1977 850 lemans

[Jonathan]

Hi
Does anybody know which NGK or other spark plug cap will work with a dual plug set up for the small secondary plug? It's a tight fit between where the cap comes out of the head and the underside of the gas tank. Thanks

 

[Chris R]

Its a long time since I did my twin plug and unfortunately the bike is not handy to check.

I assume the inner spark plug is 10mm - nothing else will fit and the 10mm only with a modified allen bolt. It is a tight fit. I also had to skim the OD of the plug socket to get it in.

I used a NGK SD plug cap. I can't remember if I used one with a straight lead or angled end. I seem to remember it being straight.

If you haven't already recurve the distributor - no doubt there are boxes out there that do it electronically, i did it manually but can't remember the combination of weights and springs I ended up with. I was able to knock it back 6 degrees at full advance. It really helps with detonation.

 

[Jonathan]

There doesn’t appear to be anywhere that you can go purchase ATU spring/weight combinations that actually accomplish this correct? How do you achieve the 6 degree change at full advance?

 

[Chris R]

In answer to your question I don't.
To be frank I was lucky - I had a friend who had an old Sun distributor machine and a box of springs.

To do it properly you put the engine on a dyno and map what advance gives you max power at select RPMs. This gives you a crudish curve that does not take into account load/vacuum. You then create the curve by playing with mechanical stops, weights and springs to give you the curve you want. So the quick and dirty route is to simply put a mechanical stop that limits maximum advance.

Obviously an electronic box that lets you change the advance in a table is way easier!

 

[Moto-Uno]

On my twin plugged Le Mans 2 I was able to use the angled distributor caps from
Lordco auto parts , they are an aftermaket performance kit that came with 8 . I found
that simply backing off my full advance by what came out to be a tooth on the ring
gear worked fine , excellent running and no detonation . This was after too much time
spent playing around on a dyno ( we had one at where I worked at the time ( Harley
Davidson of Ottawa). It ended back where I had originally put it , which was set by
retarding it until the detonation stopped , but I did love playing on that dyno. Peter