What's The Difference?

[Mumbles]

Close to 1/2″. That’s what!

While working on a ’74 Evinrude 9.9 electric start today, I was installing a mag plate with new points and condensers already on it from another ’74 9.9 it when I noticed the throttle cams looked a bit different from each other. Sure enough, the timing marks are close to being spaced 1/2″ differently on each cam. M. E. shows two different cams with two different P/N’s for two different models that year depending on whether the model number ends with a ‘G’ or an ‘S’.

The motor I’m working on was an electric long shaft converted to short and the other mag plate came from a ’74 9.9 electric which I converted to CD ignition a while back. I’m not sure why the throttle timing would be different on these two motors but I’m sure glad I caught this. It might of made for some puzzling operating characteristics!

EDIT: I just took a look at the cam on the motor and it is a 320819 cam for a 15 horse while it should be a 320434 cam for this ‘G’ suffix 9.9 motor. I believe the only difference is the shape of the ramp with the 15 horse cam opening the carb much quicker than the 9.9 cam.

• This topic was modified 4 years, 10 months ago by Mumbles.
• This topic was modified 4 years, 10 months ago by Mumbles.
• This topic was modified 4 years, 10 months ago by Mumbles.
• This topic was modified 4 years, 10 months ago by Mumbles.

[frankr]

They had a ton of problems making the 1974-76 9.9 & 15 run, and made numerous changes to correct them. You must have found one of the changs. I don’t even remember all of them. They didn’t work anyhow.

[frankr]

They also changed the carburetors. I imagine the new cam went with the carb.

[fleetwin]

Like Frank says, there are numerous “fixes” for these early engines, and some of the cams were matched to the replacement carburetors. The cam without the steep ramp looks like the original one, which I would try first…The cam with the steep ramp makes for some odd midrange running characteristics. As for the different cam pick up points, I wouldn’t worry about that either…Set it up according to the original mark and try it….Advance the throttle pick up a bit if the engine seems rich off idle, retard the original setting if the engine is lean/bogs off idle/acceleration.
I have oftentimes been confused by these engines, the early ones (with all the plug fouling issues) seem to run the nicest at low speeds, maybe it is just me…So, I am inclined to use the early stuff, and those NGK plugs (never tried them myself, the Champion UL81Cs seem to work fine) everyone raves about….Let me know how you make out….
PS Does this thing have the updated head? I wonder if the inner exhaust housing was modified, this fix involved drilling a few exhaust relief holes at the top to relieve back pressure.

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