Author Topic: B31 smoking  (Read 810 times)

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Calum

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B31 smoking
« on: 15 September, 2021, 19:23:24 »
Hi all

I have been very much enjoying my first summer of BSA ownership. 1953 B31. The gearbox rebuild proved very successful, and the mag rebuild has all but cured my hot running and starting issues. I've just returned from a 420 mile round trip to Suffolk this weekend and the bike never missed a beat. However one problem which seems to be slowly getting worse is the smoking...

It doesn't smoke from cold. It doesn't smoke when under load, nor when on the overrun. But when the engine is hot and I come to a stop at traffic lights or a roundabout it begins to emit a lot of smoke out of the exhaust! It doesn't always do this however, and I haven't worked out of there is a pattern to when it does and doesn't. As I pull away the smoke very soon clears.

My initial thoughts are the top end flooding with oil, and/or the return side of the pump not being able to pump the oil back quick enough after fast running. Oil consumption is generally quite good, so I don't think the rings or the valve guides are too bad; I also think this would cause a constant haze when moving along, but I am open to any suggestions at all!

Mike Farmer

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Re: B31 smoking
« Reply #1 on: 15 September, 2021, 20:36:59 »
 :) :) :)

What colour smoke. Generally. Blue is oil.  Black is petrol. White usually suggests a moisture content!!!!!unlikely with air cooled after a few minutes running and drying any condensation in exhaust etc

Mike 8) 8) 8) 8)

idie

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Re: B31 smoking
« Reply #2 on: 15 September, 2021, 21:22:19 »
Is the drain bolt got the right size hole in it. Sometime they get swapped with the large holed drain bolt being fitted on the exhaust rocker feed and vice versa.

Calum

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Re: B31 smoking
« Reply #3 on: 15 September, 2021, 23:53:16 »
Smoke is blue, and smells like burning oil.

Idie, I had heard of that before and it had crossed my mind. Thing is I don't know what size the holes should be, I did read it on here before somewhere but I'll definitely look into it.

idie

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Re: B31 smoking
« Reply #4 on: 16 September, 2021, 18:01:25 »
The rocker oil feed bolts have very small holes the drain bolt the hole is quite large.

martinh10

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Re: B31 smoking
« Reply #5 on: 16 September, 2021, 19:26:09 »
I replaced the rocker oil feed banjo bolts on my B31 recently with the correct 65-317 items which have a 1/64" dia hole. The drain pipe bolts have 3/32" to 1/8" dia holes, ie many times the area of the two rockers combined.

Pete C

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Re: B31 smoking
« Reply #6 on: 17 September, 2021, 08:27:38 »
I agree with checking the banjo bolt hole sizes but don't forget the lower drain pipe one in the crankcase, which can be difficult to get at.

Calum

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Re: B31 smoking
« Reply #7 on: 30 December, 2021, 14:27:39 »
So I finally got around to having a look at this... two rocker feed banjos easily removed, correct small holes. Drain banjo on head is a large hole, drain banjo on crankcase seems to be the small one >:(. Annoyingly the only one that is hard to get to, and I don't think I can quite get to it without removing, or at least loosening, the primary chaincase. Typical! I not long had all the primary stripped when I rebuilt the gearbox, I suppose I should have checked then!

Pete Gill

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Re: B31 smoking
« Reply #8 on: 30 December, 2021, 15:01:26 »
Well Calum at least youíve found the problem. I must confess that I drilled out the apertures on all those bolts on my B31 as 1/64th seemed to me to be too easy to clog up with a bit of debris. I did open up the lower ones more so. I havenít found it to flood with oil yet or smoke. If it does smoke more as time passes Iíll renew the rocker box ones, they are easy to get to.
Pete

cdsdorset

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Re: B31 smoking
« Reply #9 on: 30 December, 2021, 15:50:51 »
Have you had the head rebuilt. Sounds like wear on the valve guides. Oil in the head sucked into the engine down the side of the inlet valve.

Calum

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Re: B31 smoking
« Reply #10 on: 31 December, 2021, 00:45:59 »
Well Calum at least youíve found the problem. I must confess that I drilled out the apertures on all those bolts on my B31 as 1/64th seemed to me to be too easy to clog up with a bit of debris. I did open up the lower ones more so. I havenít found it to flood with oil yet or smoke. If it does smoke more as time passes Iíll renew the rocker box ones, they are easy to get to.
Pete
I don't suppose it's likely to flood the top as long as the drain holes are considerably larger than the combined area of the supply banjo orifices. I'll remove the drain pipe and drill the bottom banjo out to match the top and see how we get on.

Have you had the head rebuilt. Sounds like wear on the valve guides. Oil in the head sucked into the engine down the side of the inlet valve.
Nope, never had the head off. There's no smoke on the overrun or going down long hills to speak of so I don't think the guides are too bad. I do have some jobs planned for the engine in the future but for now I am just enjoying riding it when I can. Once I finally build my new garage I suspect I will spend a lot more time tinkering...

Pete Gill

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Re: B31 smoking
« Reply #11 on: 31 December, 2021, 12:52:43 »
Interestingly Poiseuille's law states that the potential flow in a pipe or through an orifice is proportional to the radius of the pipe/orifice  to the power 4 .
So a pipe with a radius of 2mm will have 16x the flow rate of a pipe 1mm radius!
Hard to believe !! and we would have to assume pressures would be equivalent to test that.
So you what Iím saying is wouldnít need to open up the lower oil drain much to make a lot of difference.

When this principle is applied to carburettor jets and especially partially blocked pilot jets  and worn needle jets it make you realise that very small changes make big differences.

Pete

cdsdorset

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Re: B31 smoking
« Reply #12 on: 31 December, 2021, 19:21:51 »
Well Calum at least youíve found the problem. I must confess that I drilled out the apertures on all those bolts on my B31 as 1/64th seemed to me to be too easy to clog up with a bit of debris. I did open up the lower ones more so. I havenít found it to flood with oil yet or smoke. If it does smoke more as time passes Iíll renew the rocker box ones, they are easy to get to.
Pete
I don't suppose it's likely to flood the top as long as the drain holes are considerably larger than the combined area of the supply banjo orifices. I'll remove the drain pipe and drill the bottom banjo out to match the top and see how we get on.

Have you had the head rebuilt. Sounds like wear on the valve guides. Oil in the head sucked into the engine down the side of the inlet valve.
Nope, never had the head off. There's no smoke on the overrun or going down long hills to speak of so I don't think the guides are too bad. I do have some jobs planned for the engine in the future but for now I am just enjoying riding it when I can. Once I finally build my new garage I suspect I will spend a lot more time tinkering...

Won't necessarily show on overnight as everything has been working quite hard  scavenge etc.  Being stood at a junction everything is just "ticking over" .
I can't remember down the oil drain purely by gravity or does it partly use the vaccum created in the crankcase?

Calum

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Re: B31 smoking
« Reply #13 on: 02 January, 2022, 13:51:00 »
Bottom banjo removed and drilled to match the top. I managed to remove the bolt once I had loosened the three bolts that hold the front of the inner primary to the crankcase. This gave me just enough room to remove it. I might shorten the bolt slightly when I refit to enable the bolt to be removed in future without having to loosen the primary case again.
The small hole (same as my feed banjos) was around 3/64" but the large was 5/32", if not slightly larger, but I drilled the bottom one to 5/32" to 'match'. I measured them trying different drill bits in which I measured with a micrometer. I had to poke the drill down the main body of the banjo bolt too to open that up to 5/32" as well. I quickly cleaned up the 45 degree sealing faces on the lathe as they were quite stepped and rough feeling. Hopefully it's all sorted now!

Pete Gill

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Re: B31 smoking
« Reply #14 on: 02 January, 2022, 19:10:38 »
Bottom banjo removed and drilled to match the top. I managed to remove the bolt once I had loosened the three bolts that hold the front of the inner primary to the crankcase. This gave me just enough room to remove it. I might shorten the bolt slightly when I refit to enable the bolt to be removed in future without having to loosen the primary case again.
The small hole (same as my feed banjos) was around 3/64" but the large was 5/32", if not slightly larger, but I drilled the bottom one to 5/32" to 'match'. I measured them trying different drill bits in which I measured with a micrometer. I had to poke the drill down the main body of the banjo bolt too to open that up to 5/32" as well. I quickly cleaned up the 45 degree sealing faces on the lathe as they were quite stepped and rough feeling. Hopefully it's all sorted now!
Letís us know if it solves the problem - for future reference.