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Negative or Postive ?

Submitted by wayne_rudolph on Monday, 6 October, 2025 - 13:43
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Electrical

Fitting a Tri-Spark MRK-0001 to my Dommi 88 . I have no wiring on the new build at present and wondered what is the best connection to use Negative or Positive ? Is there a preference ? Also the installation instructions state the fully advanced position should be 38 Degrees BTDC where as all my books say 30 Degrees BTDC . I can only assume the 38 Degrees is aimed at a different engine ? I will be fitting a new 12v Stator /rotor just for your info. 

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Submitted by kit_lewis on Monday, 6 October, 2025 - 15:41

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I had the same dilemma when sorting my bike. So far as I can tell, and I'm very new to this game, the only real benefit for going negative earth is to use something like the Motogadget wiring system and build your own loom. Which is what I did because I have always wanted one and the opportunity presented itself. I'm very happy with the result and will be rebuilding my prototype harness over the winter as it needs some changes  

 

It seems that nearly all modern bikes and cars are NEG earth so that seems the way to go as I`m going to have a very basic home made loom on mine. I`ve been so impressed with Alan Osborns site of great information sheets but need some assurance from him that I`m doing the right thing ! Hope you get yours sorted .

Submitted by alan_osborn on Monday, 6 October, 2025 - 16:29

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......is on my web site, under info sheets. A O Services.co.uk  If/when you sell, the new owner will expect Pos. If you are used to Neg then one Pos is likely to cause petty aggravation. Pos earth will show a little more corrosion especially around the Pos battery terminal, not a big issue. If you wire with the standard British wiring Lucas/Rists sytem then only the earth wire will change colour.
Negative earth will give you a greater choice of Indicators and LEDs and any other widgets you might want to fit.
The Fully advanced ignition point is dependant on the engine so follow engine advise. 
PS A bike this simple can be wired in a day or two-you don't need a whole winter!
Fit ALL the electrical bits BEFORE fitting any wiring.

I`ve been reading and printing off some of your very interesting info sheets Alan. I`ve also `saved `your web site . It would seem (having checked both my other modern bikes ) that Negative earth looks to be the most popular and like you say give greater choice of add on`s . The Tri-Spark does give a choice of POS or NEG and provides a wiring diagram for both . I have taken the advice from a note inside the Tri-Spark instructions to fit a Mosfet 20amp Voltage Rectifier/Regulator . I am still awaiting delivery from RGM and unsure if that will be polarity important ? I hope that choosing to go NEG earth is the right thing to do ? 
As for the ignition I`m going with the 30 Degrees BTDC as the motor ( Dommi 88 500cc) is standard and that`s what the Haynes and my NOC book says . Your thoughts please . 

Submitted by alan_osborn on Tuesday, 7 October, 2025 - 09:41

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The modern reg/rect is 'always' polarity important to itself, but NOT to Earth. The Pos-red and Neg-Black wire's on all reg/rects I have seen is crucial to the reg/rect, and the battery, but there is no connection to Earth inside the unit hence the case is just bolted down to whatever. 
Negative Earth will need a BLACK earth wire and it might be good idea to have a written note inside the battery box, as some 'wiseguy' will reverse the battery as 'all Norton's are Positive earth' and we are all ORIGINAL aren't we?

Submitted by ian_soady on Tuesday, 7 October, 2025 - 10:06

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... not the late 1940s and early 1950s which were negative earth. I generally like negative earth, as has been said it's much more common on modern machinery.

Submitted by michael_ball on Tuesday, 7 October, 2025 - 12:56

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Anyone working on the electrics of an old Brit bike must know that some have had polarity changed and some haven't.

Therefore, we check!

 



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