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Hi,

I bought last year a good looking honda CB750 custom, 1981 for a pretty cheap price.
After a few test drives it was clear the engine needed a revision.
Only 2 cylinders of the 4 were properly used and the bike spitted out a lot of black smoke, with an oil consumption of about 1l / 300km

In attach a pic of the bike

The bike is revised now, engine was rebuild, but I still have a worrying oil consumption. At least I think so.
The bike still consumes about 1l per 300km and spits out lots of white smoke.
The guy who rebuild the engine told me this is normal and the engine still needs more km's, some bikes need to drive 12000km after a rebuild, before they stop consuming so much oil.
I have driven now about 3000km...still the same.
Apart from that, he drives really great. I have back the power, the gear is top, all feels good actually....just that crazy oil consumption.

Anyone experience with this bike? Driving together with a buddy isn't relaxing, people really keep a distance because of the smoke/smell and I worry constantly about having enough oil. I also already changed oil and filter after 2000km.

Any insight would be great.

Thanks!

Never heard of that break in should be about 3/6 hundred miles thats it, white smoke is a bit of a lean condition which could be bad. Please pull you spark plugs and see if you have white sooty deposits on them. You may have an air leak causing a lean condition which will cause your motor to run hot and burn more oil than normal. The other possible condition is valves not properly seated but a look at the spark plugs can give you a very good insight into whats going on.

Thanks for the info. I'll check it out. I can't take them out at my current place because I miss the proper tool.

Hi,

I've checked the sparks...there's a bit white on it at the bottom (which was told to me is good - combustion).
What I didn't like was the black stuff on my sparks, going up till about 10mm up. Shows again the oil consumption.
I've taken out the air filter, all very clean and perfect condition.

The white smoke is especially when I go 7000RPM and above. But I do drive a lot in high RPM, since this is a 4cil and only evokes it power at 5000RPM and above.

Make sure you don't have an air leak Spiv and what oil are you using? You see oil burning will be blue smoke unless its mercon or Dexron (trans fluid) which will give off white smoke and some synthetics as well. An old air head like yours make sure you are using a Dino oil not synthetics as it will hold up better I'd try using a slightly higher viscosity oil and see what happens like a 15W40.

Hi Brumbear,

I'm using 15W40 Mineral oil since I bought it. I know these bikes don't handle (semi)synthetic oil well.
My fuel is 95 octane.

Would this be a good method to check for leaks? http://www.youtube.com/watch?v=XzjSEYB9RDU

I must say my engine idles perfectly at 1000RPM, even after driving a few hundred km's, it idles at a slightly higher 1200rpm.

The smoke is from oil, maybe it's blueish (i find this hard to distinguish), what I do see is small oil spots at the end of my exhaust, so that's oil for sure.
It's just not black anymore like it was before the rebuild.

Would eg 30W40 make much difference? It's a bit hard to find in my area...and I guess it's still no solution for the real problem.

Thanks for your insights! It helps me narrowing down where the problem could be. I'm not that technical

no the oil you are using is ok. I would do a leak down test at this point and see if the heaviest of the smoke is on start up perhaps valve seals a leakdown will tell you if there is a bad valve seat or ring issue. you can cheapo do it by screwing in a 12mm air chuck or one that fits whith the cyl on TDC connecting air it should hold steady set your compressor light 80psi and listen at the exaust and crank case and carb for an air leak

Thought of giving a small update here.
Engine runs perfectly, no leaks, no irregular sounds,...

However, the oil I used was wrong. Bel-Ray 20w50 seems to do the trick. Has much better viscosity at higher temps. They're also heavily in the mining business.
I ride a lot in hi rev, so especially those long speed rides (eg 200km at 150km/h average- which is about 7500 rpm constant) sucked the oil right out my CB.
Seems other brands I used all sell car oil as well and are no good, become way too thin at high temperatures.

All in all...My oil consumption dropped dramatically.

Oil.

If that were my bike I'd run Shell Rotella T 15W40.