Topic: 750 coils on a 350?

[ceruzziracing]

I've got a set of '74 CB750 coils laying around somewhere, was wondering if they'll work in my '72 CB350K. Part #'s are different and I'm not at home to look at them side by side. Thoughts?

[HondaMan]

The Four's coils have 2 plugs per coil, where the CB/CL/SL350 had single-plug coils. That is, except for some of the ones we modified into 360 degree cranks and cams, for hillclimbing... 

The CA77 (305 Dream) and CB/CL160/175 could use the Four's coils, though, as they were a 360 degree design.

The 350 twins were all 180 degree designs.

[ceruzziracing]

Chalk that one up to brain fart-must remember to think before I post. Forgot about the stupid two cyl vs 4 cyl thing. Thanks H-man for the non-flamming reply (even though I deserved it  )

[Terry in Australia]

Can you "twin plug" it? That's what I thought you were getting at? Cheers, Terry.

[okie]

Why does it matter whether it is a 180 or 360 degree crank?   The coil is going to discharge when the POINTS tell it to.

[Soos]

Isn't a dual output coil how some racers ran 2 spark plugs per cyl in the early days?(mainly 2 stroke racing I believe)


l8r

[Jay B]

Why does it matter whether it is a 180 or 360 degree crank?   The coil is going to discharge when the POINTS tell it to.

On our 4's the plugs that fire together are 360* off. One cyl is firing, the other is on the exhaust stroke, the extra spark isn't doing anything. On a 180* twin, that extra spark would hit on the bottom of the intake stroke, would mess up the charge.

[mlinder]

Isn't a dual output coil how some racers ran 2 spark plugs per cyl in the early days?(mainly 2 stroke racing I believe)


l8r

Quite a few companies did it. 2nd plug at a slightly different angle, with a slightly delayed spark on the 2nd. Was good for burning all the fuel in less than perfect combustion chambers.
The r90s comes to mind.

[Terry in Australia]

The BMW R90S didn't come with twin plug heads, but a lot were converted to twin plugs for racing. This was very effective due to the large bores and the offset of the spark plug in relation to the shape of the combustion chamber.

I had an R100RS for 12 years and had it twin plugged, using 2 Harley coils. There's a company in Britain called "Moto-Bins" that does twin plug conversions for all the old BMW boxer engines. Cheers, Terry.

[HondaMan]

Isn't a dual output coil how some racers ran 2 spark plugs per cyl in the early days?(mainly 2 stroke racing I believe)


l8r

Yep, sure is...that's one of the mods I used to do to the Suzy's we sold for Enduro racing. I put a CB750 or CB160/175 coil on them, and ran it from the magneto, because the Suzy's had the extra hole for a compression release (or you could use a short-reach plug). It made an adequate spark, but a mag-type coil was better (and 3 times the cost, because it was a "racing" part.  ).

Sometimes, we put a colder plug in the short-reach hole instead, so the riders could run FWO on the hiway for that leg of the Enduro without overheating the cylinder head. The rider would just swap the wire before hitting the road leg (single-wire coil setup, though).