Blue Smoke on decel and accel

Information relating to the Matchless G85 500cc Heavyweight, AJS 7R, Matchless G45 and Matchless G50
Posts: 197
Joined: Sun Oct 25, 2015 8:19 am
Location: AUSTRALIA

Re: Blue Smoke on decel and accel

Postby Matchymarty » Mon Nov 14, 2016 9:25 am

Well, Now I'm really scratching my head.
Split the cases, checked all oil openings, and they all are clear. Funnily, the main crank bearing seems 'dry' compared to the rest of the crankcase which seems very wet. Mainbearing also seems to be a bit noisy so I'll replace them

Oil pump and openings all seem fine. The pump bore has a score in it from the cam chest hole, but not sure that would cause much issue, as the pump seems to work well enough.

So if the crankcase is pressurising, I can't see how, that said, oil is getting past the rings, and, loading up the head heavily, so I"m not sure why....

Looks like I can save the piston and bore with a hone and new rings for now, and big end seems very good. So looks like it can go back together with a minimum of cost - Though I'm none the wiser as to the burning oil situation, as the rings all had gaps to spec and aren't worn. (so they shouldnt', piston rings and bore only 3000 miles old.)

Member
Posts: 59
Joined: Mon Jan 01, 1990 12:00 am
Location: NORWAY

Re: Blue Smoke on decel and accel

Postby bjorn » Mon Nov 14, 2016 9:43 am

Marty,

"Any other ideas where to look for blockages that cause crank case pressure build up?"

Apart from massive blow-by from broken or missing piston rings, as far as I know, there is nothing other than malfunction of the crankcase breather that can cause crank case pressure build up.
What thickness is the petal shaped disc?

regards
Bjørn

Member
User avatar
Posts: 949
Joined: Sat Jun 20, 2015 8:38 am
Location: Chester, UK

Re: Blue Smoke on decel and accel

Postby clanger9 » Mon Nov 14, 2016 10:38 pm

Matchymarty wrote:So if the crankcase is pressurising, I can't see how, that said, oil is getting past the rings...

Think about it: positive pressure underneath the piston, vacuum above (during the intake stroke). Piston rings aren't so good at sealing that way around. If oil is getting pushed past the gaskets, it'll also get past the rings...

Result is blow-by up-overs into the combustion chamber. Together with lots of blue smoke.
1966 AJS Model 14 250 CSR "cafe racer"
1993 Ducati 750SS

Posts: 197
Joined: Sun Oct 25, 2015 8:19 am
Location: AUSTRALIA

Re: Blue Smoke on decel and accel

Postby Matchymarty » Mon Nov 14, 2016 10:54 pm

clanger9 wrote:
Matchymarty wrote:So if the crankcase is pressurising, I can't see how, that said, oil is getting past the rings...

Think about it: positive pressure underneath the piston, vacuum above (during the intake stroke). Piston rings aren't so good at sealing that way around. If oil is getting pushed past the gaskets, it'll also get past the rings...

Result is blow-by up-overs into the combustion chamber. Together with lots of blue smoke.


Fair point Clanger. Thanks. From my investigations last night, seems that there's no way the crankcase can be pressurising as all galleries and relief valve are clear. So excessive amounts of oil must be getting sucked down through the head, particuarily, as I found oil in the carb throat. So, I'm suspecting the valve guides. I guess it can ONLY be that, as there's no other way oil can get into the combustion chamber apart from past the rings?

Cheers all... You are all amazing and I'm learning so much!

Member
User avatar
Posts: 949
Joined: Sat Jun 20, 2015 8:38 am
Location: Chester, UK

Re: Blue Smoke on decel and accel

Postby clanger9 » Tue Nov 15, 2016 8:04 am

Oil in the carb throat? That's interesting (and possibly important).
I can't think how or why oil would get there. Is the head cracked?
1966 AJS Model 14 250 CSR "cafe racer"
1993 Ducati 750SS

Posts: 197
Joined: Sun Oct 25, 2015 8:19 am
Location: AUSTRALIA

Re: Blue Smoke on decel and accel

Postby Matchymarty » Tue Nov 15, 2016 10:32 am

clanger9 wrote:Oil in the carb throat? That's interesting (and possibly important).
I can't think how or why oil would get there. Is the head cracked?


Clanger ,

Head cracked? Hope not, but don't think so. Nothing evident. Oil in the carb throat? Well the amount that is getting sucked down into the combustion chamber... anything is possible....

I have news. Seems the problem has been found. I pulled the valves out of the head, and the intake valve shows a big clump of burnt oil on the top, so clearly that is where my oil is coming from. Funny thing is, both valve guides seem, very nice and tight.
Any way, now I have to go looking for why the oil is coming down the valve in huge quantities. The Intake valve oil set screw seems to be set right around 1/8th turn. so I have to blow out all the holes and see what the story is.

See photos.
20161115_185144 (Small).jpg

20161112_171251 (Small).jpg
You do not have the required permissions to view the files attached to this post.

Member
User avatar
Posts: 949
Joined: Sat Jun 20, 2015 8:38 am
Location: Chester, UK

Re: Blue Smoke on decel and accel

Postby clanger9 » Tue Nov 15, 2016 10:43 am

Crikey, that is a lot of oil up there. Just looks like over-oiling to the top end - maybe turn the feed screw down a bit?
I don't think this engines have a stem seal on the inlet valve (?), so too much oil around the valve guide will produce plenty of smoke,

Looks like you have homed in on the cause. Progress!
1966 AJS Model 14 250 CSR "cafe racer"
1993 Ducati 750SS

Member
Posts: 59
Joined: Mon Jan 01, 1990 12:00 am
Location: NORWAY

Re: Blue Smoke on decel and accel

Postby bjorn » Tue Nov 15, 2016 11:28 am

Marty,
You may have other leakage issues on your motor, but here is just one final comment on the crankcase breather. It may not necessarily be working even if the breather bore and pipe are open. The breather disc is lightweight because it is intended to move back and forth with pressure pulsations in the crankcase. When the piston moves down the disc is blown “outwards” by the pressure build up in the crankcase. When the piston moves back up, it creates a suction in the crankcase which pulls the disc back against its seat in the crankcase. This action builds a slight (continuous) vacuum in the crankcase.
If the breather disc is not light enough it will not move back and forth with crankcase pressure fluctuations. Also, if the disc is not circular, it does not have sufficient “working” area for the suction from the crankcase to pull it back against its seat. The result? – The crankcase pressure is not pulled down to below atmospheric and oil leaks and burning of oil will result.
BSA singles have a similar breather, but use a fibre disc for lightness.
All classic British singles and parallel twins have a breather system which creates a slight vacuum in the crankcase. That is why they are reasonably oil tight on the lower part of the engine. Also, it prevents oil going past the rings and into the combustion chamber.
Quote from a Norton Owners forum:
“As you may be aware, the Norton motor acts like a giant air compressor during running. As the pistons go up and down the air within the case also expands and contracts. If the breather is not functioning properly oil will almost certainly leak out of the cracks and crevices. Most notably will be the timing chest as it seems to suffer the most. It could result in poor sealing of the main seals, and possible leaking around the primary chaincase where it attaches to the drive side case. If you have oil leaking out of the cases at any point (other than around the cylinder barrels and head) I would suspect breather problems, providing the motor is assembled in a proper fashion (Norton Service Release Notes October 1971).”
Hope you manage to sort it out and can get back on the road again! (Slightly envious as its Winter here in Norway and no more riding until Spring!)
Regards
Bjørn

Posts: 197
Joined: Sun Oct 25, 2015 8:19 am
Location: AUSTRALIA

Re: Blue Smoke on decel and accel

Postby Matchymarty » Tue Nov 15, 2016 8:45 pm

bjorn wrote:Marty,
You may have other leakage issues on your motor, but here is just one final comment on the crankcase breather. It may not necessarily be working even if the breather bore and pipe are open. The breather disc is lightweight because it is intended to move back and forth with pressure pulsations in the crankcase. When the piston moves down the disc is blown “outwards” by the pressure build up in the crankcase. When the piston moves back up, it creates a suction in the crankcase which pulls the disc back against its seat in the crankcase. This action builds a slight (continuous) vacuum in the crankcase.
If the breather disc is not light enough it will not move back and forth with crankcase pressure fluctuations. Also, if the disc is not circular, it does not have sufficient “working” area for the suction from the crankcase to pull it back against its seat. The result? – The crankcase pressure is not pulled down to below atmospheric and oil leaks and burning of oil will result.
BSA singles have a similar breather, but use a fibre disc for lightness.
All classic British singles and parallel twins have a breather system which creates a slight vacuum in the crankcase. That is why they are reasonably oil tight on the lower part of the engine. Also, it prevents oil going past the rings and into the combustion chamber.
Quote from a Norton Owners forum:
“As you may be aware, the Norton motor acts like a giant air compressor during running. As the pistons go up and down the air within the case also expands and contracts. If the breather is not functioning properly oil will almost certainly leak out of the cracks and crevices. Most notably will be the timing chest as it seems to suffer the most. It could result in poor sealing of the main seals, and possible leaking around the primary chaincase where it attaches to the drive side case. If you have oil leaking out of the cases at any point (other than around the cylinder barrels and head) I would suspect breather problems, providing the motor is assembled in a proper fashion (Norton Service Release Notes October 1971).”
Hope you manage to sort it out and can get back on the road again! (Slightly envious as its Winter here in Norway and no more riding until Spring!)
Regards
Bjørn



Thanks so much Bjorn. Very insightful!

That said, would it be fair to say, one could do without the disc in the valve, if you had a decent length of pipe and a piece of gauze over the end? This would ensure free breathing crank case??

Member
User avatar
Posts: 949
Joined: Sat Jun 20, 2015 8:38 am
Location: Chester, UK

Re: Blue Smoke on decel and accel

Postby clanger9 » Tue Nov 15, 2016 9:45 pm

Yes, you can run an 'open' breather (into a catch tank).

This isn't as elegant as the negative-pressure arrangement (you get more pumping losses and oil leaks), but at least it works. It will also keep working when the engine is thrashed hard and/or has more blow-by from a racing piston, hence this arrangement is more common on track bikes...
1966 AJS Model 14 250 CSR "cafe racer"
1993 Ducati 750SS

PreviousNext

Return to Competition / Racers

Who is online

Users browsing this forum: No registered users and 2 guests