Topic: Honda cb700 1986 Nighthawk -- has good compression, smokes like a 2-stroke

[Hondawggie]

I have smoke coming out the pipes like a 2-stroke.
So I checked compression -- I don't have a cb700 manual or Clymers but the readings sound okay:
Cylinder #1:   120psi
Cylinder #2:   125psi
Cylinder #3:   123psi
Cylinder #4:   125psi

I checked the oil level right after running the bike's engine and it was low, so I topped it off.

Now get this -- when I ride it and get on it, it feels normal, it's got lots of power.

And I don't know if the smoke is burning oil, but what else could it be?

The bike is all stock.  No modifications at all.

[MoMo]

eliminate the obvious first( air filter, possibility of gas in the oil, contaminated gas)...Larry

[Hondawggie]

I just discovered the cb700 Nighthawk has *hydraulic valves.*

I'm wondering if a prior owner overfilled the engine with oil -- I've read that can damage the valve train if you have hydraulic valves.

Reason I thought about valves is -- valve guides may be leaking, 'blow by', but that would hold if the bike had a normal valve train.

I don't know the ins-and-outs with hydraulic valves, especially on this model.

I need a shop manual -- does anyone know where to find one for the '86 cb700?

[Hondawggie]

eliminate the obvious first( air filter, possibility of gas in the oil, contaminated gas)...Larry

Gas in the tank checks out fine.  The odor of the oil at the fill point has no gasoline odor, but that's not conclusive -- how to know if the crankcase oil has gas in it, and why would that make it smoke?

I pulled the plugs and checked them for oil -- they're all a golden brown, look good.

[ekpent]

How many miles have you put on it so far ? Maybe somebody before you fogged the inside of the pipes with oil. Any oily residue at the end of the pipes ?

[Hondawggie]

How many miles have you put on it so far ? Maybe somebody before you fogged the inside of the pipes with oil. Any oily residue at the end of the pipes ?

   I will check that today, had not thought of it.   The bike has a bit over 18k miles.

The prior owner's grandson sold me the bike -- grandpa put it away without doing proper storage.  The gas tank  looks nearly perfect on the outside -- but the old gas inside the tank over time rotted out the tank innards so I had to buy a new tank.   Also, the hydraulic clutch and front brake master cylinders had solidified brake fluid in them and had to be cleaned out.

The other thing is,  the smoke from the pipes *only happens* when the bike is really warmed up -- when cold, the exhaust looks fine pretty much.

I don't have a manual -- does anyone know if the compression readings I got are good for this model? 
I did the compression check after riding the bike a bit and got these numbers with a warm engine:

Cylinder #1:   120psi
Cylinder #2:   125psi
Cylinder #3:   123psi
Cylinder #4:   125psi

[MoMo]

that depends on the gauge used, you should be fine with those readings especially since they are within 10%

[Hondawggie]

that depends on the gauge used, you should be fine with those readings especially since they are within 10%

Dang that's what I thought too.   


I've never had a problem like this before.  I've had smoking engines but it's always been "low compressions/needs boring".   I've never owned a bike that was smoking like this and the valve train was the issue.

The other telltale sign about a low-compression situation is - the engine will be hard-starting.

This Nighthawk starts right up.  And when she's on the mains in the carbs, throttle wide open, wow what a beast.
The bike has plenty of power.

And the plugs show no signs of oil or oil fouling.

Could this somehow be an excessively rich carb situation, like maybe the chokes are staying engaged a bit or something? 

The bike *is* all stock.  But it was not stored properly -- maybe the excess smoke isn't oil at all, maybe its excuse fuel?

[ofreen]

Is the crankcase ventilation unimpeded?

[Hondawggie]

Is the crankcase ventilation unimpeded?

Not sure -- I will check it, I'm not 100% sure where it is but I'll look on the top rear of the engine -- my dang job is keeping me away from the bike today, I'm itching to get at it.  I have to check the airbox for oil, and the crankcase ventilation.

Could carbs having the choke stuck 'on' a bit cause this smoking?   I don't think it's oil -- I do not believe the smoke is blue, I think it's a brownish color, and the spark plugs are all golden tan colored, no sign of oil.

[ofreen]

I take it when you say smoking like a two stroke you mean blue smoke. Positive crankcase pressure  build up can cause problems pushing oil into the combustion chamber and past seals. A common place for that to happen on a Nighthawk S is in the driveshaft area as I recall. You say the smoking is worse after the engine is warmed up. Another way of saying it has been running for awhile. So the oil would be thinner and crankcase pressure will have had an oppprtunity to build up. There's a couple of ways of checking for it, including loosening the oil filler cap. Be careful if you do that. Been awhile since I looked at a Nighhawk S but I think the breather is in back of the engine with a catch basin and filter.

[Hondawggie]

**PROGRESS MADE.**

I checked the airbox bottom, and the air filter, expecting to find oil in the bottom of the airbox and on the air filter. 

Nothing.

So because this bike is a California model, it has all sorts of cannisters to collect engine stuff such as the crankcase breather.  I followed the large rubber tube from the top rear of the engine, which I assume is the crankcase vent, up to a sizable plastic 3-nozzle catch-bucket to the right under the gas tank (rear of the tank on the right).

The crankcase breather tube runs to this plastic catch-bucket and there are 2 output hoses from it -- one leads to the valve/cam cover on the right (rear) side of the cover, and one tube leads to yet another cannister that is mounted under the freaking swingarm, at the very front of the arm.

Each of these extra cannisters and hoses should be labeled with a politician's name in Sacramento who imposed this crap on the bike makers.  Only California models required all this schitt.

So back to the 3-nozzled plastic bucket being fed by the crankcase breather hose.  I pulled all three hoses off the plastic bucket and removed the bucket - I wanted to turn it upside down to 'let all the oil out' -- I figured the crankcase breather must have filled it up with oil and the oil was being piped over to the cams/valve train, voila that's where the smoke was coming from, right?

I turned the plastic catch-bucket upside down, nothing came out.

Put the catch bucket back on the frame and re-connected the 3 hoses to it.  Started the bike.  NO SMOKE.

Took the bike out for an extended ride, about 1/2 an hour, a mixture of long stretches of open road then into the city with lots of stop and go's.  NO SMOKING.

Finally about 3 blocks from home the smoke came out.  It's *not* blue smoke, not oil.  As I mentioned before, the plugs are golden brown like normal.

Then I remembered -- when I flushed the gas tank, I used lacquer thinner to cut the old gas to more easily remove it from the tank. 

The smoke disappeared before I finished those 2 blocks to home. 

So I think I'm just burning off the lacquer thinner remnants, I know there was some left in there, I couldn't get it all out.

The smoke was light brown.  Smelled funny too.  Didn't smell like oil.  Didn't smell quite like gas.

I'm also assuming that all the  Sacramento Tree-Hugger cannisters on this bike have some gunk left inside, just like the gas tank had stuff that had progressed way beyond varnish.  The bike sat a long time.

That's why I rode it a lengthy bit today.  Clean out its guts.  So the source of the smoking was either:

- remants of lacquer thinner still left in the gas tank after flushing the tank with the thinner

- and/or old gunk/vapors that are still inside the Tree-hugger pollution cannisters after the bike sat for 25 years.

It was last registered and run on the road in 1990.