I hate it when this happens

[Hondacrrider]

Well, you know that feeling when you have taken a long break from motocross amd you finally get your bike back together, and everything finally falls in to place? The soil is just perfect, and the weather is awesome, and you are blasting by all the four strokes, and then your bike blows on you again! I had my top end done, and I did it myself, I was so proud of myself, I paid for all the parts myself, and had to wait a while for them, I finally got it together, and my bike blew. The thing is, I didn't blow the top end this time, I looked, the top end is fine, it's the bottom end. What happened was I was ripping super fast along the trail and was way ahead of everyone else, because I was just so excited to have everything fixed(I was beating two 250f's by the way) and I was going up this hill, and I started to clutch it, and the thing started acting kind of funny, like it wanted to die out, but I wouldn't let it, I got up the hill, kept going a ways, and the bike just stops, I wasn't going slow either, I was in the powerband in about 3rd, and the bike just stops and almost shoots me over the bars. I go to kick the bike over again, and it is extremely hard to kick over, it isn't seized, but it is super hard to kick over, I thought the bike may have ate a ring, but I took the top end off, and the ring is fine, it's the bottom end, because it is still hard to kick, without the cylinder on. I ran the bike at 50:1, I broke the bike in using the guide on this site, and I don't know what I did wrong, do you guys know what I may have done?

[JohnN]

Sorry to hear that your bike blew... that stinks!

When you had it apart did you check the big end bearing? After a while those things wear out. SO depending on how much time is on the bike, you have to replace those and the main bearings. When replacing the top end you should always check the bottom end for play. The specs and instructions should be in your manual.

Most faster guys will replace big end bearings once a year on a 125. Just to be safe.

[meger z]

what bike? Plus 50/1 dose sound a bit lean to me ,I run my bike at 40/1 smokes a bit but but seems ok when stripped .its an RM 250,05.Also jetting is a big one i try and run standard jetting.

[JETZcorp]

We run our bikes at 50:1 (we used to do 45:1 because of how big a bottle of Red Line was, but then we switched to cheaper stuff).  The bikes last forever.  Of course, they don't get revved to the moon at the track, but they still git opportunity to rev as high as the engine will go.  For hard track use, I'd go with 40:1 just to be safe.

[metal_miracle]

i always use 32:1 and many motocross manuals recommend that ratio

[KXwestYZ]

i run my kx 250 on 25ml of oil to every litre of fuel... i go through a plug every about 4 hours but think its better to be safe than sorry.. 

[admiral]

i run my kx 250 on 25ml of oil to every litre of fuel... i go through a plug every about 4 hours but think its better to be safe than sorry.. 

that's a 40-1 ratio. the reason you foul plugs is not the oil/fuel ratio, it's the fact that your bike is jetted too rich.
HondaCR, that sucks. if you were in my area i would help you out with the crank rebuild. i've got one of those press together jigs that assemble the crank true with no blows.

[scotty dog]

That sure sucks! I have run all my bikes at 40:1 with good quality oil and have had one big end failure in over 10 years racing. Bad luck mate, it sucks when that happens after youve done work to ya steed!!
Mine was an 89 RM 250 which siezed the big end , snapped the rod and put it thru the cases.................that hurt...

[grumpy]

Ouch, always hurts to hear of bad news on a 2t. I run 24:1, but then I just use mine as play bikes and don't need to pull every ounce of hp out of 'em and would rather have more oil for less likelihood of failure. Though, until I see better research I actually believe 20:1 provides the most power and least chance of failure of all ratios. Check out this article at Bridgestone http://www.bridgestonemotorcycle.com/documents/oilpremix6.pdf though it's old, it's the most scientific testing I've seen on premix ratios.

[Hondacrrider]

Well, I have done some research, and come to three options; Option one is that I go to bikebandit.com and buy every OEM part(top and bottom, because it turns out that I also detonated the piston), which, I am not sure I want to do. I can also go to motosport.com and order a wiseco top and bottom end kit which includes everything(has anyone had any experience with this kit?), or I can look for a used engine and swap the engine out(Does anyone know if other brand engines will easily fit in my frame? Like a 250 2 stroke RM?). Now, I think that wiseco probably has the best performance parts, but isn't OEM more reliable? Also, would the wiseco set come with the bottom end put together already, because I don't have the machinery to put it together otherwise, if so, does anyone know of a place that can help me out? Also, is a 2001 cr250 engine interchangeable with a cr125 frame, I heard that the 2001 was the best year. Any advice would be appreciated, 2 strokes still rule, but believe me, I got hassled about it from my 250f friends, although their bikes are only 2 years old, and mine is a 2001, and as far as I know hasn't had a top end until last week. Well, I suppose it is time to take the old fart out of the shed(1984 xr 200), say hello to good gas mileage, and heavy weight, no power riding, at least it isn't a loud four stroke...

[TMKIWI]

Bad luck mate.
I have found Wossner forged pistons to be very good and reliable.
I am not a fan of Wiesco pistons ( I have seen too many fail in outboards) . When I rebuilt my RM I went OEM and counter sunk the piston ports to retain oil. Its a trick recomended to me by a pro RM rider and Suzuki mechanic.
I always run 40:1 Synthetic and have no problems.
Just run what oil mix you are comfortable with.

Grumpy that is a interesting article but that is for aircooled motors that have no relevence for water cooled engines. Thermal efficiency is chalk & cheese between the 2 types of engines.
Thats why modern 250s have twice the horsepower of the old aircooled engines and can run much leaner oil rates.
 

[JETZcorp]

Twice the horsepower?  What about Maico's air-cooled 250 in 1981, which made 42 horsepower?  And I know the 490 gained a lot of power when they made the new '83+ engines, so I wouldn't be surprised if the '83 250 (also air-cooled) made more than 42, but of course I don't have a number to post on that.  Yes, the '81 is four horses down on a new YZ, which also has a powervalve, but you can't say they've got double, unless you look at a 250 that doesn't have an expansion chamber, or something.  And remember, all my bikes, as well as my dad's and my uncles', are air-cooled and running 50:1 ratios, and they haven't had lubrication-related failures.  I'd probably put more oil than that if I were to go actually racing, but still, anything that can last thirty years on 50:1 probably doesn't mind lean ratios.

I don't want to start another new vs old debate, I'm just saying, don't short-sell the fins.

[opfermanmotors]

I run 32:1, here's an article by Super Hunky:

http://dirtbike.off-road.com/dirtbike/Tech+Archives/Two-Stroke-GasOil-Ratios/ArticleStandard/Article/detail/400487

[JETZcorp]

Well, I should say, if it were totally up to what the engine likes, I'd probably mix right about like you, especially if I were to go racing.  The main reason we keep it lean is that oil ain't cheap and we don't exactly make six digits.  And, considering that I've still got 43-year-old rings in one of my bikes with no troubles, it seems to be working well enough for government work (although admittedly, we run more and higher-quality oil in that particular bike).

[TMKIWI]

I was talking about the 27hp the PE made in the dyno run.
Calm down boys.