Topic: CB750 K4, Burning Oil After Top End Rebuild

[bompxc]

Im stumped here guys.  I rebuilt the top end on a 750 K4.  New rings, honed the cylinders, and all new gaskets.  Also lapped the valves.  The engine was NOT burning oil before the rebuild, did the top end for a project and peace of mind.  I did not replace the valve guides or have any machine work, as there was not a problem before.  Also replaced valve seals. 
Anyway, I've had 800 miles of pure bliss.  Running great. 
Then, I started getting a backfire at idle from cylinder 3.  The backfire went away after a bit.  Then as of last week, that same cylinder is smoking after warm up and riding a bit.  Smoking as in definitely burning oil.  Rode great for 800 or so miles. 
I take the #3 plug out, and clean it.  Oily but not overly dirty.  I put it back in and now the same cylinder is burning oil immediately after startup and gets worse as I ride!
I know the issue has to possibly be valves or rings, however, how did it ride great for the past 800 or so miles, why now?? It wasn't burning oil before the rebuild. 
Has anyone ever had this issue and it turned out to be something else.... perhaps an easier often overlooked issue?

[Airborne 82nd]

I know this doesn't help but. If it ain't broke don't fix it! But here is a bump maybe someone has a more useful idea.

[Airborne 82nd]

OK have you done a compression check? leak down? We need to focus on what is different about #3.

[bompxc]

Thanks for the bump!  I'd ignore it, but it not running as smooth since the oil has been burning.  I have not done a compression test or a leak down test yet.  I was first wondering if it was possibly any other issue??

[Davez134]

Well, if it does not pass a compression or leak down test, no use in moving on anyways. Besides valves and/or rings, nothing besides (possibly) a head gasket that I can think of. Either way, oil from a cylinder means some work ahead.

[disco]

Hi I would suggest your 1st move would be invest in Franks weldless frame kit, so you can pull the head & cylinders without Removing the whole motor. Frame kits make a lot of sense. I have a Gordon kit in my new K2 re-build....it's good insurance, just in case it goes tits up! I also have a Frank kit, in case I ever need to pull the head in my K6.

[Redline it]

Honing looks easy, and sometimes the finished hone looks so machine shop quality. It is very crucial to know each cylinder dimension before honing, and the type of hone and how it's run. The little short process of ridge removal is better with lots of experience with older junk cylinders before a trial on an inline 4 set of small bores, they can remove too much material so fast.   

[bompxc]

Thanks for the solid input guys.  Regardless, it looks like I need to get into the top end again.  Was just hoping by small chance there was something easier I overlooked.  Never is the case!