Topic: CB650 burning oil

[Flatlander80]

Hey everyone!

I have a 1980 CB650 custom that I rebuilt 5 years ago, and I’m having an oil burning issue. I dubbed the bike Ol’ Smokey a while ago, but I’ve been doing my best to keep the smoke in where it should be.

Recently I redid the top end (with the help of my Dad), since I had low compression on #1 cylinder (60psi). Here’s the list of what was done:

Cylinders honed and new piston rings gapped and installed.
Cleaned head bolts and head bolt threads (with a tap and die)
New top end gaskets, o-rings, exhaust gaskets, etc. The only gaskets that we’re reused we’re the valve cover gaskets (there are 3 on this bike)
Reassembly went smooth with the exception of one head bolt (front middle) that started to pull threads, and 2 bolts in the head cover. We didn’t fully strip them, but we couldn’t torque them fully.
New spark plugs, wires checked, and timing reset.
Checked the carbs, and they were pretty clean... no clogged jets or passages I could find, blew carb cleaner and air through everything I could find. We installed aftermarket needles, and raised their position 1 notch (the bike would bog when you opened the throttle, and that drastically improved it.
Synced the carbs, and set the idle mixture screws at 2.5 turns out. (it runs and idles best at this setting)
No oil leaks that I’ve seen since the rebuild, plugs look great (all tan in color), and did a compression test (115, 110, 105, 115)

Modifications:
K&N single filter in the stock airbox, and a 4 into 1 Mac exhaust. Main jets are 95 (stock in the carbs, and these are the PD carbs, stock idle jets)

Problem:
The bike is still burning oil. Less than it was before, but I’ve burned about 3/4 L of oil in 500km. Bike accelerates and drives better than it ever has. But when I let off the throttle from high RPM, I get quite a bit of smoke coming out of the exhaust. From what I can tell, this is the only time I’m getting smoke from the exhaust.

I know this is a loooong post, but I’ve tried everything I can think of. I’d like to figure out what the next step is... any suggestions would be greatly appreciated!!




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[scunny]

smoking on deceleration would point to oil coming past the valves, did you replace the valve guide seals ?

[scottly]

smoking on deceleration would point to oil coming past the valves,

1+, especially the intake seals, as the valve stem is exposed to a high vacuum during the intake stroke with the throttle closed, sucking oil into the combustion chamber. Leaking exhaust guides are not affected by deceleration.

[Flatlander80]

We did replace all the valve guide seals... in fact we had done the head 2 years age and replaced the seals and head gasket then... and then replaced with new this time around.


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[bryanj]

Honed too big, piston to bore clearance on Hondas is very tight

[Flatlander80]

Honed too big, piston to bore clearance on Hondas is very tight

I checked the cylinder bore after, and while I can’t remember the numbers, they were well within spec... we also gapped the rings and there weren’t any rings where the gap was too big...


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[bryanj]

I have never seen a set of std Honda rings in a std bore that were not on minimum gap.

There are only 2 causes of oil burning
1 piston to bore clearance too big

2 oil down valve guides and it can be either, 750 F2 will billow blue smoke at 150 miles per 20 ounce pint with just exhaust guides gone