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2old2Bfast

Smoking after rebuild, how much is too much?

17 posts in this topic

I recently upgraded my trusty 2004 yz250f to a 2007. I moved my 290 kit to the new bike and put a fresh piston and rings in the old 250. For the 290 on the new bike I just put in new rings and gaskets.

New bike with 290 runs good, but now smokes a bit. I did not notice until about the second time I warmed it up, but when you crack the throttle it puffs out blue smoke like a two stroke. No visible smoke other times. I probably have run it about 1/2 hour and everything feels OK, except for the smoke.:thumbsup:

Now the oil rings are pretty thin, and it is possible I garfed one on the assembly. Before I tear back into it and burn a $60 something gasket set, has anyone had similar smoking after a rebuild and had it clear up?

Before you ask, yes - the ring gaps were staggered.

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It should not smoke its not "normal". If ist smokes it could be the rings as you suggested or maybe valve stem seals if it just after extended warm up ie idle. Its could also be the rings not bedded in but I have never seen this before on a 250f as some here have. Does it have excessive blow by??

MM

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all of the 250f i have rebuilt do not smoke after rebuilds. They only smoke that happens is from the assembely oil. Check your valve seals, make sure your oil ring is right, check the cross hatching in your cylinder as well.

Also I am wondering how a head gasket, base gasket and flywheel cover gasket set you back $60

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Did you run a ball hone through the barrel and clean it surgically clean?

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Moose: No blow by, but that is more a function of the compression ring than the oil control rings.

Zurch, Athena gasket kits are pricey. The whole big bore kit is pretty innexpensive, but the replacement pistons, gaskets and rings make up for it.

William: I did not hone, just cleaned with carb cleaner. Original cross hatch was still fresh. On the 250 rebuild I did the same and it was fine.

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Not giving the barrel proper prep is probably the cause.

agreed. Even though the orginial cross hatching was in good shape. There can still be scale on the cylinder and if it wasnt perfectly clean you can have issues.

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Moose: No blow by, but that is more a function of the compression ring than the oil control rings.

Zurch, Athena gasket kits are pricey. The whole big bore kit is pretty innexpensive, but the replacement pistons, gaskets and rings make up for it.

William: I did not hone, just cleaned with carb cleaner. Original cross hatch was still fresh. On the 250 rebuild I did the same and it was fine.

If the rings are not bedded in you will get excess blow by. I was asking to see if you did! This would answer you question of why you have smoke i.e the rings are not bedded. Its a bit hard to test behind a keyboard :thumbsup::lol: I'd also be worried about using carb cleaner on the bore it does leave traces behind and is not the right product to use to clean bores. Use Warm soapy water then a high flash point solvent to clean bores and then engine oil :lol: :lol:

MM

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agreed. Even though the orginial cross hatching was in good shape. There can still be scale on the cylinder and if it wasnt perfectly clean you can have issues.

Thanks for the replies.

I guess I am living in the past. I was a mechanic quite some time ago (25+ years) and nikisil was never honed back then. If you honed it you then had to chemically etch the bore to eat away a little aluminum and leave the silicon standing proud. No doubt they have improved manufacturing, or just found that the bores are more durable than they thought. However, I have never honed a bike cyl and this is the first time I have had issues. The same jug and piston on another bike was fine, but I did change the rings.

I did not run the bike that much before putting the 290 jug on, but I don't think it smoked before then. That would lead me to think it is not valve seals.

Since it is not puffing all the time, I may just run it one more time and see if it beds in.

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I would give it a few good rides. watch the oil level. It may take a little longer than normal to seat the rings with out the cylinder glaze being broken. I also know dry installing the piston and ring is all the rage now days for quick ring break in. I cant get myself to do it. I lube the piston with motor oil and install. I just give it longer to break in.

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When ball honing NikaSil, you are not actually honing it as you would be with an iron bore. All you are doing is scraping the surface, removing and glaze. To really hone a NikaSil barrel, requires a diamond hone.

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Here is an update.

After a couple of rides the smoking was unchanged. When I changed the oil filter I thought there was a bit too much metal. Not tons, just more than normal.

Soooo, pulled it apart and found the oil rings stuck. Seemed to be in correct position but not moving freely in the land. The bore looked ok on first inspection, but on second look I can see a slight bit of wear. There is about 2-3mm wide vertical swath where it has removed the cross hatch. Can't feel it, but you can see it.

My big question now is, what does the cylinder need? Just honed or should I have it plated?

Once I resolve that, I think a new piston and ring set is in order and we should get this ready to ride.

Thanks!

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After closer inspection, I believe the oil ring was sitting on top of the expander, rather than in front of it. :ride:

The cylinder was unscored, but I think it would have been in one more ride. No smoke after new piston and rings.

So to answer my own question, Prolonged smoking is not normal and indicates a problem.

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After closer inspection, I believe the oil ring was sitting on top of the expander, rather than in front of it. :)

The cylinder was unscored, but I think it would have been in one more ride. No smoke after new piston and rings.

So to answer my own question, Prolonged smoking is not normal and indicates a problem.

thanx for the whole story :ride:

It is allways good to hear what the problem was!

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