siezing is expected?

jaguar · Feb 5, 2004

If you are on the highway (let's say you live out in the boondocks where there aren't any cops) and in top gear you get the engine revs into the middle of the on-the-pipe "hit" and then reduce the throttle setting to 1/4th to 1/3rd to maintain speed and not accelerate
then is it normal to seize?
That just happened to my KDX200. It was jetted great and I had practiced motocross with it just the day before. On taking it apart the piston top showed the middle of the crown black colored, and the remaining crown (probably half of the area) a light brown. The pipe/porting of the engine was for the hit to happen at upper mid range. The wiseco piston melted on the exhaust side and I really had to work it over to release the stuck bottom ring.
what do ya think?

agitt73 · Feb 5, 2004

very lean say if your jetting is correct an air leak or improper warm up wiseco pistons
need to be warmed up alot better then stock or last but not least bad fuel

WWR · Feb 6, 2004

Your jetting could have been fine for wide open throttle (main jet), but your pilot jet could be too lean making the quarter-throttle position too lean. Many people get the jetting (main, needle, pilot) confused by thinking these are tied to engine rpm instead of throttle position increments.

It is most likely that you are lean on the main, also, which overheated the piston crown. It often takes a few seconds for this to actually seize the piston. It could have been well on its way to seizing before you even chopped the throttle.

If your piston crown was melted, this is often a sign of detonation or too much ignition advance. Detonation is primarily caused by your octane being to low for the engines' requirements. Ignition being advanced too far can cause excessive heat in the combustion chamber, even if there is no detonation present. My bet is that it was detonation, though, possibly aided by a main jet that could have been a tad too lean.

CaptainObvious · Feb 6, 2004

jaguar said:
It was jetted great

No it wasn't.

Your needle or pilot circuits were too lean. A bike that is just "a little" too lean feels real snappy. That is what you felt the day before on the MX track.

Repair and rejet.

jaguar · Feb 6, 2004

It was well warmed up.
I doubt an air leak because all the seals are less than a year old.
It couldn't be the main jet because I wasn't wide open throttle.
I took the slide out to inspect things and found that the stock needle doesn't even taper very much until about half way down the needle. I think that would contribute to being a bit lean at 1/4 to 1/2 throttle. So I took my exacto knife and scraped some brass off the needle to open that area up a bit. But now the "hit" is a little less but hopefully it will be less prone to highway siezures. And I will ride a little slower on the highway to keep the revs just below the area of the powerband where it starts to really come on-the-pipe.

Enduro_Nut · Feb 6, 2004

It is the main - when you ride "on the pipe". Providing the motor is stock, you should run regular fuel.

In the 70's I called WISECO, SEIZECO because of the problems I had. From what I understand, they have improved but as previously mentioned you need adequate warm up time because of the aluminium piston not expanding at the same rate as your cylinder.

dirt bike dave · Feb 6, 2004

If you are maintaining a good clip on the highway and then roll off the throttle (like when you are going downhill or just wanting to coast or slow down) this can be a problem on a hot two stroke. Think about it - you roll out of the throttle, but the hot motor is still turning 8,000 rpm. No throttle = no cooling fuel and lubricating oil. Things can seize in a hurry, even if the bike is well jetted for normal trail work.

To avoid highway seizure on a two stroke, make sure you pull in your clutch if you are coasting at speed.

Layton · Feb 6, 2004

jaguar said:
So I took my exacto knife and scraped some brass off the needle to open that area up a bit.

:ohmy:

You are on the right track but a new needle would have been a better idea. Getting the same amount off the needle around its complete diameter would be impossible to do with a knife.

canyncarvr · Feb 6, 2004

re:'this can be a problem on a hot two stroke.'

Oh yeah. Even air-cooled twin 400cc yamaha street screamers!!

A throttle roll off is a great way to stick a 2-stroke.

Best to avoid it.

re:'Pull in the clutch'

Great advice.

Jon K. · Feb 8, 2004

Enduro_Nut said:
In the 70's I called WISECO, SEIZECO because of the problems I had. QUOTE]

You got that right!

Back when I was running enduro; if the trail opened up onto the highway; I would run with the choke on to avoid just that problem.

I learned this trick after seizing my Husky with a (you guessed it!) Wiesco piston.

jaguar · Feb 9, 2004

Guys, I put solder back onto the area I had scraped off because the mod had it running too rich (obvious by the power loss and more spooge). Now it runs great again.
Dirt Bike Dave hit the nail on the head. I always thought a friend of mine was nuts because he wouldn't ride his mid-range powered CR250 on the highway with me to get to a riding area. Now I know what he was scared of!

dirt bike dave · Feb 9, 2004

I guess there is a reason that two stroke street bikes are oil injected by a mechanical pump that is rpm sensitive. :)

jaguar · Feb 10, 2004

man, you are so on top of this subject.
yeah my bike has an injector pump but I don't use it since the plastic drive gear inside the right cover stripped and I can't order it from the states.
Next time I order from England I will get one.
thanks Dave!

dirt bike dave · Feb 10, 2004

Jag,
I dual sported a '90 KDX200 and a '91 KDX250, neither with oil injection. I did some 500+ mile rides on these, many highway miles and never had a problem.

However, I did blow up my '99 CR last year. I was trying to keep up with some fast guys with higher top speeds. After about 1.5 miles of pure WFO, I decided to back off the throttle to help my motor cool. BAD IDEA. The big end bearing went south in no time. If I had pulled in the clutch, or pulled the choke, or hit the kill button while keeping the throttle open, or just continued going WFO, I am convinced I would not have blown my motor. It was an expensive lesson - needed a new crank, piston, right side crankcase, etc... etc...

jaguar · Feb 10, 2004

ouch! expensive lesson.
well, hopewfully those reading this thread will learn from our mistakes.

canyncarvr · Feb 11, 2004

An oil injection system based on a pump regulated by throttle position and rpm will still get you a lean roll stickage!

Well...it worked for ME anyway (RDs) :(

john stu · Feb 20, 2004

lean jetting or oil injection

kdxfreeeek · Mar 9, 2004

thanks guys you realy helped me here i just got my 96 kdx 200 and rode it down the road and back and noticed that the guy bewfor me that just rebuilt it did a half a** job well i rolled off the throttel when i rode it up the road and when i took it apart to do sum much needed repairs to the fresh rebuild i noticed simalar scaring to what it turned into last time when it seized now i know to not rll off keep it throttle and pull in clutch thanks guys buy

sean_mxrider · Apr 1, 2004

your engine overheated

Someone · Apr 1, 2004

Wow this is good info. I just got a wiseco put in my 01 YZ 250 and when I hit the trails, I have to travel .5 to 1 mile on pavement to hit the trails. I will remember what I learned here! Trails open up in two weeks here, and that would have SUCKED seizing my new top end!

SULLY · Apr 2, 2004

Also worth remembering that if you are in the woods the engine gets hot, then if you pull out onto the road and accelerate the cylider will recieve a cold blast of coolant from the rads, causing it to contract, if this happens too quick for the piston to cool and contract it can seize.

silveco · Apr 26, 2004

jaguar said:
If you are on the highway (let's say you live out in the boondocks where there aren't any cops) and in top gear you get the engine revs into the middle of the on-the-pipe "hit" and then reduce the throttle setting to 1/4th to 1/3rd to maintain speed and not accelerate
then is it normal to seize?
That just happened to my KDX200. It was jetted great and I had practiced motocross with it just the day before. On taking it apart the piston top showed the middle of the crown black colored, and the remaining crown (probably half of the area) a light brown. The pipe/porting of the engine was for the hit to happen at upper mid range. The wiseco piston melted on the exhaust side and I really had to work it over to release the stuck bottom ring.
what do ya think?

I have another theory apart from your jetting being too lean.
You must remember that your cylinder barrel and your piston are two seperate components that can be cooled in two seperate ways.
Firstly, all metals expand and contract with temperature.
Your piston is being cooled and lubricated by the incoming Air/Fuel mixture.
The Cylinder barrel is being cooled by the coolant system.
Therefore, If you are cruising at high speed with only a partial throttle opening, the coolant system will be working well due to the air flowing through your radiators. This will cause the cylinder to run cool and in turn it will shrink.
With only a partial throttle opening, there will not be much air/fuel mixture to cool/lubricate the piston. Also the high rpm and small throttle opening will cause a lean condition.Both of these can cause your piston to run hot and expand.
Cylinder contracts-Piston expands.....SEIZE!!!!
I know a young guy who did the same thing to a 3 day old Honda CR125.
My advice is Good Fuel,Good Oil and ride it Hard on the Throttle to Hard on the Brakes.

WWR · Apr 26, 2004

It is most likely jetting. Your jetting is probably lean at a certain throttle position. You could be rich on the pilot and spooging alot, but lean on the main (or vise-versa).

But you do have a point with the coolant. My NSR (street version of the RS250) has a thermostat, which would keep the coolant in the cylinders from dropping below 55C. As for my RS250 (GP bike), no thermostat but we use duct tape on the radiators to keep them up to temp. Then again, the radiator size is much larger than what a dirtbike uses.

Both these bikes go wide-open at high speed ALOT, and the cylinders are extremely similar to a CR125, just in a v-twin format. The NSR peaks out around 125mph and the RS about 150mph.

siezing is expected?

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