Fred, HELP

Ron Brown /

Fred,

I realize it is not a moped, but I have been helping my neighbor to rebuild his 20hp, 2 cvl. Johnson outboard motor. When we got around to adjusting the timing, I noticed that this motor has the stator plate, carrying the points, connected by a linkage to the throttle. As you open the throttle, the plate rotates and advances the ignition. I am not sure of the actual timing at either end of the range as the book does not specify it. However, the range of motion is greater than 45 degrees.

Given the posts we have had here in the past about fixed vs variable timing, this seems like a huge variation from idle to full throttle timing. Any ideas?

Ron

not much help

I have not done a lot of work on outboards... but I have seen that large amount of advance rotation for the ignition... I don't know why they need so much.

But as far as checking it.... I would think you would use a strobe ignition timing light and look for marks on the flywheel and see if they line up while idling... there might be another mark for full advance also.

If you are trying to set it before it is ready to run... just set it near where it was (or in the middle?)... because with such a wide variation it should run almost anywhere you put it... then strobe check it and adjust it to suit.

I also have a neighbor who's home operated business is outboard repair.. and any questions can easily be answered by him if need be... he does cylinder boring and honing and boat motor warranty work for dealers also

This is the guy who has all the tools for bore and hone's on small bore ped engines.

Re: not much help

Ron Brown /

Fred,

Excelent answer, unfortunately, wrong question. :-)

I need to express myself better if I am ever to overcome my "issues".

We had the shop manual and timing bar for the engine so that was no problem.

My confusion arises out of why an outboard 2 cycle can make use of all that advance when a small motorcycle can not, or apparently does not try.

Ron

some guessing

Aaah well... different story then.

There is a lot of guessing here >

I believe the huge range of advance and amount of advance outboards run is partly because they have rather inefficient and large bore combustion chambers (compared to what we know of as high performance 2 stroke streetbikes.. (old ones.. of course))... and the requirements of an engine running through a single speed power transmission, pushing an object with a lot of drag is quite different from an engine pushing a lightweight low resistance vehicle with a 5 or 6 speed.

In other words... the huge advance curve is necessary for these "tractor (boat)engines" with their large low compression combustion chambers... and these engines are also expected to idle around all day long getting halfway decent gas mileage and not overheating or fouling plugs.

The high performance motorcycle 2 strokes run in a narrow range at much higher efficiency (hp to cc) ratio... and a large advance curve is no advantage whatsoever.. it would be a detriment.

Correct Ignition timing in any one situation is determined a lot by the largeness of the combustion chamber and the burn rate of the mixture, and the RPM you are running at... It is like a fuse... if you want the bang to occur at the most advantageous time... you have to light the fuse early enough.

The larger the combustion chamber... the earlier you have to light it for the flame front to travel across the chamber and generate its peak pressure right about TDC... and as rpm goes up you have to light it earlier to let the burning be done in time for a near TDC peak.... and yet in these boats you still need to idle around all day with no trouble... That... is why I think they need the large range of timing advance... Boat 2 stroke pistons are way bigger bore than bike 2 stroke pistons (in general)

Re: some guessing

Ron Brown /

Fred

Thanks, that seems reasonable. I did notice also that this engine still uses a deflector type piston, presumably another "idle all day long" compromise.

Ron

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