Jump to content
Sign in to follow this  

Snowed in, dirtbikes on the mind

Recommended Posts

My second crf230 that I bought broke and rebuilt for one of my sons, smokes after the rebuild, not on the pull but once you let off the throttle. Does this sound like valves? I assume I tested the valve seats by seeing if fluid would leak through... but maybe I forgot. The bike was ran hot, likely without oil when I bought it. Thought it would be a simple rebuild but I had to go into the bottom end to find portions of the piston. I have refrained from learning valves, but if this sounds like valves, then I will study up and redo the head

Share this post


Link to post
Share on other sites

If you do a "leak down test" you can pin point the problem without removing anything other than the spark plug, the cover that hides the flywheel retaining bolt and both valve covers. ImageUploadedByThumper Talk1453517893.063569.jpg

  • Like 1

Share this post


Link to post
Share on other sites

Smoke on decel wouldn't likely have anything to do with the way the valves are sealing but more the valve stem seals leaking. Added vacuum from high rpm with the throttle closed pulls oil through the intake valve guide. Make sense?

Stem seal leakage won't show up in a leak-down test because it's not a combustion leak.

  • Like 3

Share this post


Link to post
Share on other sites

Smoke on decel wouldn't likely have anything to do with the way the valves are sealing but more the valve stem seals leaking. Added vacuum from high rpm with the throttle closed pulls oil through the intake valve guide. Make sense?

Stem seal leakage won't show up in a leak-down test because it's not a combustion leak.

Was talking with a stranger at the track  about bikes one day, rebuilding and such, and he also said valve stems, without giving explanation. Thanks for explaining. I am a quick learner if I can envision it in my mind. So, wonder if I can find this fix on youtube? LOL

Share this post


Link to post
Share on other sites

Valve guides for sure.  Under closed throttle decel the engine creates very low pressure (very high vacuum).  This pulls oil down through the valve stem seals and into the combustion chamber where it is burned.

  • Like 1

Share this post


Link to post
Share on other sites

Valve guides for sure.  Under closed throttle decel the engine creates very low pressure (very high vacuum).  This pulls oil down through the valve stem seals and into the combustion chamber where it is burned.

Now to find instructions..... wonder if this is covered in my manual?

Share this post


Link to post
Share on other sites

Smoke on decel wouldn't likely have anything to do with the way the valves are sealing but more the valve stem seals leaking. Added vacuum from high rpm with the throttle closed pulls oil through the intake valve guide. Make sense?

Stem seal leakage won't show up in a leak-down test because it's not a combustion leak.

IMO: totally disagree

If your leak down test is less than 3% past the rings into the crankcase, both valves are at 0%, then that pretty much seals the fate of the stem seals since it indeed smokes so bad on decell?????

I suspect that his engine will NOT indicate only 3% leakage (which is pretty much ideal) and will be more than that? ( like closer to 10% which is max) the stem seals in 230f engines are pretty much bullet proof unless it's been severely overheated (a lot)... If so and if that's the case, the leak down test will certainly show more than 3% leakage.

Keep in mind, I talking about a 230f, not every gasoline powered engine ever made...

Again, IMO:

The tool, when used correctly, offers way more insight than even an "educated guess"

Share this post


Link to post
Share on other sites

If your leak down test is less than 3% past the rings into the crankcase, both valves are at 0%, then that pretty much seals the fate of the stem seals since it indeed smokes so bad on decell?????

Exactly what I was trying to say. Valve stem seal leakage won't show up on a leak down test because it's not a combustion leak.

I rebuild engines for a living. Obviously I perform leak-down and compression test and check oil pressure on each one before it is sold. All of mine get new valve seals. However, the guys in the department next to me clean, inspect and test used engines. Same tests. Valve stem seal leakage doesn't show up on the leak-down test but it does show up if you look into the ports after the engine has been spinning and flipping oil onto the valve stems.

https://www.facebook.com/mikerbike/posts/10154027449352223

  • Like 1

Share this post


Link to post
Share on other sites

Assuming you did proper rebuild then an oil leak through the piston rings is not the case. On decel you shut the butterfly and the engine is trying to suck but the throttle is closed, the oil then leaks through the valve stem seals. 

(yeah I know there is no sucking, it's high pressure and low pressure but you get the point)

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×
×
  • Create New...