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Question about swapping 'L' and 'N' motors & Are high RPM's a killer?

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Hey all, I finally got the forks and shock back for my project 2002 TTR125L that I bought off some kid. I've been over the bike and all checked out, including valve clearances and compression. Unfortunately, on my second weekend of riding it (which was on the TT track at Hollister, CA) the motor seized. After wringing it's neck out on the TT track for a couple half hour sessions, I thought I'd heard some very faint noise in the motor. Kinda like a slightly noisy tappet when you're valve clearances are loose. I pulled off and killed the bike, then after a few minutes (after re-checking oil, etc) I kicked it over again. Started just fine, no noise, but stalled when letting out the clutch, no power. Kicked over again, revved, idled, and then stopped dead. Motor was seized at that point. Haven't tore it down yet, but I plan to do it in the near future.

I have two questions:

1. Is there any difference at all between the L motor and the N motor? I have a 2001 TTR125N that has a good motor and I want to swap it into the '02 TTR125 L model. Any thing I should be aware of?

2. When I ride MX and TT, I run the RPMs pretty high, kinda like a road race bike. Are these motors sensitive to that, causing the need for frequent rebuilds? Or do they handle that kind of abuse pretty well? BTW - I maintain my bikes very well, and I run fully synthetic oil.

Thanks for your input.

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There is no difference in the two engines. L stands for large wheel I believe.I've never had a problem with high rpm's but never ran the revlimier more than a few seconds!

By the way. The seized up motor how bad is it? would you be interested in selling it? Im starting to get into motorcycle engines more and more and want to rebuild a 4stroke! Let me Know

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Yeah, figured they were the same, but I wanted to check for the little things that I missed. For instance, I noticed last week that the kickstands are different lengths, so I couldn't swap those. :cry:

The seized motor? Dunno. Haven't pulled it apart yet. I'll give you a shout back when I do, but I'm also interested in finally doing a top end and whatnot.

Say, where are you at? I'm in San Jose, CA.

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i use castrol 4 stroke motorcycle oil (the non-synthetic stuff), and change it about once a month. my bike is street plated and i tend to cruise at max redline for 15 min's to an hour and a half (time to get gas) at a time (i'm doing about 95 km/h in 5th at redline). i'm installing a big bore kit and a 49 tooth rear sprocket in attempt to bring my cruising rpms down and my top speed up (probably won't happen, i'll just cruise at the max rpm again (but at least i'll be getting there quicker)).

:cry: :cry: :cry:

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High RPM's are a killer. It stretches the rod and the noise you heard was likely the wrist pin in the loose top rod eye! I know where you can get a high quality steel rod and you'll never have to worry again.

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I am in Baltimore, Maryland. If you do decide to sell it I would pay whatever shipping charges their is since its hard to find a small fourstroke to work on and toy around with to get expierence with the insides.

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What I found on high reving TTR125's is that the nut on the primary drive comes loose and the oil pump filter comes adrift and you loose a lot of oil pressure. Have a check when you strip it down. It may be that, it might not be. I use green loctite and primer and increase the torque about 50%. No problems since doing that. Mine go out to 13,600 rpm but in a MX situation not motard or road race. To date I have not done any rod damage but I keep an eye (and an ear) on both motors from duglasfurs recommendation.

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What I found on high reving TTR125's is that the nut on the primary drive comes loose and the oil pump filter comes adrift and you loose a lot of oil pressure. Have a check when you strip it down. It may be that, it might not be. I use green loctite and primer and increase the torque about 50%. No problems since doing that. Mine go out to 13,600 rpm but in a MX situation not motard or road race. To date I have not done any rod damage but I keep an eye (and an ear) on both motors from duglasfurs recommendation.

Exactly. That's what I figured happened. I've already seen that primary nut get loose on another bike and I used red loctite on it, set it to spec torque. So far so good.

The bike that I'm currently riding hasn't had it's primary drive nut threadlocked yet, and though I didn't hear the sound of the nut milling into the case cover, it probably left the oil pump undriven enough to melt my rings.

I swapped the motor from my TTR125 N into the TTR125 L last night. Tonight I'll pull the seized motor apart and see what's the haps.

ACS, it's good to hear that you ride at high RPMs on MX. I was wondering if maybe I was expecting too much outta these motors. I keep an eye and ear on these things, and sometimes get lucky to catch failures early, but this time there was barely any warning or noise.

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try going to a 14 tooth front sprocket to bring the revs down a bit

Well, it's not that I'm winding out in top gear, it's that I'm trying to wring as much power out of the motor as I can. But thank you.

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Well, I looked at my seized motor last night. First I pulled the clutch case and checked to see if the primary drive gear nut had come loose. Nope, it's perfectly set. No play on any of the components, oil pump drive is nice and taught.

Next, I pulled the head and cylinder and inspected the top end. Piston looks nice, not even that much carbon on it. Valves are good, rings are good, cylinder is perfectly unscratched. In fact, before I pulled the head off, I set a wrench on the crankshaft end to bring the motor to TDC. Right then, the motor unseized and became buttery smooth once again. Unless I'm missing something, it looks like the top end wasn't the issue after all.

I guess that my next step is to check out the tranny. I hope that I find something obvious, I hate the thought of running a motor with an inconsistent urge to seize up. Although, if the clutch is disengaged and the motor still wont spin via kickstarter, that seems to say that the issue is with top end, not tranny. Dunno, we'll see. :cry:

Maybe a good question is: if my rings did weld themselves to the cylinder wall, would it be terribly apparent?

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Yeah, the cylinder walls would have very obvious marks. When you pull the cylinder off, check to see if there's any play in the piston on the end of the rod. It shouldn't have any side to side movement at all. Did you look at the piston top for any signs of contact with the valves? That could be subtle, w/ only a small scratch near the pockets on the piston. :cry:

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i've found that it makes more power in the upper mid range not completly wound out. my bikes power felt more useable with the 14 tooth

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Could well be gear box problem. Look for blue shifter forks, blue gears and chipped gears (or even stripped gears)

Piston should be scuffed if it seized. If its the rings they will be damaged on their faces or broken but that generally only happens if there isnt enough ring gap. Bore may even have metal transfered to it.

Double check to see if its a piston valve problem but that generally is not a lock up but rather a no compression issue.

Let us know how its going. :cry:

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I did notice some blue coloration on the 'tips' of the crankshaft balancers. But I still have to pull apart the bottom end to see what's going on down there.

Just a stupid question, the oil level gets checked when the bike is held level (on a level surface, of course), right? As opposed to sitting on the sidestand?

I'll fill you all in when I find out more.

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