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XR250R smokes and plug is black

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Got an 02 xr250r recently starts and runs great, seems like it has good compression I'm waiting for proper adapter to,check the numbers.

still has factory tires, chain, and sprockets, not very worn. It may have sat for a while, some of the o rings in the chain are cracking. I'm guessing the bike has around 500-1,000 miles at the max?

it smokes on decel a little, whitish-blue, smells like oil. Startup it doesn't smoke until after I give it throttle.

I put UNI filter and 135 main jet in. Stock filter didn't look too dirty. Slow jet is on the way. Came with FMF exhaust. No other mods.

Should I try replacing just the valve stem seals without taking the head off, or should I do a full top end rebuild (rings, hone or bore, check valve seats etc)? Wait to see the compression test numbers to make rash decisions?

I feel like the cylinder, piston/rings, and valves seats can't be in bad shape this young in the bikes life.

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Have you been able to warm it up enough to do a proper oil level check on it?

Be sure it hasn't been overfilled, which typically happens when new owners check level for the first time and add to it, thinking its low due to a improper check procedure.

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Yes I've checked it a couple times according to the manual it was right at the upper limit mark. The previous owner changed the oil and filter right before I bought it. I've checked the crankcase vent tubes they are all clear as well.

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I think I was overfilling my 86-95 wet sump 250 motor.  It does not have the reservoir in the frame. 

 

What I figured out:

When the bike is on the side stand leaned over to the left with the engine running, oil is pumped up into the head and some of it pools in the head cavity.  As the oil pools, it finally drains down the chainway.  If you stand the bike up straight or lean it to the right, it no longer has a place to pool so the oil freely drains down the chainway.  You can easily see how this pooling can change your oil level.  

 

My new procedure for this bike, is to fully warm the bike so the oil is hot and thin.  I shut the bike off and lean it to the right for a moment.  Then I rest it on the sidestand and pull the dipstick which I clean, dry, and replace.  I do not thread it into the clutch cover.  Next, on level ground, I pull the bike to its vertical balance point and then rest it on the sidestand.  Then I read it.  This gives me a consistent reading without overfilling.  The warmer the oil, the higher the reading because the thinner oil drains more easily from all the internal engine components.

 

The dry sump XRs with frame reservoir have a similar procedure.

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IMO (seems to be a trend lately and I am such a follower)

For the cost of a gasket kit, you could pull the head and gain first hand knowledge of the engine's condition. Check bore, do a leak down test on the valves, clean up carbon on the piston crown, etc. While in there if everything is within spec, replace the valve seals with the head off. You stated it has good compression, once you verify actual numbers it's likely you might not need much of anything else. Cometic gasket sets are in the $25 range online. it will come with every gasket you'd need, valve seals, intake and exhaust gaskets, etc.

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Smoking on deceleration would lead me to believe valve seals. if the carb/throttle slide is closed, vacuum is created and the seals will pull oil through. Doesn't take much oil to make a mosquito abatement machine.

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IMO (seems to be a trend lately and I am such a follower)

For the cost of a gasket kit, you could pull the head and gain first hand knowledge of the engine's condition. Check bore, do a leak down test on the valves, clean up carbon on the piston crown, etc. While in there if everything is within spec, replace the valve seals with the head off. You stated it has good compression, once you verify actual numbers it's likely you might not need much of anything else. Cometic gasket sets are in the $25 range online. it will come with every gasket you'd need, valve seals, intake and exhaust gaskets, etc.

 

I had two successive Cometic head gaskets that continually leaked oil on my 250 all over the cylinder fins.  I changed to the stock laminated all metal gasket and problem solved.  The machinist told me he trims the stock laminated metal gasket for use on big bores.

.

Edited by Baja Rambler

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I had two successive Cometic head gaskets that continually leaked oil on my 250 all over the cylinder fins.  I changed to the stock laminated all metal gasket and problem solved.  The machinist told me he trims the stock laminated metal gasket for use on big bores.

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Leaked oil where? Into the bore or outside the cylinder? This is good information, I have a 284cc and about to put the cometic in because the laminated gasket I have was for stock bore and too small for the 78mm piston

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Found an OEM Honda gasket kit for $54 shipped, it has valve seals and all that so I bought it, I'll rebuild this thing at some point in the future if not soon, I wanna keep her forever they are great bikes.

I'll wait until after the compression test to make any decisions.

But if I'm going to do the valve seals it seems reasonable I should check the bore and piston? Or if it has good compression just remove the head and clean the crown of the piston? Rings are only another $30

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Smoking on deceleration would lead me to believe valve seals. if the carb/throttle slide is closed, vacuum is created and the seals will pull oil through. Doesn't take much oil to make a mosquito abatement machine.

 

I thought the same thing on mine, turned out to be rings instead.

 

Found an OEM Honda gasket kit for $54 shipped, it has valve seals and all that so I bought it, I'll rebuild this thing at some point in the future if not soon, I wanna keep her forever they are great bikes.

I'll wait until after the compression test to make any decisions.

But if I'm going to do the valve seals it seems reasonable I should check the bore and piston? Or if it has good compression just remove the head and clean the crown of the piston? Rings are only another $30

 

I would. Pulling the jug isn't too difficult to check ring gaps, and a quick hone and new rings won't kill the engine either.

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Still smokin after 100 miles, if the oil was overfilled the excess would have burned off by now correct? The PO seemed like he knew what he was doin he said he used to race motocross and had a really clean tidy shop I can't imagine him overfilling it that much? It's not burning a ton of oil as the dipstick reading is still near the upper limit, I'm comparing to the stock dipstick because I put the XR's only temp gauge dipstick to monitor temps.

I'll tear it down soon enough and find out what's goin on in there. Just sucks I didn't notice any smoke when he was ridin it in his yard. it'll be good to inspect the cam and valves anyways I guess. Once I'm done it should be a solid smokeless bike for many years to come.

What about a cam chain while I'm there? Wait to see how far the tensioners out I guess? Hard to tell good sounds from bad on a four stroke with so many moving parts

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When I was removing the cam gear the two pieces behind the gear on the shaft spun when I was trying to remove one of the bolts. They are right next to the plunger with the spring underneath. Is this normal, it only spins one direction but the camshaft is stationary.

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Figured I'd update if anybody out there cared.

Replaced piston rings, honed the jug, replaced valve seals and cam chain was worn as expected, tensioner showed a big improvement with the new one on. The valve train was in great shaped, guides had zero play, all the seats looked good and passed solvent tests. Everything looked barely worn which was what I was hoping for.

First ride today went great! Fired right up and I didn't see any smoke behind me when glancing back! Glad I did everything myself I think the bike will be solid for years to come.

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Can you give a little detail on the seats and the passing of the solvent test?  I'm getting ready to crack mine open and want to get it right the first time.

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A manual will guide you through a solvent test, but you basically dump some solvent into each port and hold the head to see if any leaks into where the combustion area is.

I just visually inspected the valves and seats, the mating surfaces are supposed to be at ~45 degree angle. A worn valve seat will not have a flat surface it will have a ridge somewhere. Try to research and find some pictures of worn valves it will make more sense, I'm no expert.

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OK, so by "seats" you mean the sealing surface between the valve and the head?  Makes sense.  While you have it apart, now is a good time to lap those valves in.  The tool and compound are cheap, you can find them at any auto parts store probably for less that $10 total.  The process is simple, and ensures a really good seal.  Lots of youtubes around on the process, takes about 5-10 minutes while the head is off.  Just be sure to clean the compound off very well before reassembly.

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