium old pump gas, and it was ripping and revving again like previouse 250cc bore, now with a 293cc bore  good increase in power torque. I need to focus on riding not tuning before I can think about if I like it over stock bore. Seems good as so far just more torque.

175/45 was a good combo back when I ran a mid/top ported 72mm top end with race fuel and high compression head.   PC pipe and V-Force as well...     That was near sea-level and between 65°F to 75°F.     I think you're on the right track.

There are some Suzuki needles that some guys choose...   
I ran the 2002 spec Yamaha needle.  I think it's N3CJ or N3EJ, but memory could be mixed up.   I think the current YZ250s use the N3EW maybe?    I have a bag of a billion needles including the ones I bought from Suzi in the mid 2000s.  They are worth trying depending on your compression, pipe, riding style, etc.

1
humidity
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3/29/2024 7:41pm

I put a new plug in after warm up and did a top end run to check plug. It was full lean with some detination particals on the ground strap. that was the 175 main, 45 pilot, 20 timing.

  I will test the 178, 50 combo next. I think the timing might need to go back to stock .18mm. But I will do small changes first to figure it out.

 I need to find the parts you all suggested to try also.

cwtoyota
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Tacoma, WA US
3/29/2024 11:05pm

All the jetting I tried was at stock timing setting using an average quality dial indicator by a competent machinist.

Also, I run 50/50 mix of Trick 114 and ethanol free premium fuel here in WA at just slightly above sea level and no more than 100°F ambient temps.

Sounds like you're close.   Those Yamaha 250 engines detonate and erode the head around the  outside circumference of the squish band.   Good on you for checking / catching it.

1
humidity
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4/14/2024 7:07am

293cc YZ. I feel kind of lost again because when I get it to over rev like I want it seems to be lean and hotter temp 150*F Eng. temp at radiator cap gauge. plug has no color.

If I go richer main jet the over rev sputter gets worse.

I'm thinking plug heat range needs dialed in. I tried 8 and 9 heat plug.

This is a 293cc big bore kit YZ. I tested many jet options my best set up at this moment is ,

needle #1 clip. I tried #2 and #4.

Power jet 60. I tried 50.

pilot 52

178 main. I tried 175-180

#8 slide

Every time I try to richen anything the over rev sputter gets worse. I want a clean over rev pull, no sputter.

I think most any combo runs good but if you want it to over rev also then it gets difficult. 

Do you have any thoughts on ways to clean up over rev tuning ? Plug heat range ? Timing ? Or what could be bad damaged?

I'm worried if it ran hot 150F on a cool day just testing jets, it might over heat on hot track day.

 I'm not sure if you can get a 293cc to run like the 250cc over rev  great as 250. I'm trying to get the 293cc to do same clean over rev.

 I don't feel confident but this is my 293cc jetting, Parts orders take long time, I forget but I think I have the needle in it that was advised earlier and on clip #1. I tried all clip positions. power jet 60. I ordered a 40, but I think the 60 might be right. I tried 50. Pilot 52. Main 178. #8 slide. plugs 8 and 9 range tried.  

I just use premium pump gas. Thanks for any help on improving over rev stumble.

FGR01
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AZ US
Fantasy
1312th
4/14/2024 9:27am

Consider the design of how the power jet works on the YZ.   At very low rpm it is closed so you have clean initial response and it does not affect the pilot circuit.   Then it opens across the mid range under large throttle openings to give extra fuel when the bike is under load.  Then it closes again on the extreme top end to allow clean overrev.  

So, I think you'd be best going with the main jet that gives you the overrev you want and then richening the power jet or needle to get the middle richer.

You also need to make sure the power jet is functioning correctly.   Remove the solenoid on the carb and inspect the tip of the plunger.  It has a small rubber tip on it that closes and seals the circuit when the power jet closes.  Many times, on older bikes this rubber tip is dry rotted or ripped off.  If so, the circuit will never close correctly or may even be clogged up from debris when it is supposed to be open.  Unfortunately, Yamaha does not sell this itty bitty piece of rubber separately.  

3
humidity
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Location
vanwert, OH US
4/17/2024 7:01pm

Thanks for advice.

 I think I landed a good tune up after trying everything suggested. 

My best jetting for my 293cc big bore YZ two stroke...

Main 178

pilot 52

#8 slide

needle N3CJ #2 clip

40 Power jet

stock .18mm timing

8 and 9 heat range plug tested

32:1 premium pump gas

2.5 hours testing many combo.

 I think this is best. seems to run great and over rev can be held with no sputter. The 40 power jet was the last thing I tried and it cleaned up the high rpm sputter issue.

Now I have a little more torque power over the stock 250cc bore.

obiwan461
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Location
Cypress, TX US
4/18/2024 4:39am
humidity wrote:
 I thought I could post about this and ask for help and advice . Thanks. I rebuilt my YZ 250 engine with wiseco crank and a...

 I thought I could post about this and ask for help and advice . Thanks.

I rebuilt my YZ 250 engine with wiseco crank and a 293cc big bore kit.

I changed to 182 main and 55 pilot in carb.

I think my carb. floats valve stuck because it had a flutter at mid to high rpm and fuel draining after test run.                                             I opened up float bowl and found a large chunk of aluminum casting material from carb. Casting stuck in the float valve hole. 

I set timing to stock. .18mm. but I might try .10mm

I use the Hitlis power valves. pro circuit pipe. 

I still like my Rekluse clutch full auto. It has effortless lever pull and helps everywhere.

I matched the cases no ledges impeeding.

I used the V- Force 4 reeds.

I will test the bike again after I check floats and valve. But it seems like a completely different bike with a lot more power maybe 30% more feeling not sure. I just wanted to try out a big bore kit. I really thought my stock bore was perfect excellent running as 250.

Make sure the center power valve is moving free and full range first then dial in smooth movement on side valves. Needs attention during build.

I found a 3/16" ledge between cyl. and case so I blended that together, removing ledge. both stock and new cyl. are same, only cases need blended.

I used tools and videos from RMATV to help project and get my skills back.

I also use a super tacky marine type grease chassis bearings, other moving parts. I think this is best to avoid moisture in moving parts and bearings. I also use fully coated fastener lube ARP is best but anti seize is good, coat washer and head flange and threads. This will keep all fasteners better with proper torque values. I cringe how horrible fasteners are treated. Most are one time use nowadays with production, but we reuse.

Ring gap .018" - .022"

 

Man I wish I hadn’t read this thread now I want another big bore I did Athena bolt on 330cc kit on a 2001 CR250. Ran more like a 500. Than a 250. Only thing is it was so big out eliminated the HPP power valve  
 

back to the metal shavings Did you find the source ? Was going to see if you checked your chrome on the slide ?   You Totaly sound like you know what you’re doing.  I think it’s critical to keep a power valve.   My 330 was a blast on Loam but hard pack forget it   It was a sit and spin like the CR500 was not near a  good of a bike as the KX 500 because of the kipps power r valve. The metering of power is critical. Hated my CR500 because it was an on /off like a light switch.  Had to roll On in 3rd like my 330. Had zero bottom flubbed a little on mid range then pulled ridiculously hard and clean on up.   Here I go looking for aYz 250 to build.   What’s the name of the big bore kit ?  What elevation are you at? Im in Houston. 1 foot above sea level hot and humid  going to build me one of these   Enjoyed your read. Lots of good information. I’m saving it all 

1
humidity
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vanwert, OH US
4/18/2024 9:04am

 Yes fun project in the end. I was old and rusty skills so I made costly mistakes more than I did when I was a child doing same service. I pushed through and seems good. I found a cheaper cost kit on mx locker website and thought I could use it while servicing stock parts. Trying to keep the bike running always. I never found any company support box labeling seems legit not sure what. If you buy a unknown kit you, should plan on mic. Checking everything verifying all piece's tolerances and such. I think mine was magnum performance brand label.  I've been away from riding for 30 years, but I had a 1981 maico 490 as a kid crazy to get started but fun engine wheeling. I was always hooked on 250 two stroke race bike. I rode all the good years models. The YZ is old memory type bike. 

humidity
Posts
31
Joined
1/17/2022
Location
vanwert, OH US
4/18/2024 11:36am
humidity wrote:
 I thought I could post about this and ask for help and advice . Thanks. I rebuilt my YZ 250 engine with wiseco crank and a...

 I thought I could post about this and ask for help and advice . Thanks.

I rebuilt my YZ 250 engine with wiseco crank and a 293cc big bore kit.

I changed to 182 main and 55 pilot in carb.

I think my carb. floats valve stuck because it had a flutter at mid to high rpm and fuel draining after test run.                                             I opened up float bowl and found a large chunk of aluminum casting material from carb. Casting stuck in the float valve hole. 

I set timing to stock. .18mm. but I might try .10mm

I use the Hitlis power valves. pro circuit pipe. 

I still like my Rekluse clutch full auto. It has effortless lever pull and helps everywhere.

I matched the cases no ledges impeeding.

I used the V- Force 4 reeds.

I will test the bike again after I check floats and valve. But it seems like a completely different bike with a lot more power maybe 30% more feeling not sure. I just wanted to try out a big bore kit. I really thought my stock bore was perfect excellent running as 250.

Make sure the center power valve is moving free and full range first then dial in smooth movement on side valves. Needs attention during build.

I found a 3/16" ledge between cyl. and case so I blended that together, removing ledge. both stock and new cyl. are same, only cases need blended.

I used tools and videos from RMATV to help project and get my skills back.

I also use a super tacky marine type grease chassis bearings, other moving parts. I think this is best to avoid moisture in moving parts and bearings. I also use fully coated fastener lube ARP is best but anti seize is good, coat washer and head flange and threads. This will keep all fasteners better with proper torque values. I cringe how horrible fasteners are treated. Most are one time use nowadays with production, but we reuse.

Ring gap .018" - .022"

 

obiwan461 wrote:
Man I wish I hadn’t read this thread now I want another big bore I did Athena bolt on 330cc kit on a 2001 CR250. Ran...

Man I wish I hadn’t read this thread now I want another big bore I did Athena bolt on 330cc kit on a 2001 CR250. Ran more like a 500. Than a 250. Only thing is it was so big out eliminated the HPP power valve  
 

back to the metal shavings Did you find the source ? Was going to see if you checked your chrome on the slide ?   You Totaly sound like you know what you’re doing.  I think it’s critical to keep a power valve.   My 330 was a blast on Loam but hard pack forget it   It was a sit and spin like the CR500 was not near a  good of a bike as the KX 500 because of the kipps power r valve. The metering of power is critical. Hated my CR500 because it was an on /off like a light switch.  Had to roll On in 3rd like my 330. Had zero bottom flubbed a little on mid range then pulled ridiculously hard and clean on up.   Here I go looking for aYz 250 to build.   What’s the name of the big bore kit ?  What elevation are you at? Im in Houston. 1 foot above sea level hot and humid  going to build me one of these   Enjoyed your read. Lots of good information. I’m saving it all 

713 ft. Elevation 

TDC
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Location
HTown, TX US
4/19/2024 5:32pm

Congrats.

Yup, any big bore kit generally requires leaning out.

humidity
Posts
31
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1/17/2022
Location
vanwert, OH US
4/20/2024 2:18pm

 I'm still jetting, I thought I was good a few times now but it can alaways be improved it seems.

 I ran it hard for 1.5 hours and the plug was darker tan. So I put the good running 250cc JD jets back in but left the 40 power jet. It ran great , but I forgot to look at plug. Need to do that. So now my 293cc is 172 main, 45 pilot, 40 power jet.

 So the power jet should have been changed first from 50 to 40. That seems to be the main issue at this point. 

 They need stronger main fuel tank bolts. They don't need to be tightened, just snugged but I've broke them off twice now. Difficult to remove. I'm going to look for Ti. bolts for tank. I've been wanting to start buying Ti. bolts.

I still miss my 250cc it revs and stirs the gear box more which I think is fun. The 293cc is quite a bit more torque and gets there before I need to shift. 

TDC
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190
Joined
8/4/2023
Location
HTown, TX US
4/20/2024 4:32pm

Seems like adding base gasket thickness could stretch that powerband out some. 

Have you adjusted the timing for better top end?

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