Topic: Smoking Breather Pipe - cb750F1

[750goes]

Thank you in advance to all who reply with some help.

bought a 1976 cb750f1 - a real mess in every respect at this time.

would like to get the bike running to put back on the road. it was basically recovered from the tip, not a complete unit, but mostly all there.
previous owners have done some alterations to the frame, and some accessories changed or altered.

siince purchase the following has been done
new plugs, - old ones pretty oiled up & black
replaced spark plug end caps where required
checked static timing - 1 & 4 ok, 2 & 3 adjusted a bit.
points gap ok on both sets, points not pitted.
drained fuel from tank and some rubbish came out, ran it till it was clear, did this a few times
checked drain plugs on carbs - fuel flowed but ony a trickle
purchased battery and tried to start - fired up but only ran a few seconds then no more
needless to say pulled carbs off and cleaned all jets, bowls, lines etc..
Jets are #40 on idle screws and #105 mains, did not pull sliders or choke assembly. reset air screw to 1 turn out on all. runs with air pods
fired up- ran ok and idled fine - a little bit of backfire to be expected.
have 4 into 1 exhaust
when bike was warm smoke coming out of breather tube under battery like steam out of a kettle (colour blue and definitely oil).
i know this is a long winded opening but i would like to start simple and hopefully not have to rebuild anything major.
not a mechanic but have some tinkering ability.

can anyone provide k.i.s.s. list of options to pursue

[bryanj]

You have two options:-
(1) take it apart now and redo the rings
(2) Run it and see what happens and probably take it apart later.

Rings and gaskets aint cheap but pistons and rebore are even more expensive

[Jim Shea]

Welcome to the forum, if the answer is out there these guys will provide it.
I am not a techy but I have a 76 F1 and wondered where you are located?
Cheers,
Jim.

[jurob]

do a compression check and go from there. sounds like piston ring blowby.

[scondon]

Didn't see an oil change in your list of things done. Contaminated oil, either gas or water, can cause excessive smoking from the breather tubes. Although probably not to the extent of "blowing steam from a kettle". My two cents.

[TwoTired]

new plugs, - old ones pretty oiled up & black

when bike was warm smoke coming out of breather tube under battery like steam out of a kettle (colour blue and definitely oil).

Oily plugs?   How did oil get in there?  Preservative storage, valve guides, rings?

The crankcase can be pressurized by:
Water/condensation boil off into steam vapor.
Leaking piston rings. on compression and power stokes.

I'd like to see the results an engine hot compression test, with another test imediately following with a teaspoon of oil dumped into each spark plug hole.  If you don't already know the proper proceedure for a compression test, ask.

Possibilities:
1 - After the engine heats thoughoughly the water get competely vaporized and the smoke stops.
2 - Stuck or seized ring(s) free up with use, reseats and seals, reducing the smoke volume.  This can sometimes be "encouraged" by dumping some Marvel Mystery oil into the spark plug holes and allowed to soak overnight.
3 - Permantently seized/scorched rings, broken rings, or scored cylinder walls must be renewed to seal properly.
4 - worse

Cheers,

[750goes]

Thank you for your comments and suggestions

will shortly do a wet & dry compression test, and will post the results
also will change the oil as it is very dirty and seems kind of thin, maybe even a bit overfull according to dipstick
probably change the filter as well.
there is no blue smoke coming from the exhaust after warm up or on start up.
is there some way of determining if smoke is coming from the head ie valve seals or if it is from  rings

i forgot to mention that this oily smoke also comes out from the oil tank when the cap is removed, oil in tank shows small bubbles like it has been aerated.

Jim I live in Sydney Australia.

regards and thanks again, will keep posting with your assistance appreciated

[TwoTired]

is there some way of determining if smoke is coming from the head ie valve seals or if it is from  rings

The dry / wet compression test will show an increase in compression if the oil seals the rings.  Oil won't seal the valve guides, though.  But, leaky valve guides don't usually cause crankcase pressure.  But, it can cause exhaust smoke.

i forgot to mention that this oily smoke also comes out from the oil tank when the cap is removed, oil in tank shows small bubbles like it has been aerated.

The 750 is dry sump. The scavenge pump pumps oil out of the engine faster than it puts it in.  It sucks air while doing the scavenge thing and this puts air bubbles in the oil tank.

Cheers,

[Sam Green Racing]

You may find, when you have the correct amount of oil in the bike all your problems will will disappear.
Your motor needs the correct amount of oil to lubricate, it also needs a certain amount of breathing space.

Sam.