two-stroke rebuild questions

[burnham]

So I have an '02 KTM 300exc that I bought new in '03. It has maybe a hundred hours on it. Most of those hours I'm just lugging it around in tight trails. The bike still runs great, but should I rebuild the top-end just for sport? I was thinking of checking the compression to see what I have. Roughly how much PSI should this thing produce?

The bike was also using a ton of fuel the last time I used it. Roughly twice what it would normally burn. Is that an indicator that it needs a rebuild? It wasn't leaking and I haven't done anything with the carburetor....ever.
thanks, todd.

[Paul_E_D]

Yes, I would rebuild it. You CAN run em forever, but you'll never know what your missing in a fresh motor. Compression (im not positive about a 300) should be higher than 160psi. I know my 125 would be like 195 when fresh.

The fuel consumption is definitely odd. twice as much as normal? I'm not even sure what could cause that, but rebuilding would be first on my list. Be prepared to buy some parts for the powervalve system. There are a lot of wear points that will be in tough shape after 100 hours.

top end plus powervalve freshen up and you won't believe its the same bike!

[R7]

What Paul said

A tell tail sign that a smoker is getting tired is the cold start-up. Fresh, 1 or 2 kicks and it should be running. If it takes a bunch of kicks to get it running, it's probably over due.

As for fuel consumption, could be something as simple as how or where you rode it last. My 200 xcw smoker sucks on fuel so far, but I'm guessing it's mostly because all I've ridden is slow slingle track trails. I think I'll be lucky to get 40 miles out of the 3 gallon tank with this bike

[Rambunctous]

Freshen it up with a new wrist pin and bearing, piston and ring. You will also need a cylinder base gasket and head gasket or Oring. Rather easy job.

[gregp]

I am the lone disenter here. I would probably leave it alone. 100 hours is nothing for that bike, especially at low rpm. If you have the money (it will cost you $200 or more, by the time you get all of the gaskets) it is always nice to do, just to have peace of mind.
My '04 KDX 200 is still running on its original top end, and I rev it out quite a bit and have way more than 100 hours on it.
A worn top end is not causing your excesive fuel consumption. A stuck power valve might have some impact, though.
I suggest taking the top end apart, and looking everything over closely. You can clean the power valve parts, and pop in a new ring (or 2, depending on the design). The head gasket on that bike may be a simple orange O-Ring (inexpensive, possibly re-uasable), the base gasket you will have to buy, though. The forged pistons that KTM uses are of very good quality, and usually last a long time.
JMO.

[SSearchVT]

Check the compression and see what you get. Personally - I'd pop it apart and at the very least deglaze the cylinder and re-ring it. Call the shop and see if they have all of the top end parts in stock or order them ahead of time. There's nothing worse than watching a great riding day go by because your bike is apart and a $2 base gasket is on back order.

The fuel consumption could be a stuck float or bad needle and seat in the carb. Take a look at the area by the overflow tubes and see if it is washed nice and clean.

[Paul_E_D]

Well, my guess is that the powevalve needs attention more than the piston. I'm willing to bet that Greg's KDX powervalve also needs attention. the spindles and linkages wear out fast in my experience. No way do they last 100 hours. Now, the powervalve may still function to some degree, but there will be tons of slop in it, making response mushy or inconsistent. While in there, there is no reason not to put new rings in. That is an expensive failure when it happens. Been there, done that. Pistons do fail at the skirt with age, and that is also an expensive fix. 200 bucks now sure beats 700 bucks later.

[seth399]

Ya what they said. I just did the top end on my yz at 50 hours and it was running perfect. Just preventative/keep it fressssssh

[SSearchVT]

Pistons do fail at the skirt with age, and that is also an expensive fix. 200 bucks now sure beats 700 bucks later.

Not to mention fixing it on your schedule instead of pushing it out of the woods and scurrying to find parts...

[breakdirt916]

imho, do it!

[noob] as I understand it, for 2-stroke bikes there is no required interval for servicing the top end...the only tell tale rule is that preventative maintenance is much cheaper than corrective maintenance.

According to the books, 100 hours would be due on a smaller bore MX bike, but on a bigger bore woods bike, that figure may be more lenient.

my last comment is on compression: iirc there is no magic number...a bike with a fresh top end could have 150psi while a worn bike could have 165. The only real way to assess top end wear is to have a baseline compression reading to compare against

considering that you're way past that, a backyard mechanic's test is to read the compression, add a few drops of oil through the spark plug hole, test again, then test after a few kicks. If the latter kicks drops the compression significantly, the compression increase was from the oil, and you're past due for a change.

but if it were me, I'd just do the top end, get a reading and take it from there...it could cost like $150 to do it all (piston/ring/wrist pin/gaskets)

[/noob]

[seth399]

^ Your rm doesnt have an interval for top end?? I forgot on my last yz125 but I think they recommended every 20 hours. My 250 is recommended at 40 hours.

IMO, it is good to do piston/rings at intervals as provided nothing else goes wrong (you clean your filter all the time and change oil) then the cylinder should remain in pretty tip top shape if the rings never get super old. I have seen several cases where the stock yz piston (or other bike for that matter) basically explodes once it fatigues too much kind of like what Paul mentioned.

[burnham]

so I tore it down and sent out the cylinder/head today. the bore and piston look great, there is some carbon but no build-up, and there is still a good cross-hatch on the cylinder wall. what I didn't like was the action on the power valve, it took a good amount of effort to move it. I don't know what it should feel like, but it didn't feel right. the parts are going to a place in PA for some porting, and they'll send me a new piston and gaskets when the work is done. I dropped the forks off at computrack today, so hopefully the bike will be back together in a couple weeks. thanks guys.

[seth399]

so I tore it down and sent out the cylinder/head today. the bore and piston look great, there is some carbon but no build-up, and there is still a good cross-hatch on the cylinder wall. what I didn't like was the action on the power valve, it took a good amount of effort to move it. I don't know what it should feel like, but it didn't feel right. the parts are going to a place in PA for some porting, and they'll send me a new piston and gaskets when the work is done. I dropped the forks off at computrack today, so hopefully the bike will be back together in a couple weeks. thanks guys.

nice, gotta love a freshened up bike!

[breakdirt916]

^ Your rm doesnt have an interval for top end??

it does! my bad...I worded that wrong...what I meant is there's a "recommended" service interval for the top end based on the number of hours/races you do, but I think a lot of people don't follow it. So if they're past the recommended 40 hours, aside from "yes change it", there's no way of measuring specs on the bike to figure out if they should keep running that top end or not. Compression is only useful if it's compared to baseline

is that right?

[seth399]

it does! my bad...I worded that wrong...what I meant is there's a "recommended" service interval for the top end based on the number of hours/races you do, but I think a lot of people don't follow it. So if they're past the recommended 40 hours, aside from "yes change it", there's no way of measuring specs on the bike to figure out if they should keep running that top end or not. Compression is only useful if it's compared to baseline

is that right?

ya for sure. I think other than some race bikes that are being rung out to hell, the only reason most 2strokes seize is a combo of the following:
a) never changing oil
b) never cleaning filter
c) just never doing top end/rings until the damn thing blows
d) horrible jetting and the thing is running lean and/or has air leaks

[burnham]

So I have the bike mostly back together. New tires, chain, hand guards, grips, odometer cable, and a few other small odds and ends. What I don't have is the cylinder and head back from the shop yet. I sent them to Cycle Playground in PA. The stuff was supposed to be turned around in three days, it's been a month now. They don't return calls or emails, or follow up and do the things they say they will. I was told the parts should be here last Friday but they didn't show up yet. So anyway, the bike hopefully will be up and running this week, and fuck Cycle Playground.

[breakdirt916]

It sucks...I know...it's hard to find good dirt bike mechanic and machine shop nowadays. I dropped off engine cases in January and only got it back a couple weeks ago to finish up the job

It's good to see that you made progress on everything else in the meantime so that it's ready to go. Nothing worse than waiting a month to get a part, then wait another month to finish off the rest. It will all be worth it when it runs

And yes f cycle playground

[seth399]

as far as coating cylinders, I hear this place Millenium coatings is the bomb

[Stromper]

I think you better drive there next Saturday

Let them know by phone and email your coming loaded for bear

Take it back REGARDLESS of what they say

Crystal Meth its not just for breakfast anymore

[Paul_E_D]

Yeah, screw the porting. Just get the stuff back. You'll notice more improvement just in getting the squish set right.

[burnham]

Well there was a note on the door from the post office, I guess (hope) my parts made it there and I just need to go sign for them.

I'm sure something is either missing from the box, or fucked-up.

[rolker]

as far as coating cylinders, I hear this place Millenium coatings is the bomb

I just had my RS125 cylinder replated over the winter by Millenium and so far so good with it.

[burnham]

Yeah, screw the porting. Just get the stuff back. You'll notice more improvement just in getting the squish set right.

Do I set the squish by getting the X height set? I think the only what to make an adjustment is by stacking/selecting base gaskets, the head is o-ringed.

[peterz85]

Careful on the squish, stacking/reducing base gaskets may change your port heights/timings and cause some adverse effects. You should be good to go with your stock set up as long as they didn't port your cylinder.

Pete

[burnham]

they ported it. I was told the KTMs want a zero X height (piston level with top of cylinder at TDC). I figure by adjusting the X I will also be setting the squish.