[K3 Kev.]

CB350K3 1971
OK so I bought this bike with a smokey left pot, and a very oily muffler.
I checked compressions and they were way down below 150psi, and I had to take the bore out to final oversize to remove the ovality and fitted new pistons and rings.

Compression now at a healthy 180psi

Started up and we still have smoke, :twisted: so for me the only thing left to cause this, is a worn valve guide/valve.
Sure enough removal of the inlet valve saw a lot of burnt oil on the back of the valve face, result!

Now I cant do this sort of thing as the guide has to be reemed out to the valve diameter. So the head went off to a specialist who suggested fitting a phosphor bronze sleeve.
He calls me later that week and says that the guide diameter is within tolerance, no need to fit a sleeve, and fitting a new valve would solve the smoke issue.

Engine back in and started up, after about 5 mins (or once the engine gets warmed up) the smoke returns, just as bad. :twisted: I then thought ah! maybe the header pipe is oily and I need to clean it out, so I did that and still no good.

Anyway I have a couple of questions please.

Has this "specialist" got the specs wrong?
Is there another point of possible oil ingress?

I'm going to get a second opinion from a honda CB 350 race engine company. But I thought I'd ask here for some advice, as the engine is coming out

 

[Yendor]

I would not replace a valve without repalcing the valve guide.

Even "IF" the internal guide diameter was within spec. (and I have no idea how he managed to measure that throughout the entire lenght of the guide, that is a very delicate task).

The internal lenght of the guide has worn to the the stem that was riding in it.
Much like Pistons and Rings.
You wouldn't Bore out and/or Hone a Cylinder then put the old pistons and rings back in. (at least not normally)

Did he tell what the measurements were?
I would have wanted to know so I could look them up in the FSM myself to check.

 

[K3 Kev.]

I would not replace a valve without repalcing the valve guide.

Even "IF" the internal guide diameter was within spec. (and I have no idea how he managed to measure that throughout the entire lenght of the guide, that is a very delicate task).

The internal lenght of the guide has worn to the the stem that was riding in it.
Much like Pistons and Rings.
You wouldn't Bore out and/or Hone a Cylinder then put the old pistons and rings back in. (at least not normally)

Did he tell what the measurements were?
I would have wanted to know so I could look them up in the FSM myself to check.

That's a very good point, he just said they're ok.

I've found another place that seems a little more savvy.

I will ask what the guide measures up to when the head goes in to be fixed

Thanks

 

[mike in idaho]

It's better to sleeve an existing valve guide than to replace the guide. Likely the new guide won't be concentric with the existing seat so the valve seat will need to be re-cut, the valve will sit lower in the head, which affects the breathing in that cylinder and lowers the compression ratio(more volume in the combustion chamber).The average "machinist" will most likely hammer or press the old guide out and the new one in without properly heating up the head first. That adversely affects the interference fit between the guide in the head. A loose fitting guide means the valve won't seal properly and oil will run down the od of the guide.