Triumphrat

Trumpet1

I have a old '63 Iron head sporster that I am restoring. I am being told by the "experts" that because of the high dome piston design detonation will be a problem because of the low lead gas unless I modify the heads to accept two plugs plus buy a special spark coil that will handle the additional plugs. I also am advised to change out the valve seats to low lead type.

I am also restroring a 61 pre unit Bonnie. The pistons on the Bonnie look just like the old sporster and compression is 9:1 for both engines. It seems to me that if detonation is a problem on the sporster it will be problem on the Bonnie as well. There are plenty of threads and discussion on this forum dealing with the need to install low lead seats, but I have seen nothing about the need for dual plugs/cylinder so I figure it isn't a problem but not sure why. Of course the sporster is a V twin rather than a vertical twin, so is that the difference?

GrandPaulZ

"experts"?

Baloney.

By any chance, are these experts offering the dual plug mods through thier shop(s)?

__________________
GrandPaulZ
Author of "Old Bikes"
Born Again Bikes
My Photos

NHOJ from Maine

i would avoid the dual plugs..
a pain. improvement is questionable.
9:1 dosen't seem that high to me.
you could always reduce the compression with a cylinder base plate.
but i think it would be just fine stock.
besides, just use premeum gas. that's what the added octane is for.
to keep pre-detonation at bay.

panda

Dont get too confused. the BMW forums talk about this double plugging a lot. Some models have pinking problems. I think partly due to piston/head design and partly because those engines are very short stroked and have hot spots.
The Triumph will run with the higher compression partly because the alloy head allows it to run cooler. BUT, simply by retardingthe ignition a little will compensate for poorer gas. The reason or thinking behind twin plugging is by running retarded there is less time for teh mixture to burn. starting a flame at two points speeds up combustion. At best the improvement is marginal.

-IF- you really have problems do as suggested and slip a 2mm shim under the cylinder barrels.
Avoid running weak or rich. You can sometimes alter the timing curves by using stronger springs in teh ignition auto advance mechanism. That way the advancing is held back harder and longer.

__________________
A wise man learns from his mistakes..
A genius learns from other peoples mistakes.

Trumpet1

Thanks all for your input. The dual plug mod was being highly recommended as important for the old Sportster I have. The Harley is a V twin, not a vertical twin, so perhaps that makes a difference. I'll just try the engine as is and use premium fuel or octane booster I see advertised.