Author Topic: BTH Magneto  (Read 3275 times)

Online groily

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Re: BTH Magneto
« Reply #15 on: 07.10. 2012 17:35 »
Dunno about the BT-H thingy, but ref the alloy pinion Kiwi, just went and had a look at 2 of mine. Exact same thing as yours - maybe a tad worse even (these were SRM-supplied but likely the same maker?) - I have to admit I never even thought to check that carefully before fitting. One's plain, one I attacked to mount an ATD on, and they're both the same.
The good thing is, they've run fine, the plain one for a lot of miles. No weird wear patterns either on the teeth.
So maybe they're all like that?
Just checked them for mesh by running them round a cam gear on the bench, and they seem smooth enough. So with any luck, not a problem.
Cheers, Bill
Bill

Online KiwiGF

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Re: BTH Magneto
« Reply #16 on: 08.10. 2012 02:47 »
Hi Orabanda (plus anyone else with a BTH mag).

I think I've found the explaination to the problem I observed, mainly because I found an "Ignition Timing in mm Before TDC Calculator" on the net. The link is below if anyone is interested.
 
www.lambretta-images.com/archive/porttiming.php
 
Eyeballing the mag pinion there is about 2mm movement at the pinion perimeter when the BTH mag is locked, I calcualte this equates to about 3 deg, which in turn equates to 6 deg at the crank pinion.

Using a dial guage, and after locking the mag and setting the timimg to 8.73mm (11/32") I observed the variation in piston height between magneto stops was 6.15mm to 9mm (2.85mm variation). In a perfect world the crank would of course be locked by the mag and not move at all.
 
According to that calculator using  rod length 165mm, stroke 84 mm
 
34 deg btdc is 8.86mm
28 deg btdc is 6.10mm
               ======
               2.76mm

So this more or less proves that the (observed) play in my new BTH mag locking/timing mechanism can result in the timing being up 3mm out.

The 3mm that really surprised me, that such a big piston height difference results from just 6 degrees crank rotation.

Anyway, I guess this is just something to be aware of if your (new style) BTH mag has any play in the stops like mine has. I'd still buy the BTH mag knowing this.

I'll just experiment with the timing once the bike is going, I'll just use the mags slotted mounting holes to make "fine" adjustments to the timing should the engine pink etc. 
New Zealand

1956 A10 Golden Flash (1st finished project)

1949 B31 rigid “400cc hot rod” (2nd finished project, + favourite bike)

GL1800 Goldwing, well, the wife likes it

KTM 950 ADV, cos it’s 100% nuts

1952 Armstrong Siddeley Whitley for rainy days (with wife)

Online orabanda

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Re: BTH Magneto
« Reply #17 on: 08.10. 2012 09:41 »
KiwiGF,
Interesting feedback; Mine does not show this problem.

I assume you will make timing adjustments with a timing (strobe) light?

32 BTDC was best for me!

Online KiwiGF

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Re: BTH Magneto
« Reply #18 on: 08.10. 2012 11:14 »
Hi orabanda, I probably won't do the strobe thing, I might but I'll probably just set the timing so it's 11/32 in the middle of range allowed by the mag stops.

It does not make me feel great that my BTH mag is not quite right compared to yours  *rant*. I'll mention it to BTH.

Groily /Bill I persisted with The alloy gear after your findings (thanks!) rather than send it back, and with some wet and dry wrapped a lolly stick and a couple of hours "fettling" it now fits ok without binding. Levering the mag away from the camshaft gear and then doing up the mounting bolts helped a lot towards getting it to fit.

So onto the primary drive next assuming no issues with the dynamo drive....



Ps sorry about the lamberetta link just avert your eyes from anything else in that site  *razz*
New Zealand

1956 A10 Golden Flash (1st finished project)

1949 B31 rigid “400cc hot rod” (2nd finished project, + favourite bike)

GL1800 Goldwing, well, the wife likes it

KTM 950 ADV, cos it’s 100% nuts

1952 Armstrong Siddeley Whitley for rainy days (with wife)