Author Topic: B31 ignition  (Read 263 times)

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chrisday59

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B31 ignition
« on: 23 May, 2022, 09:22:11 »
Stupid question. I  just bought a B31, and can't start it. The previous owner gave me his routine - Petrol on, timing lever back a small amount - just a couple of mil, air control lever at 4 o'clock, use valve lifter to take it just over TDC and give it a good kick.

Have to say, compared with my B44, the I don't need the valve lifter, it will turn over TDC with the kickstart. The return spring on the lifter control at the valve end seems very weak, so I've adjusted it and also ensured it's closed with my fingers. I also tried checking the difference between plug in and plug out, so it's definitely got good compression.

Just for sanity's sake, I'm not missing anything obvious like an ignition switch somewhere am I?

Cheers
Chris 

idie

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Re: B31 ignition
« Reply #1 on: 23 May, 2022, 09:33:06 »
Try not using the choke, hold the float tickler down until it wets your finger. Open the throttle further. Try it with no advance, does it kick back if not there is a problem. Has it wet sumped. See if there is a drain plug on bottom of engine. It will be a small bolt undo and see if oil comes out, make sure you have a container to drain into. If it has drained oil into the sump it can cause quite a drag on the flywheel so making it harder to swing engine over.

Greenfield

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Re: B31 ignition
« Reply #2 on: 23 May, 2022, 09:39:00 »
Hi Chris
You'll need to go through things step by step to sort this out.
Start by removing the plug and turn it over to check for a spark and use the tickler to check you have petrol getting through.
Every bike likes a different starting procedure and its a matter of finding the correct fuel/air mix with the correctly timed spark. The relationship between timing and mixture seems to be fairly critical with my B33.

idie

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Re: B31 ignition
« Reply #3 on: 23 May, 2022, 10:37:00 »
My B33 and M33 both need a different starting procedure, mind you I have alleviated the problem by fitting an electric starter to both. Saves my poor old knee.

chrisday59

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Re: B31 ignition
« Reply #4 on: 27 May, 2022, 14:15:19 »
I trailered the bike down to my friendly local bike mech (Thanks Alan from MotoTempo), and he immediately sussed that the air control lever operates in the reverse direction than expected (i.e. full choke with lever in forward position). Once we sussed that, a quick prod gave a big backfire. As previously mentioned, the valve lifter assembly is very slack, the return spring isn't long/strong enough to close the lifter without help. There's now no compression, so I need to strip the assembly & check it out. I'm not a spanner virgin, but I haven't touched an engine in anger for over 40 years, so I need to follow an idiots guide for peace of mind. I can't find any pictures/steps online, and the manual available online is pretty sketchy.

Does anyone have any pics or a step-by-step?

Rog1

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Re: B31 ignition
« Reply #5 on: 29 May, 2022, 17:45:06 »
You are not the first to be caught out by the air control lever. I wasn't the first either!  :)

Now here's a couple of photos. This is what the decompression mechanism looks like on my M33 head, so near enough the same engine, I should think. I faddled around for ages setting the thing up but as shown it works a treat. The first photo shows it at rest, the second how it looks when the handlebar lever is pulled all the way in. Note the angle of the arm compared to the gasket face of the rocker cover, in both shots.

If you need to dismantle the mechanism (it might be stiff) you'll have to remove the rocker cover. The arm fits on to splines so its angle can be adjusted, but I found I had to remove the rocker cover every time I adjusted the angle, because pushing the arm onto the splines just pushes the pivot in; the pivot needs holding on the inside. (Until I got it right I just secured the rocker cover with two of its four screws)

When the cable is properly adjusted it should pull the arm round a bit, so take that into account when getting it on the splines. When you do this, make sure the engine is in a position where the exhaust valve is closed.

I found my cable to be frayed, which doesn't help. That's a new cable on there now.

chrisday59

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Re: B31 ignition
« Reply #6 on: 29 May, 2022, 18:49:05 »
Thanks Rog, very helpful. Your pics & explanation match what I can see. As mentioned, the return spring is so weak it doesn't do it's job, but when I removed the rocker cover I could see that the lifter wasn't holding the valve open, so  shouldn't have done any damage. I poured a bit of petrol into the gap above the valve, and it did  disappear so there is a leak, but after I'd taken off both rocker covers, and top and bottom inspection plates to establish that both valves and tappets were moving freely, I bolted all back up again, gave it a hail mary last kick before giving up for the day and bless me it started first time. Maybe the exhaust valve was a bit sticky and the petrol cleaned it up a bit, who knows. Anyway, it's LOUD, way louder than my B44, but ran sweetly.

Needless to say, it wouldn't be a BSA if the ending was perfect, because there's now a large pool of oil on the drive. It's dripping from the left hand side behind the primary drive cover. Funny it's only started since I started the engine, which makes me think it's engine oil under pressure, but what would that be doing in the primary drive?

Photos to follow when my email decides to behave, the wonders of a 2 Mb broadband connection :(

 

chrisday59

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Re: B31 ignition
« Reply #7 on: 29 May, 2022, 19:49:15 »
Here's a pic of the general area where the leak/drip is coming from. I've managed to zoom in to the pic and it's captured a drip just about to fall. Looks like it's coming from a brass bolt but I don't know what it is. I couldn't see it by eye as I don't have a lift, but the flash on my phone has frozen it

Pete C

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Re: B31 ignition
« Reply #8 on: 31 May, 2022, 11:49:06 »
That looks like the primary drain plug. If it is leaking frm there you could try checking the seal (fibre washer?). Alternatively that is the lowest part of the inner primary case and any oil flying out from behind the clutch is likely to end up dripping off there. If the oil is originating in the primary, it is now coming out because it is being thrown about by the chain when the engine is running.

Jim S

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Re: B31 ignition
« Reply #9 on: 31 May, 2022, 13:08:53 »
That could be gearbox oil also.

Jim