CB350 smoking breather pipe

L

lhemrick

Active Member
I have a 71 CB350k Twin that when it gets warmed up it smokes from the breather pipe that vents the crankcase. I have just taken the engine apart and replaced all the gaskets, I have checked to make sure the top is oiling and the compression checks out at over 160lbs. On the advice of others I changed the oil just in case I had left the petcock open and fuel fouled the oil. I have double checked the timing, points and plug gaps. I don't think the engine is running hot because it starts back to easy after shutoff once its fully warn. No other bike I own has this symptom and since this is the only twin I own I have to ask "Is this normal?"

Thanks
 
I took the it for a spin today myself and the longer I rode the bike it seemed to get a bit sluggish at the end and finally it wouldn't stay running when you pulled in the clutch. I got it back to my shop and opened the tappet covers and discovered that very little oil could be seen flowing on the rockers. I know I've seen it oiling before but when cool but wondered then if it was enough. I hate to have to take the engine out and back apart. I know both the screen under the pump and the "filter" are clean. Could it just be the pump? How can I check it?
 
Well...You don't know screen and the filter are clean.
You know for sure when you've taken it apart and concluded that they are indeed clean.

When the breather pipe is smoking, that's not good. The breather tube is the engine's highest 'exhaust' so when smoke comes out of there something is melting or getting very hot at least, creating smoke. What kind of smoke was it? White, black, gray? Smelling like gas, oil or aluminium?

I'm afraid the damage's already done since it started to smoke. Weird point is the clutch. With lever pulled the engine has in fact less resistance, so theoraticly it should run easier.
Have you even checked the oil level? When there's not enough oil in the engine the oilpump intake is not submerged in oil, thus sucking air instead of oil. The pump's intake needs to be submerged to get the oil into the engine. Check the oil level and let us know!
 
I've concluded that I'm gonna pull the oil pump and make sure its clean and not obstructed. I know the part pf the screen I can see from the bottom is clean. With the oil pump removed can I force oil through the engine somehow to make sure there is not an obstruction to the top? I just changed the oil to make sure there was no fuel in it and am pretty sure its full but will check again.
 
You said you juat rebuilt the motor... Are you sure it's actually smoke? You will get condensation built up in the motor and when the bike warms up it will release steam from the vent tube.
 
I pulled the right side cover and the pump is pumping oil, the the filter and cover are in or on correctly (all the holes line up) with all the gaskets and o-rings installed. I still can not get oil to the top of the engine. I know it was oiling when I first put it back together because I accidently used a lock washer on one of the top engine bolts and when I cranked it it was forcing oil through the slit. What has happened or where do I need to look to correct this issue?
 
Some cb's have VERY little holes where the oil gets to the top. It could be clogged. No oil means cetain death.
Take her apart and check the oil holes. The pump is strong enough to get oil to the top, so something must be wrong. The right sode is the side where the oil gets to the top, via the long rod in the right back corner. Check the oil canal for that rod. Its VERY small, less that a mm.

I circled the hole on the drawing. The little point in the middle of the circle is the oil hole. Oil goes from the pump in the right side cover thru that hole up the cilinder bolt and to the camshaft.That's the only hole for all the oil going up. 99% of the times people have used aftermarket gasket kits wich unfortunatly lack that hole or the right side cover gasket prevents oil going into the cavity beneath that hole. If there's no oil going up, you'll have to take her apart again. I'm sorry.

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You also say you 'cranked' the topbolts. Did you use the correct route of wich bolt to tighten first? And at wich Nm?
 
i am having the same problem. i rebuilt my 1977 cb400f with an athena gasket kit and its smoking heavily from the breather pipe. it never did that before. i have oiling good compression. im baffeled. any direction would be appreciated more then ever. i have poured tons of time and emotion into this build!
 
I know I've worked harder on this bike than any other I have so I feel the pain. I'm due to make a phone call to a Gentleman from the Honda Twins site who is going to help me with my oiling issue. I hope that helps and if it does I won't mind a bit sharing the love!
 
For all it's worth, I've been building a K3 I found upside down in a yard, and it's been doing the same thing. So that's an indication that it is indeed that (tiny) oil passage that's full of crud....
 
My problem ended up being a worn oil pump that wouldn't pump the thin hot oil to the top of the motor. It destroyed the entire top end. I got that fixed struggled with the carbs a little and got it running great. I broke the bike down for powder and paint and now it's backfiring when you throttle it on the right cylinder. I didn't touch anything that should have mattered while it was apart so I'm very confused. Any ideas?
 
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