Author Topic: Uneven Revving B40 - Any ideas ?  (Read 5727 times)

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Mike Farmer

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Re: Uneven Revving B40 - Any ideas ?
« Reply #45 on: 08 December, 2019, 10:25:16 »
 :) :) :)

Something else that I may have missed. Have you tried opening the throttle by simply reaching in and pulling the cable up at the top of the carb. If so did it return to normal revs straight away??? Could be a simple cable problem. May even be something as simple as the carb to not "sitting " exactly square.

Mike 8) 8) 8)

Crazy Mike

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Re: Uneven Revving B40 - Any ideas ?
« Reply #46 on: 08 December, 2019, 14:26:24 »
Mike, the mechanism from throttle to carb is all new and the cable works as it should. The slide can always be heard to make a nice returning 'click' so I'm fairly certain it's not a carb problem. I'm really looking for views about the level of grease in the advance / retard mechanism of the distributor as seen in the photos.
I will take up an earlier suggestion to contact the Distributor doc for advice as well. Just for clarification here, the distributor is mounted on top of the crankcase.
Thanks for the responses.

berniej

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Re: Uneven Revving B40 - Any ideas ?
« Reply #47 on: 08 December, 2019, 17:10:29 »
I haven't greased my A/R mechanism, just oiled it. However, if it has been professionally overhauled I'd assume that new springs have been fitted and even if the mechanism is heavily coated I would have thought that the springs should pull the bob-weights fully back in when idling. If in doubt however, wash all the grease off, reassemble with some light oil and give it a whirl.

I have had the same problem since my S90 since rebuild and not yet diagnosed it. I did have a charging problem which would have been affecting the spark so now that's fixed I'll be testing it all again as soon as the weather allows. I'll update if there's any change due to resolving the charging problem.

Cheers,
Bernie
'49 M21/B31 hybrid
'56 M21 combination
B40 Super Star
A50CC project

Crazy Mike

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Re: Uneven Revving B40 - Any ideas ?
« Reply #48 on: 08 December, 2019, 17:32:52 »
Thanks BernieJ
I will do exactly as you advise as soon as I can.

There is one thing I've only just thought of and it's when I removed the bob weights I noticed some 'scoring' on the underside of the weights. There are, what look like rivet heads, on the body of the distributor underneath the bob weights. I wonder if there's some fouling ? Once it's all clean I will be able to see.

Also, I assume that by removing the points and A/R mechanism, this should not affect the ignition timing.
Will report back once done and tested.
Cheers !

berniej

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Re: Uneven Revving B40 - Any ideas ?
« Reply #49 on: 08 December, 2019, 18:08:28 »
Scoring doesn't sound good and would suggest that there has likely been sticking in the mechanism if parts are fouling.

I haven't been able to find an exploded parts diagram of the distributor but did find this: http://www.brightsparkmagnetos.com/library/Parts%20lists/Lucas%20motor%20cycle%20parts%201936-57/G02-07%20Contact%20breaker%20units%20distributors%20and%20suppressors.pdf which might help.

Is the scoring on the top of the 'shaft and action plate'? If so maybe that suggests either wear in the hinge post/bob-weight interface allowing some downward rocking or possibly missing washers under the bob-weights (I don't know if there are supposed to be any but hopefully someone might know or have a diagram??). Or is it maybe due to the cam rubbing on the top of the plate?

Might be an idea to disassemble, clean and examine the parts for wear and see if the bob-weights have been fouling the plate - or if they can be rocked such that they would.

(We may be moving closer to understanding why BSA, in a blaze of enlightenment realised in around 1966 that perhaps a distributor for a single-cylinder bike (oxymoron?) wasn't the most sensible design option and moved to side-points for the '67 models onwards....)

But we are where we are I guess and have to solve the riddle!!

Cheers,
Bernie



'49 M21/B31 hybrid
'56 M21 combination
B40 Super Star
A50CC project

Phil C

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Re: Uneven Revving B40 - Any ideas ?
« Reply #50 on: 08 December, 2019, 18:23:22 »
Re sidepoints, to be tiresomely pedantic, at least some BSAs were sidepoints before '67. My B40 is 1965 and is sidepoints.  I just mention it as a point of interest.  Phil.

Phil C

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Re: Uneven Revving B40 - Any ideas ?
« Reply #51 on: 08 December, 2019, 18:30:10 »
By the way, I don't doubt for a second that Bernie is aware of that.   Phil.

Mike Farmer

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Re: Uneven Revving B40 - Any ideas ?
« Reply #52 on: 08 December, 2019, 21:49:58 »
 :) :) :)

Purely as a matter of interest I had a BMW R65 that did exactly what you are experiencing. I NEVER found the cure. I sold it to a guy who was quite happy with the problem and I don't think he ever found the cure either.

Mike 8) 8) 8)

Crazy Mike

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Re: Uneven Revving B40 - Any ideas ?
« Reply #53 on: 11 December, 2019, 17:59:00 »
I have now removed the points, condenser, springs, weights and cam plate and de-greased the unit as best as possible without actually removing the distributor from the crankcase.

I mentioned earlier about some 'scoring' on the underside of the weights. It look like the 4 raised rounded areas, as can be seen in the photo, may be the cause but I think they are there to reduce friction for movement of the weights, so it may be normal to see some markings.

The components, in my inexperienced opinion, look in good condition but I have two questions, perhaps  silly ones...

1) There is a screw which goes in the top of the rotor shaft, which I thought was there to secure the cam to the shaft, but it does nothing. What is it for as it doesn't secure the cam in any way unless I have a missing part !

2) I have not removed the distributor from the crankcase. Can I assume that the ignition timing will not be affected by what I have done so far ?

Thanks all.

KEV441

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Re: Uneven Revving B40 - Any ideas ?
« Reply #54 on: 11 December, 2019, 21:11:19 »
Is the screw there to put a washer under to lock the cam on full advance to set the static timing. I would try to clean more of that grease out and reassemble with a light touch of oil on the moving parts then see how it runs.
  Kev

LightningAndStarfire

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Re: Uneven Revving B40 - Any ideas ?
« Reply #55 on: 11 December, 2019, 21:26:22 »
If you measured maximum opening width of the points they should be set the same at reassembly. Any difference will alter the timing to some extent.

berniej

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Re: Uneven Revving B40 - Any ideas ?
« Reply #56 on: 12 December, 2019, 08:52:06 »
There is another way to check if your advance/retard is erratic, by using a strobe.

To set the timing accurately this way you would need a timing disc fixed onto the alternator rotor and a reference mark on the stator (I use typing correction fluid to make a white base and then pencil a line over it) but that's been covered elsewhere on the forum.

However, you wouldn't need a timing disc to check for irregular behaviour, just a pair of reference marks on stator and rotor roughly lining up at TDC.

If you then run without the primary cover on and strobe you should be able to see how the A/R is behaving and if it always returns to the same position at idle by observing how the offset between the two marks varies with engine speed. Might help work out if this is distributor-related or not.

Cheers,
Bernie
'49 M21/B31 hybrid
'56 M21 combination
B40 Super Star
A50CC project

AdrianS

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Re: Uneven Revving B40 - Any ideas ?
« Reply #57 on: 13 December, 2019, 11:58:39 »
The timing will be affected by the points gap on re assembly. Did you measure the gap before dismantling?

Make a mark across the distributor body to the crankcase and undo the clamp screw and pull the distributor and clean it properly. Note which way the offset cam drive comes out or you can put it in 180 degrees out. Check the spindle bushes and wash everything out with paraffin or what ever you prefer and then re oil it if sound. Easier to work on when on the bench. Advance and retard springs do wear and it is difficult to assess if worn. Distributor Doctor supplies new ones but they are very expensive for what they are ( I think about £14 plus postage and plus the twat). Any other components can be supplied by that company.

I refurbished my distributor on my B40 and it made a difference to the tickover and running. To be fair, I have never worried too much about the timing. I set mine with a marked rod before TDC down the piston hole and then fine tune it my turning the distributor after running the bike to give a smooth engine response. I probably run mine slightly retarded as I feel that the settings suggested make my engine feel a little harsh.

Crazy Mike

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Re: Uneven Revving B40 - Any ideas ?
« Reply #58 on: 13 December, 2019, 12:36:19 »
Adrian, thanks for your thoughts.

The points were 0.015" before removal and that's what I set them at after re-fitting so I don't understand why the timing should then be out.

To gain access to the distributor clamp screw on my bike involves removing exhaust, gear lever etc and then crankcase as there is no clamp on the outside of the crankcase which would have made life so much easier !

I agree with what you say about the cam drive plate as it can be fitted in 2 ways. The springs are in good condition as are the weights, as the unit was overhauled by the Distributor Dr in 2017 (300 miles ago).

Using the rod in the hole method with both valves open, I have checked the 1/16th BTDC and it looks like the cam is at the position to be just starting to open the points, so that looks ok. The weights pull back nicely if twisted by hand.

However, the only way I can start the bike now is by putting a washer under the spindle screw which clamps the cam and prevents movement. This surely indicates the static timing has changed , but how ?

I'm reluctant to remove the distributor if it's not necessary but would like to understand if and why the timing may have changed ?

Thanks

Crazy Mike

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Re: Uneven Revving B40 - Any ideas ?
« Reply #59 on: 13 December, 2019, 15:50:21 »
Following on from my earlier post, I have now 'discovered' a screw / bolt on the outside of the crankcase which, I now realise, is the distributor clamp screw !!!
I never really looked at this screw as I just thought this was another crankcase screw / bolt. Now it all becomes somewhat easier.
With the slightest turn of the distributor the bike now starts and runs. I still don't fully understand how the timing can change by just re-setting the points but hey-ho !

As for the uneven tick over, a road test will answer that in due course.

One question, now I have discovered the distributor clamp screw (cough-cough!) if I need to remove the unit, do I unscrew the screw all the way out or is just loosening it enough to presumably just withdraw the distributor from the case? Is it that easy ?

Thanks