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Confussed on loss of power 02 CR250?

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So I'm riding a 02 CR250. I've had the bike for about three years. Last season I decided to rebuild my top end. I pulled it apart and the piston looked ok, light brown carbon build up on the crown and some blow by down the sides and undernieth. The plating on the exhaust bridge was pretty much gone, which explains the blow-by. I had sent the cylinder to Eric Gorr at Forward Motion. I mostly ride tight rocky single track and occasionally ride massive sand dunes. I was after more bottom end. EG turned .01in. off of the head, .03in off off the cylinder base and appoxied the ports for many reasons to change power distribution??? I also had the cylinder replated. Along with these mods, I put on a new gnarly pipe, turbine core spark arrestor, 10oz flywheel weight. The bike ran great all season, so I never got into the jetting. I went to the dunes in the fall and my bike ran great the whole first day. The second day it started to bogg a little and I really started to notice a huge loss in power. If anything I thought it was running a little rich all summer, the power was there, I never once fouled a plug in the tight stuff and usually in the dunes it loves to be on the pipe and doesn't have any issues. The dunes I ride are a little lower altitude than the mtns at home. I got home and pulled the jug and the pics below are what I got. The last pic is after It was honed. It actually looks good in person. There is a circle mark in the carbon build up on the crown? I think my other piston also had this? Is this normal? Right now I'm replacing the crank bearings for good measure, everything else looks good, water pump ok. Seems like it got hot? I rode it for a while with the power loss and had no coolant loss or leaks. Any clue why I would have such a loss of power? I've had this bike in the dunes before the head and cylinder work and had no issues. I can't diagnoise it? Could it be an air leak in the crankcase? Would that matter? Do you think the top end mods affected this? Sorry to ramble. I'm just about to put this thing back together. I never did test the compression, it seemed good, and the piston was only about 4 months old. I mostly just rode the woods fast at low rpms. And advice would be a huge help. Thanks.

[html]http://s1159.photobucket.com/albums/p629/Ziggler27/?action=view&current=SANY0087.jpg[/html]

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[html]http://s1159.photobucket.com/albums/p629/Ziggler27/?action=view&current=SANY0087.jpg#!oZZ3QQcurrentZZhttp%3A%2F%2Fs1159.photobucket.com%2Falbums%2Fp629%2FZiggler27%2F%3Faction%3Dview%26current%3DSANY0085.jpg[/html]

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[html]http://s1159.photobucket.com/albums/p629/Ziggler27/?action=view&current=SANY0087.jpg#!oZZ1QQcurrentZZhttp%3A%2F%2Fs1159.photobucket.com%2Falbums%2Fp629%2FZiggler27%2F%3Faction%3Dview%26current%3DSANY0018.jpg[/html]

Edited by BzmnKTM

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The only thing unusual I see is the pitting on the crown. It looks like it has been lightly sand blasted. That says detonation or sand ingestion. Otherwise our piston simply looks toasted. How many hours did you have on it? You know Honda says 7.5 hours for replacement in the manual. That's a little conservative. 20 hours is good for a strong rider. 40 hours is good for an average rider.

Edited by 1987CR250R

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I would also check your RC valve. Make sure the cables are tight and adjusted properly.

Also make sure that it actually works. The servo motor on my died out.

Edited by Crazyced

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The trench on the piston where it runs against the exhaust bridge suggests to me that the bridge needs to be relieved several more thou. It was probably fine for low-speed work, but after continuous high-rpm use it got hotter than usual and grew into the bore causing the trench. It probably notched the ring, causing blowby, weak compression, and loss of ring tension. The wash on your piston suggests that your jetting was safe and not a factor. You got lucky, the ring will often break in that situation and catch the port, ruining the cylinder. I would get the bridge relieved more.

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This is a good example of why OEM cast pistons are superior to forged pistons. Even with the lubricating holes drilled in the forged piston, the piston and rings will seize there first if things get a little too hot. The rings are now shot.

I would recommend going back to OEM piston and rings to prevent this problem in the future. With 2-stroke engines, cast pistons can handle the higher engine temps that occur on sand better than forged.

Also note that the hand drill job on the piston resulted in one hole being way out of line and this will result in lower secondary compression and loss of power.

piston.jpg

Edited by CamP

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Ugh, just bought a Wiseco for my '98 to fix any rings that might spin due to poor port placement. If I have trouble with it I suppose I could just buy a '99 jug some day.

You'll probably be fine. Just run leaded fuel if you ever ride in sand.

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I know its probably somthing having to do with the motor , but i know one time i lost the muffler packing in my 01 and the center tube cracked in half and the bike had a bad bog and ran like garage. May be something to look into?

ALso, I 2nd the RC cables/valves mine were badly carbonized and i had a poor idling/low end performance and after cleaning it is impresively cleaned up although i think a leaner needle will fix the probs all together.

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Ugh, just bought a Wiseco for my '98 to fix any rings that might spin due to poor port placement. If I have trouble with it I suppose I could just buy a '99 jug some day.

your 98 has no exhaust bridge. no bridge, no holes, no worries

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your 98 has no exhaust bridge. no bridge, no holes, no worries

Major :smirk:

:bonk: Haven't even pulled cylinder to measure the bore. Will do ring end gap with new rings prior to install though.

Looking at the 02 pics, I remember that exhaust bridge. Running a cast piston on that bike.

Edited by shrubitup

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There is no problem running forged pistons with a properly relieved exhaust bridge. A cast piston/oem ring won't hold up to a tight exhaust bridge any better than a forged one will. Your piston doesn't look like that simply becaused it is forged. In fact cast pistons tend to crack when run past their time moreso than a forged.

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A cast piston/oem ring won't hold up to a tight exhaust bridge any better than a forged one will. .

Wrong...

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Wrong...

Sorry, pal, but it is you who is wrong. Just as wrong as saying leaded fuel would have prevented this, or the misaligned hole caused a loss of crankcase compression. Just thought the OP should understand the misinformation.

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The trench on the piston where it runs against the exhaust bridge suggests to me that the bridge needs to be relieved several more thou. It was probably fine for low-speed work, but after continuous high-rpm use it got hotter than usual and grew into the bore causing the trench. It probably notched the ring, causing blowby, weak compression, and loss of ring tension. The wash on your piston suggests that your jetting was safe and not a factor. You got lucky, the ring will often break in that situation and catch the port, ruining the cylinder. I would get the bridge relieved more.

+1 to this. IMO, call Eric and tell him what happened. Send him the pics. I would bet that he would take your cylinder back and fix it for you for free. He is very good to work with.

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Sorry, pal, but it is you who is wrong. Just as wrong as saying leaded fuel would have prevented this, or the misaligned hole caused a loss of crankcase compression. Just thought the OP should understand the misinformation.

Fact. Using leaded gas in a high load situation, like sand dunes, will help keep a 2-stroke alive when pushed to it's thermal limits.

Fact. Drilling holes in the exhaust side of the piston will bleed off crank pressure if they are out of line with the bridge. This reduces the efficiancy of the pump and reduces power.

Fact. Cast pistons are more thermally stable than forged pistons, so they are less likely to stick on the exhaust bridge when the exhaust temps get too high.

In the dunes, I'll take the bike with leaded fuel and a cast piston over the bike with pump gas and a forged piston.

Edited by CamP

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Fact. Using leaded gas in a high load situation, like sand dunes, will help keep a 2-stroke alive when pushed to it's thermal limits.

Fact. It won't make any difference if there isn't adequate bridge clearance at high temps

Fact. Drilling holes in the exhaust side of the piston will bleed off crank pressure if they are out of line with the bridge. This reduces the efficiancy of the pump and reduces power.

Simpleton view. Fact: The bridge doesn't plug the hole, it is there to recieve the cooling mixture that bleeds through. At 8000 RPM there is no percievable difference in crankcase compression between a drilled or nondrilled piston.

Fact. Cast pistons are more thermally stable than forged pistons, so they are less likely to stick on the exhaust bridge when the exhaust temps get too high.

True but not what you said to OP. You said a cast piston will avoid what happened, like cast aluminum alloy will somehow withstand nicklesiliconcarbide intrusion better that forged aluminum alloy. Really? Absurd!! Carbide will win regardless of the method of piston manufacturing.

In the dunes, I'll take the bike with leaded fuel and a cast piston over the bike with pump gas and a forged piston.

And I'll take the one with a PROPERLY RELIEVED EXHAUST BRIDGE.

Good day. The OP can choose for himself.

Edited by 36MotoMarc

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Right. Looks like we'll have to agree to disagree on this one.

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In the dunes, I'll take the bike with leaded fuel and a cast piston over the bike with pump gas and a forged piston.

+1

I run my 2005 CR250R in the Buttercup California dunes, and I use 110 race gas, and the engine has a Pro-X cast piston in it.

Runs awesome, no issues.

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+1

I run my 2005 CR250R in the Buttercup California dunes, and I use 110 race gas, and the engine has a Pro-X cast piston in it.

Runs awesome, no issues.

My guess is, you are not running on a replated modified cylinder. IMO Eric didn't relieve the exhaust bridge properly after the replate and that caused this issue. It would have done this with either a cast or forged piston. Call EG and let him make it right. In my dealings with him, he has been more then willing to go the extra mile to make sure someone is happy. Just call him, be nice and let him make it right. End of story, cast or forged.

Edited by frdbtr

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