Vent Pipe Smoking

[Garryk]

Was wonder if someone can help me with the following issue.
There is what looks like a vent pipe coming from the top of the engine then runs over the top of my airfilter and then runs down the back of my airfilter,ive noticed smoke coming from the pipe at the open end behind airfilter,bike seems to be running fine and no smoke from exhaust,im just hoping it something simple.
1975 CB750K5
Garryk

[hairygit]

Is it smoke or steam/water vapour? If the bike is used infrequently or for short journeys condensation forms inside the engine, and can give the inside of the oil filler plug/dipstick a coating of greyish gunge, then if ridden for a while and the engine gets properly warmed up, the condensation boils off, giving steam through the breather which is often mistaken for smoke. Take a look at the dipstick, it can tell you a lot! Also, a little bit of blowby on the rings and general pressure build up in the crankcases is not uncommon on older bikes, the Honda manual actually mentions checking the end of the vent pipe to clear any oil or muck which may have accumulated there, so probably nothing to worry about unless it is smoking like a 2 stroke!

[Garryk]

Hairygit many thanks for your help.
I only bought this bike 8 weeks ago,previous owner had her for 4 years and never really took the bike out as he was getting probs keep her running on 4 cylinders,i sorted that out with help from others on this forum,it was the dirty carbs causing the problem at the time,the bike has nearly 14,000 miles on the clock,unsure if they are genuine miles or not but she is in perfect order and most bolts and screws seem untouched,i noticed the smoke/steam coming from the pipe a few times and it always seemed on short journeys of 7 to 8 miles,longest distance ive had the bike so far was 80 miles and never noticed any smoke/steam,i havnt been using the bike much due to the weather,so im hoping what you said above about the steam is what the problem is,or should I say praying that's what the problem is,i will keep an eye on the issue and see if it vanishes with the better weather on it way.
Cant thank you enough for your help
Respects
Garryk

[K2-K6]

Good advice from Hairygit there and if used more regularly then it should improve if that's the case.

The pipe is, as you've already mentioned a breather pipe......this is because engines can pressurise themselves during normal running, so to prevent that happening and trying to force oil out via seals etc then having a vent pipe allows the internal pressure to vent to atmosphere.

As hairy says, if there is water contained in the oil (it gets into the oil from short runs and or too much running on choke) as the engine heats up, then it can be seen as steam from the pipe.

In addition to hairy's advice of looking at the dipstick, just smell it as well.....it should be just oil (I know that sounds obvious) and not of petrol. If there is a noticeable petrol smell then it needs changeing.

More use should definitely improve it, and if the weather stays like today then you may get more chance to ride it as well.

[Garryk]

Hi K2-k6
Many thanks for your reply and help,will check dipstick tomorrow for petrol smell,gave the dipstick a visual check  the other day and no signs on it of anything other than oil...
Im hoping to change the oil in next few weeks.
Here's to better weather.
Garryk

[steff750]

 hi
      I  have this problem with my CB750 sidecar outfit, which I use as a winter hack, so it never really warms up and has I have not used it yet this year, the oil tank still has a bit of gunge around the filler cap and it is well overdue for a oil change anyway, so that will be my first option as it has always puffed out steam from under seat
          I did have to rebuild my first CB750 years ago when I first had it , it had a hard life as one of the previous owners had used it for hill climbs and sprints and had thrashed it  a bit in the 70's , he told me it had snapped the gearbox main shaft . it had a bit of steam and back pressure and it never went away even when the bike was warmed up and it seemed to be down on power . when I did get around to rebuilding the motor about a year later #4 cylinder was well scored because a piston ring had broken up into about twenty pieces and damaged the bore and the piston groove,  I had to bore out the barrels two sizes over (+50) and added + 50 pistons ,so try all the other options first , a compression test should tell you if there is a cylinder down.

[Garryk]

Steff,many thanks for your reply and help.
Checked for gunge round my filler cap and there doesn't seem to be any,just hoping a few days out on the road will clear the steam/smoke then i'll know all is well,will be buying a compression tester from Ebay.
Cheers
Garryk

[Garryk]

UPDATE
smelt the oil tank and dipstick today and I think I can smell petrol so im guessing petrol is getting by rings.
Its summer here at the moment as you all know,so I was thinking if I change the oil would it be ok to ride the bike until winter when I can change the piston rings ?,I will  still keep my eye on the steam/smoke from the pipe just incase its not petrol im smelling from oil tank.
Your thoughts on the above matter please.
Garryk

[Bryanj]

Petrol in the oil is not bad rings but short journeys on choke

[Garryk]

Bryanj,Thanks for that info and for putting my mind at rest.
When I first got the bike I was having bad carb trouble which I sorted thanks to members on this forum,what I did notice in my carbs was the choke was half way open all the time till I sorted it,i never needed the choke for cold starts so that's when I knew something was wrong,so im guessing this was when petrol got into oil.
Thanks Again
Respects to everyone on this forum for help they give to others and for sharing their knowledge.
Garryk

[K2-K6]

The choke function on older carbs is quite crude and makes the mixture much richer than normal running requires.....too much of running on choke when not needed and wet fuel doesn't all get burnt but a proportion of it leaks past the top piston rings and ends up in the oil.

As Bryan says, it should be fine but you need to change the oil so that it protects the bearings and other components as required .

If you test the compression and compare to spec, you'll probably find that it's all ok so it's nothing to worry about unless other things indicate otherwise.

These motors live on fresh oil, so don't avoid a change if required.....likewise they don't give much trouble if you pay attention to oil condition.

[Garryk]

K2-K6
Thanks for the info,respects
Oil change was done yesterday.put some classic 20/50 into her,new oil filter and cleaned oil screen in the sump,so she's ready for the charity rideout tomorrow.
Cheerssssssssssssss
Garrk