Author Topic: 1946 B31 Crankcase Oil Blow  (Read 841 times)

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Keith L

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1946 B31 Crankcase Oil Blow
« on: 07 May, 2020, 16:08:00 »
Hi. Can anyone suggest why my crankcase blows out oil through the breather? Thanks - Keith

GLENN

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Re: 1946 B31 Crankcase Oil Blow
« Reply #1 on: 07 May, 2020, 18:16:04 »
Hi Keith, sounds like your engine is wet sumping.  Oil is passing from the oil tank past the little ball that is meant to prevent this and then accumulating in the sump.  When you start the bike the excessive amount of oil is forced out through the crank case breather before the oil pump is able to return all the excess oil to the tank.  All the best, Glenn

Keith L

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Re: 1946 B31 Crankcase Oil Blow
« Reply #2 on: 07 May, 2020, 19:04:46 »
Thanks Glen. I had been wet dumping but solved that by servicing the ball valve in the timing cover. Unfortunately the problem persists. Any other thoughts. Keith

Blackthou

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Re: 1946 B31 Crankcase Oil Blow
« Reply #3 on: 07 May, 2020, 20:49:37 »
Could it be worn rings thus increasing the pressure in the crankcase or simply a filter change required somewhere, also creating additional back pressure?

Pete C

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Re: 1946 B31 Crankcase Oil Blow
« Reply #4 on: 08 May, 2020, 06:11:26 »
There is quite a bit of oil flying around in the timing chest and the orientation of the hole in the breather tubeas well as the type of timing cover can influence how much finds it's way out of the breather.

Ideally you want to position the tube so that the hole is exposed to as little splash as possible - so not in the orientation shown! Hole facing the cover should reduce the chance of oil making it's way out.

I think the later cover, shown at the bottom, with the added shielding around the tube was introduced to help with this problem.

This may not be the cause of your problem but it is worth checking it.

cheers
Pete



GLENN

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Re: 1946 B31 Crankcase Oil Blow
« Reply #5 on: 08 May, 2020, 09:01:55 »
Hi Pete, The 1946 B31 will have the crank case breather located on the drive side of the crank case, not on the timing cover as in later models, so I'm guessing that the issue is still too much oil in the crank case for some reason.  All the best, Glenn

Keith L

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Re: 1946 B31 Crankcase Oil Blow
« Reply #6 on: 08 May, 2020, 09:03:49 »
Thanks to Blackthou. I had a re-bore and new piston and rings some time ago and I have only done a few miles since due to a clutch problem that is now resolved. I did wonder if things might improve as the engine gets more run in.

Thanks also to Peter C. These early models did not have a breather in the timing cover and I have wondered if fitting a later cover with a breather might help?

Keith

Keith L

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Re: 1946 B31 Crankcase Oil Blow
« Reply #7 on: 08 May, 2020, 09:12:57 »
Hi Glen, our posts both appeared at the same time. I did drain the crankcase of oil after the engine had run for 15 minutes or so and there was only a small amount of oil in the sump. There was still some oil dripping out at the end of the 15 minutes but perhaps not as much as when I first started the engine. Maybe there is still a degree of wet sumping but not enough for it to drain out through the chaincase as before.

Once these stay at home restrictions are eased I will get it out for some longer runs.

Keith

Pete C

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Re: 1946 B31 Crankcase Oil Blow
« Reply #8 on: 08 May, 2020, 09:29:23 »
Sorry, I hadn't realised it was the earlier model (got one myself as well as the later!).

 A friend had a similiar problem with an early M20, even after making sure the breather was working freely. It got somewhat better when he switched to straight 50 oil.

I see no reason why a later timing cover could not be used but I would probably blank off the one on the crankcase.

It would not mean an irreversible modification and timing covers are not particularly hard to come by.

paulm

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Re: 1946 B31 Crankcase Oil Blow
« Reply #9 on: 08 May, 2020, 11:35:42 »
This is a common problem caused by excessive crankcase pressure, it is either compression getting passes the rings (was it a good rebore) or valve / guides leaking.


Paul

Keith L

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Re: 1946 B31 Crankcase Oil Blow
« Reply #10 on: 08 May, 2020, 12:20:58 »
Thanks Paul. The re-bore was done at Gosnay's in Essex.  I will check the valves and guides. It was smoking before the re-bore and it isn't now.

keith

GLENN

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Re: 1946 B31 Crankcase Oil Blow
« Reply #11 on: 08 May, 2020, 16:22:03 »
Hi Keith, if you leave it overnight how much oil can be drained out of the sump prior to starting?  Is the ball in the oil pump free and seating ok?  Could it be the oil pump that isn't scavenging enough? 

Keith L

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Re: 1946 B31 Crankcase Oil Blow
« Reply #12 on: 08 May, 2020, 17:51:13 »
Hi Glen. I will check tomorrow and see what is in the sump. I get a really good flow back to the oil tank. Thanks

Keith L

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Re: 1946 B31 Crankcase Oil Blow
« Reply #13 on: 09 May, 2020, 10:51:47 »
Hi Glen. There was no oil in the sump this morning so that's after nearly 2 days since I drained the sump. I will start the engine later today to see if it still blows oil out through the breather. Keith