[Joff]

I have a CB400 and was out riding last week. I thought the bike was running low on fuel as the bike started to splutter. I have checked and there is fuel in the bike. Upon re-staring I noticed that the oil pressure light was faintly coming on. Then the right cylinder was misfiring almost dead. I let her cool down and tried again. The oil light is on and the right cylinder only works when not under pressure. When pulling away or going up hill the right cylinder is not firing at all, but re-starts when not under any load. My wife followed me home and said when the right cylinder does fire there is lots of blue smoke coming from it. I'm going to do a compression test today, but any ideas where I should start or what to look for?

Thanks.

 

[T2_Black-Wolf]

It states in the owners manual of the CB400N series (or any other motorcycle manual) that if the oil light comes on, the bike should be turned off asap. (pump failure or low oil level)
Blue smoke from the exhaust means that you´re burning oil, my uncle owns a lot of 2 stroke mz´s they are supposed to burn oil mixed in the fuel, if too much is added there smoke becomes a heavy blue cloud.
Start out by checking the oil level, if it is burning oil, then the oil level should be dropping. Next smell if there is gas mixed in the with the engine oil, if the oil can come up into the cylinder, fuel properly can flow down as well.
I do not have much experience with engine problems, but if i were to guess either the piston is broken or the piston ring seals are done.
hope this can help diagnose the problem.

best of luck.

 

[Joff]

Thanks T2. Is kinda what I was thinking. Looks like the engine will be coming out this winter.

 

[Irving]

Yes sounds like your oil pump failed, or an oil passageway became clogging or your oil level became too low.

 

[Joff]

Hi Irving, yes the oil level had got low. Has since been topped up. Will this cause a misfire?

 

[J-T]

It's unlikely that the low oil caused a "misfire", however, if it was low enough to turn the light on, the loss of lubrication could have caused enough heating to make the pistons tight in the cylinders causing an apparent lack of power.

 

[Joff]

Hi J-T, so I have ordered a compression testing kit and will do that. Then sounds like I need to get the head of and have a look inside, unless you think I should take a different approach?

 

[longdistancerider]

Moved to SOHC Engine section
The oil pressure switch triggers a .3 BAR or 4.3 PSI which is very low. This can be due to very low oil, worn bearings or an erratic low idle speed. If it comes on while riding the engine has to be shut down immediately.
If the running problem occurred while the engine was wet or riding in the rain it most likely is due to the coil or plug wires shorting out, right side is more prone to this. Solved with WD40 sprayed all over those parts with the engine OFF. It'll catch fire if you do it running.

 

[drydreamer]

As above but you do not mention if the rev counter stops working ?

Get the manual from here and study, as well as the exploded views on cmsnl , which will help you, but when looking at parts prices make sure you are sitting down !

You know of david silver spares in the UK.

If stripping down the motor you will need a few special tools, namely a flywheel extractor bolt ( use the oil filter bolt as your pattern, 20mm by ZZ pitch, I just cannot remember the pitch of that one ) and a clutch nut peg spanner. Undo the clutch nut before removing the engine from the frame using the transmission ( top gear ) and rear brakes to "lock" the engine. I have seen/used a locking tool to lock the transmission with the engine out of the frame but it is easier to do it before removing the engine. Similar for the flywheel bolt and crank primary gear bolt.

Be very aware of the centre head bolts that are open to the outside world, which can be very troublesome, also get a 10mm x 1.25 tap to clean out the cylinder head bolt holes in the upper crankcase.

Get boxes to put stuff in section by section when stripping down the engine, ie a box ( or the clutch cover ) to put all the clutch side bits in etc..

Take your time and enjoy.

What was the verdict about fuel in the engine oil and compression test ??

Also what oil where you using?

Can you post a picture of the left and right spark plugs, firing end?

 

[Bill H]

I like using gallon and quart zip lock bags marked with a black marker pen to put parts in.

Just a thought.
Bill