I've received lots of help from reading the rebuild topics on this forum, but few things seem to be unclear.
I know I have to get a:
New piston (1mm oversize Im thinking, cylinder appears boreable, piston had only marking 25 and same 3mb00 marking or something, that cylinder had) New rings New bottom gasket New circlips New small end bearing New.. thing that goes into piston.. finger? lol
Which adds up just about 110 pounds by local Yamaha dealer. Now that's fine.
I have to clean my engine as well, because a broken piece of old ring is somewhere in there. Which shouldn't be a problem, cause my friend is a motorcross racer and knows motorcycle engines and can help me with the labour.
Only thing that worries me is.. does my piston rod have to move from side to side? This truly worries me, because I may rebuild the engine and the movement could muck everything up afterwards, so I'm asking, is it normal? Or does it have to be stiff and only move when engine moves? OR if the movement from side to side is okay, how much should it move approximately?
And another thing, old piston was melted on top, just at the spot where the spark plug is. And it wasnt a small melted hole, it was damaged pretty well. May add a photo later today. Rings were damaged as well, probably from the heat, one of the rings snapped during a ride. Is my mixture wrong? what could cause that?
You used to be able to get pistons in 0.25mm steps - if thats still the case go for a .50 oversize and leave yourself room for rebores in the future. The thing that goes in the piston is the gudgeon pin.
A little bit of lateral deflection(!) is normal, in fact required, at the big-end of the conrod. The Haynes manual says the "big-end axial clearance (endfloat) should be between 0.2 and 0.7mm - basically you would want to be able to slip a feeler gauge in that range between the big-eng thrust bearing and the crankshaft flywheel. The Haynes manual does a good job of describing and checking this properly.
If you've melted a hole in the piston crown its usually due to running too lean a mixture. To correct this you'll need to up the main jet size a little. Be aware though, it can also be caused by an air-leak at the magneto side crankshaft seal, too far advanced ignition timing or faulty igniter box, too hot a spark plug range, too high a compression ratio, or too low octane fuel to name but a few. 2-stroke engines, eh? Gotta love 'em...! :)
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Just because I don't care, it doesn't mean I don't understand...!
Holy sh*t at the last part! I'm hoping it's the main jet fault, lol. What is the standard size and what would be the optimized size for me to get? Should I try another spark plug as well?
Don't worry too much about the last part - 99 times out of 100 its mixture-related that does it.
Depends - what mods do you have? Standard on most DTRs is 210, 230-240 might be better if you've derestricted it. I know many will not agree with me here, but its always good to be a tad on the rich side with 2-stroke motors, especially ones that are likely to get a good hammering every now and again... ;)
Once you've bedded it in again (remember a rebore will require 500 miles to run in again) and start to thrash it, fit a 240 and do a plug-chop, take it from there. Until its run in, a 210 will be fine cos you are NOT going to abuse it, are you...
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Just because I don't care, it doesn't mean I don't understand...!
well i have just done my top end and i got the same size piston as standard and it worked fine same power if not a little bit more but i guess it depends on miles were etc
Don't worry too much about the last part - 99 times out of 100 its mixture-related that does it.
Depends - what mods do you have? Standard on most DTRs is 210, 230-240 might be better if you've derestricted it. I know many will not agree with me here, but its always good to be a tad on the rich side with 2-stroke motors, especially ones that are likely to get a good hammering every now and again... ;)
Once you've bedded it in again (remember a rebore will require 500 miles to run in again) and start to thrash it, fit a 240 and do a plug-chop, take it from there. Until its run in, a 210 will be fine cos you are NOT going to abuse it, are you...
Ofcourse I'm not going to abuse it. Then I'll get the 210 main jet and 240 main jet. I'll use the smaller one first, then upjet, as I seem to have full Arrow system installed. Any idea how much these jets cost? I don't know much about what last owner did to it, but I'll be sure to take care of it well.
Any good motorcycle shop with a dyno will be able to supply you with a 240 main jet, I'd be surprised if you paid more than £2.50 - £3.50 for one.
Have you any photographs of your knackered piston?
I'll upload the pictures of my piston + rings and cylinder tomorrow.
My conrod (where the piston is attached) moves more than 0.2mm - 0.7mm from side to side, it's obvious. I asked Yamaha and they said one conrod bearing costs about £30, does that mean it has two conrod bearings or even more?
If you push and pull on the conrod you shouldn't feel any movement if the big-end roller bearing is good. Similarly, any "rocking" should be an absolute minimum, about 1.0mm maximum. The 0.2 to 0.7mm mentioned above refers to clearance between the conrod and the thrust bearing - this is sideways movement, or to put it another way, the big-end sliding along its pin. If I had more time I'd knock up a couple sketches to try to explain this better for you but right now I've not got time. Will do it tonight.
At the big-end there is one roller bearing and two thrust bearings. Only downside to it all is rebuilding a crankshaft with new bearings is a tricky job beyond virtually all home mechanics, and you will need to shell out for a specialist to rebuild the crank assy for you.
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Just because I don't care, it doesn't mean I don't understand...!
Today I inspected the conrod closely and found out it actually isn't so bad. Previously I measured side to side movement where the small end bearing is, today I measured it where the conrod is attached and it was about millimeter or a bit more, so I'm not too worried about it anymore, as there is no excess movement when I move the conrod up and down, it seems to be working well. But a broken piece of my old piston ring should be somewhere in there, is it a hard job to get it out? I think it's somewhere under the crankcase, but there's a good chance it went into muffler, lol. Or not. Need to clean the engine as well, really good, to get all the excess dirt out.
Well, here are some pictures of my worn out piston.
Piston: http://www.hot.ee/marko727//DSC00056.JPG
Another one: http://www.hot.ee/marko727//DSC00057.JPG (I wonder what does the marking 25 mean, also attention to melted area)
Rings: http://www.hot.ee/marko727//DSC00058.JPG
Cylinder: http://www.hot.ee/marko727//DSC00059.JPG (Am I right if I say that the inner dark gray area is mostly boreable area? Because I'll certainly buy a bigger piston)
Sorry about the quality, I had nothing but my phone as a camera when I went into garage, hehe.
PS. Forgot to take a picture of the spark plug and the head, but both were quite black beneath, not coffee brown, as suggested, I guess that refers my bike's "hotness" came from too rich mixture. Will change the main jet, try different sizes as well.
-- Edited by marko727 on Thursday 2nd of July 2009 02:34:16 AM
My parts are hopefully on its way to my country, hehe.
But here's a quick question. I'm pretty sure I know how to reassemble everything and everything is already cleaned up.
But there's a skeleton in my closet, my last piston was melted as you can see from my previous post, I'm trying to figure out what it was. It can't be the cooling system, I've basically checked all the elements.
I cleaned the air filter and the carburettor today and found a fishy little something. Pilot air screw is totally ****ed by previous owner. It's almost unscrewable, but I managed to take it out. Head of that screw is almost broken, but the rest is fine. CAN it be that it was that SAME goddamn screw that made my bike hot really quickly and even overheated it? Like.. causing the mixture to be too lean/rich, or is the mixture screw something else? And do they sell it at dealers, or do I have to go for whole goddamn carburettor?
Thanks and peace, can't wait to get back on the road.
=)
-- Edited by marko727 on Tuesday 7th of July 2009 12:57:47 AM
