Topic: CB350F still smoking after 2 rebuilds... Ideas anyone? NEED HELP.

[dave500]

these have a wet sump no scavenge.

[KRMK3]

Thanks, not familiar with the 350F !

[dave500]

no sweat,only the 750 was dry with the separate oil tank.

[trueblue]

White smoke can be caused by a rich condition, although it isn't common. 
Blue smoke is caused by burning oil, although synthetic type oils can look almost white.
Black smoke is caused by a rich condition where the mixture can only partially burn.
White smoke is caused by a rich condition where the fuel doesn't burn at all and is usually accompanied by a miss.

[craywm]

It was a blown head gasket all

[MCRider]

Just a thought but did you check the ring end gaps in the bores to see whether they were within spec before re-assembly ?  From the video it seems to be burning oil and it looks to me like it is a piston/bore/ring problem and not valves/guides. A rich fuel mixture wouldn't show itself with blueish exhaust smoke. Is the oil pump scavenging properly, if not that may be the problem.
   Good luck with it.   

What is the relationship of the oil pump scavenging to smoking? I ask as I have a rebuilt CB750 that smokes and I just realized the stopper valve on the scavenge side of the pump was stuck open. The symptom was the oil tank kept draining out into the crankcases.

I thought the 2 were unrelated, till you made the comment.

[750resurrection]

Nice, answers !!! Love it I love it.

Good luck cray on the next head gasket and McRider very interesting - that's a nice easy fix if it turns out.

[KRMK3]

Just a thought but did you check the ring end gaps in the bores to see whether they were within spec before re-assembly ?  From the video it seems to be burning oil and it looks to me like it is a piston/bore/ring problem and not valves/guides. A rich fuel mixture wouldn't show itself with blueish exhaust smoke. Is the oil pump scavenging properly, if not that may be the problem.
   Good luck with it.   

What is the relationship of the oil pump scavenging to smoking? I ask as I have a rebuilt CB750 that smokes and I just realized the stopper valve on the scavenge side of the pump was stuck open. The symptom was the oil tank kept draining out into the crankcases.

I thought the 2 were unrelated, till you made the comment.

Probably more appropriate to classic British machinery than a CB750K Honda MC (as the Honda has a reasonable sump below the pump) but any build up of excess oil in the bottom of the engine crankcases through poor scavenging or gravity leakage through the oil pump when left standing will usually end up being sprayed up into the cylinders by the flywheels/crank webs causing over-lubrication. This can cause the motor to emit smoke when running. With 'wet-sumping' through standing the symptoms soon disappear when engine has run for a few minutes i.e. when the pump has shifted the excess oil back to the oil tank.         

[MCRider]

Just a thought but did you check the ring end gaps in the bores to see whether they were within spec before re-assembly ?  From the video it seems to be burning oil and it looks to me like it is a piston/bore/ring problem and not valves/guides. A rich fuel mixture wouldn't show itself with blueish exhaust smoke. Is the oil pump scavenging properly, if not that may be the problem.
   Good luck with it.   

What is the relationship of the oil pump scavenging to smoking? I ask as I have a rebuilt CB750 that smokes and I just realized the stopper valve on the scavenge side of the pump was stuck open. The symptom was the oil tank kept draining out into the crankcases.

I thought the 2 were unrelated, till you made the comment.

Probably more appropriate to classic British machinery than a CB750K Honda MC (as the Honda has a reasonable sump below the pump) but any build up of excess oil in the bottom of the engine crankcases through poor scavenging or gravity leakage through the oil pump when left standing will usually end up being sprayed up into the cylinders by the flywheels/crank webs causing over-lubrication. This can cause the motor to emit smoke when running. With 'wet-sumping' through standing the symptoms soon disappear when engine has run for a few minutes i.e. when the pump has shifted the excess oil back to the oil tank.       

While waiting for your answer, I sortof came up with that on my own. Mine sumped it all, 3.5 qts and never refilled the tank for as little as i've run it.

Here's hoping. I should get it back or near back together tomorrow. (Pump's out and apart right now)

[craywm]

Just to be clear, it was a head gasket blown because of warped head. Remachined and should be fine, ill post upon rebuild! It didnt smoke before my first rebuild so it cant be anything else but the head gasket.

[KRMK3]

Glad you've found the problem !

[camelman]

I'm glad you got it figured out.  I'm in your boat now too though.  Something's given up the ghost on my 350F (I loaned it to a friend for a few months) and it is burning oil now from all four cylinders.  The rockers are pretty where they contact the cam too... ugh!  It still runs fine, but I'll be burrowing into it to figure out the issue.  Where did you decide to source your new head gasket and rubber dowel gaskets from?  I'm planning to contact Honda since I'm not sold on the quality of the VESRAH (or other) kits these days.

Camelman

[HondaMan]

I'm glad you got it figured out.  I'm in your boat now too though.  Something's given up the ghost on my 350F (I loaned it to a friend for a few months) and it is burning oil now from all four cylinders.  The rockers are pretty where they contact the cam too... ugh!  It still runs fine, but I'll be burrowing into it to figure out the issue.  Where did you decide to source your new head gasket and rubber dowel gaskets from?  I'm planning to contact Honda since I'm not sold on the quality of the VESRAH (or other) kits these days.

Camelman

I just finished a 400F, got those parts from Honda. Good thing it was Sochiro's favorite bike!

[lucky]

How did you check the valve guides???
Did you measure the wobble the way the book shows you?
Or did you just put a valve in a guide and say that feels good.

Not one word about measuring the cylinders and the pistons.

[craywm]

Just so people know, I had to rebuild again and redid the entire top end again. used same valves and valve seals. Its still smoking now but I only put 4.5 miles on since I rebuilt it. hopefully the rings break in.

[nivlac]

I hate that for you buddy.

[MCRider]

Just so people know, I had to rebuild again and redid the entire top end again. used same valves and valve seals. Its still smoking now but I only put 4.5 miles on since I rebuilt it. hopefully the rings break in.

Seems replacing the valves would be desirable, and certainly the seals mandatory. Why did you not?   

[craywm]

I actually meant to put new valve seals but same valves. I cleaned the valves and they still had a lot of life. just sucks that even though they were pretty before they aren't now! I just put 20 miles on the new engine and the smoke has reduced considerably. still a good amount left. most likely crappy exhaust residue from burning oil before was causing a lot of smoke. anyway im hoping by atleast 50 miles it will be broken in fully and not smoking so I can change the oil and do some fine tuning. how long do your guys engines usually take to break in? i.e. rings seating? Ive never had smoke like this before on an engine rebuild.. Ive done a few in the past. is this a Four thing?