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Question About Rebuilt Engine Vibration

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As some of you know I recently rebuilt the engine in my 2001 CR250. Ken O'Connor rebuilt the crank, and I put all new bearings, gaskets, seals, Wiseco piston, rings, Pro X rod, wrist pin bearing, new V Force 3 reeds, re-plated cylinder, milled .030" off the cylinder, and some mild port work, etc;

 

Well, I got the bike together but have not been able to ride it yet. I have done a couple heat cycles letting it get up to temp with light blips of the throttle. The bike on initial start up took 3 kicks and is purring like a champ. It has what feels like a LOT of compression.

 

Today I was doing the 3rd heat cycle and I notice that if I rev the motor & bring the RPM's up the engine reaches a certain RPM point where the bike vibrates excessively. I have not been able to pinpoint the exact point yet where it does this but it will do it on engine acceleration & deceleration at the same point. It also feels like the engine is going to jump out of the frame when it does this.

 

I've been waiting on wheel spacers before I ride the bike but now I'm nervous that my new engine is going to blow up on the first ride. I first noticed the bike "quivering" a little bit as I revved it-but I was not sitting on it. Once I sat on it and revved the motor you can actually feel the bike shaking under-neath you. It feels like crank bearings (which are new) or the crank is out of balance.

 

What would you guys do at this point? If I pull it back down, I doubt I will be able to see anything. I did soak the crank bearings in 2 stroke oil before installing them.

 

Should I just ride the bike and see how it runs before I jump to any dangerous conclusions? Possible the vibration will go away after break-in?

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Sounds like resonance. Resonance is when the Hz of the engine and the Hz of the bike are in phase and the amplitude will multiply giving you the vibes. We call this "critical speed" in the engineering world, and we try to avoid running machinery at critical speeds. All rotating machinery has a critical speed, but from the sounds of it you are experiencing more than normal resonance. A full spectrum vibration analysis would be a great tool diagnosing stuff like this, but multiple baseline readings on a fresh engine would be needed.

Options; hopefully it goes away on complete break in and doesn't come to an abrupt stop in the meantime...open up the clutch side cover to see if there is anything obvious going on...split the case and start from square one...

Good luck

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I think I've decided I'm just going to run the ever-loving' snot out of it and enjoy what ever time I have left on it. If/when it blows-I'll put a CR500 engine into the frame, keep it for weekend riding, and then buy a KTM 300 for my hare scrambles.. Yea baby!

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I'd get in contact with the engine builder & see what he thinks. Recheck all bolts It could be something simple as that causing the vibration. You mention it's a new motor rebuild, but did you check the specs of the crank to the manual ? Don't want to speculate just trying to rule some things out. Never heard of Ken O'Connor is he in the East Coast ?

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H4L--I'm the knuckle-head that assembled the engine.  Ken O'Connor rebuilt the crank to include new thrust washers, Pro-X rod, and assembling it, and straightening/balancing. At this point, I doubt it is the crank. I'm leaning towards bearings.

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I used the Tusk crank installer and gently pulled the case half into position. The bottom end went together perfectly with no hiccups.

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Why did he balance the crank if you were satisfyed with it before? Maybe he changed the balance factor of the crank the wrong way. Did he balance it with the piston you are using? For what revs is it balanced? I guess you have checked the Engine bolts and head stay :).

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Why did he balance the crank if you were satisfyed with it before? Maybe he changed the balance factor of the crank the wrong way. Did he balance it with the piston you are using? For what revs is it balanced? I guess you have checked the Engine bolts and head stay :).

Was also thinking initially the crank, rod & piston as not being balanced correctly, but didn't want to speculate. I'd start with the basics first as in the crank being in spec & go from there.

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Was also thinking initially the crank, rod & piston as not being balanced correctly, but didn't want to speculate. I'd start with the basics first as in the crank being in spec & go from there.

You can also call the crank guy and ask him how he balanced it and if he changed the balance factor for higher or lower revs. He should know what piston you are using and if that one is different in weight to the one you had Before.I don't like to change a cranks balance factor if i feel that i have one that is low on vibrations. I have less vibrations in my 550cc CR500 than i have in the stock 500's i have.

Edited by Johnnymannen

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I got the piston from him along with the crank work being done by him at the same time. I would hope that the crank is balanced with that piston as it all came out of his shop.

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The way he assembles them they're near perfect true every time. Idk what else could affect the balancing. Are the stuffers didn't materials?

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I think there is a difference between balancing a crank and truing a crank. If truing of the crank was done sometimes the shipping and how it was handled during might have a role...who knows. I bought a brand new OEM crank from Honda and supposedly they come very close to spot on. But I took it in anyways to be trued by a local shop to make sure. I can't remember the exact numbers but it was off by a few. Cool thing is that there was no shipping involved after being trued.

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Well Ken puts them together balanced. He uses a jig that basically only lets it go together true. Then after its checked as well

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Like I mentioned above, it is probably not the crank. I did replace all the bearings as well and maybe one of the bearings is either not seated correctly, or bad right out of the box. Don't know for sure.. Got everything together on the bike finally and will take for a short shake-down ride today.

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Well, I got a chance to ride the bike yesterday. It runs awesome! You cannot feel any vibration while riding the bike. I shifted through all the gears basically short shifting it before it got into the power-band. No over-revs for me at this time until the motor is broken in properly.  Good news is that I don't think the motor is going to come apart any time soon.  I have a race this weekend so I will really have a chance to break it in then.. Hoping everything goes as planned, but bringing a back-up bike just in case it does not run the way it should, or breaks down.

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Well, I got a chance to ride the bike yesterday. It runs awesome! You cannot feel any vibration while riding the bike. I shifted through all the gears basically short shifting it before it got into the power-band. No over-revs for me at this time until the motor is broken in properly.  Good news is that I don't think the motor is going to come apart any time soon.  I have a race this weekend so I will really have a chance to break it in then.. Hoping everything goes as planned, but bringing a back-up bike just in case it does not run the way it should, or breaks down.

 

Glad to hear this is working out for you.  I have had the cranks in both my '01 CR250 and my '07 CRF450R done by Ken O'Conner and they turned out great.  The CR250 naturally has some vibration, it's and "old school" 2-stroke, no balance shaft, but it is still better than in was stock.  On my '07 CRF450R, if the muffler wasn't barking behind me or if I was deaf I wouldn't know the engine is running it's so smooth. 

Edited by bhaugeberg

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BHAU I hear what you are saying. I think I might just have been stressing about running a basically new engine. When I rode it yesterday I did notice that there was like a pop through the exhaust on deceleration so it might have been exhaust noise I was hearing.. Who knows. I am going to run the snot out of it in a hare scramble on Saturday. That should be a good way to break it in. I'll run it good on the practice lap to be sure that she will hold together during the race.

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