2-cycle Timing Advance (Small Book but Interesting)

[Tao of Funk]

1976 70hp Evinrude

My 70hp fouls the plug when I run it at low rpm's for too long. I have checked wiring, plug type, ignition coils, and compression (I have fixed orifice plugs for the high and low speed jets on these carburetors and have not changed the opening diameter on the low speed orifice plugs...yet). The following discussion may not be the root cause of my problem but it is something I'd like to rule out.

My understanding is that the relationship between timing advancement and throttle advancement is critical for two cycle outboards.

Full timing advance is set to 17 degrees per engine placard. I read (somewhere) that the engine should achieve full timing advance at 4500rpm during throttle advancement when operating in the water. This is a tricky setting to check because it requires 2 people and the obvious engine-operating-while-cruising-on-the-water element. I have set the throttle cam per instructions (cam engages throttle roller at the proper mark) but this setting seems to be in vein since it does not necessarily ensure coordination between engine rpm and timing advancement.

Whenever I adjust one aspect of the cam engaging the throttle roller (forward or retarded ignition) I change another aspect of the timing/throttle advancement relationship. Is there another way to set this relationship up or is it paramount I check the timing advancement/rpm setting in the water while cruising?

Thanks!

GB

P.S. I don't have a tachometer so that's why I can't/haven't performed the on the water check.

 

[Tao of Funk]

Re: 2-cycle Timing Advance (Small Book but Interesting)

Bump!

GB

 

[jonesg]

Re: 2-cycle Timing Advance (Small Book but Interesting)

Look in the FAQ files at the top of this forum for Joe Reeves method.

 

[Tao of Funk]

Re: 2-cycle Timing Advance (Small Book but Interesting)

Thanks jonesg. I believe WOT timing is fine, I'm more interested in the relationship between throttle opening and timing advance. Timing advance takes place concurrently with throttle opening. The throttle cam position can be changed in relationship to the timing advance position via the threaded connecting rod. I know the full advance point but I am curious how one sets up the point where the butterflies start opening (in relation to timing).

GB

 

[jonesg]

Re: 2-cycle Timing Advance (Small Book but Interesting)

but I am curious how one sets up the point where the butterflies start opening (in relation to timing).

GB

By the book, its very handy to get the factory service manual, not clymer or seloc books. Then all you have to do is follow simple directions.
www.outboardbooks.com
Best money you can spend.

 

[tashasdaddy]

Re: 2-cycle Timing Advance (Small Book but Interesting)

the above mentioned OEM manual, has the procedure you want. it is called synchronization, do it after Joes Reeves WOT timing procedure. it gets all the linkages set in the proper place for the carbs and timing to work properly together.

 

[Tao of Funk]

Re: 2-cycle Timing Advance (Small Book but Interesting)

I used the book to set my '69 55hp. I figured this '76 70hp was very similar to that engine except it has a procedure that requires setting full timing advance as a function of engine rpm. Since I have another book for a very similar model of engine I figured someone could enlighten me on this particular procedure thereby not necessitating the purchase of another book. "Buy the book" could be the answer to nearly all the questions in this forum.

GB

 

[clanton]

Re: 2-cycle Timing Advance (Small Book but Interesting)

There is a reason for checking the timing at the higher rpms. the timing will move 4-6 degrees because of voltage increase from the ignition system. You can set the timing anyway you wish, but the final check should be done at the rpm set in the manual. Others engines require this also.

 

[Tao of Funk]

Re: 2-cycle Timing Advance (Small Book but Interesting)

I found the answers to my questions.

First off full spark advance need not be achieved at a particular rpm. There is a necessary timing sequence but it is set according to the throttle pickup point.

Second, it is necessary to set the pickup point (point where the throttle cam engages the cam follower) in relation to timing. The cam follower is the little roller that gets pushed and opens the carburetor butterflies. In my case the pickup point occurs at cylinder #1 TDC. The procedure necessary to set this point depends upon the cam yolk position on the throttle cam yolk rod and positioning the cam follower to the pickup point tick mark indicated on the throttle cam.

SOOOoooo, if you have a 1976 70hp Evinrude and you want to set your throttle cam pickup point (which in turn synchronizes the throttle cam movement with the spark advance timing sequence) you will need to set the pickup point to correspond with the #1 cylinder spark occurring at TDC. Also note that the pickup point timing is model specific.

If you want to know your pick up point timing setting and you have established a relationship with your local outboard parts supplier/service center they will most likely give you that information.

GB

 

[Tao of Funk]

Re: 2-cycle Timing Advance (Small Book but Interesting)

Also I installed Boyesen reeds on this motor. They are two stage fiberglass reeds. Because of the increased air flow I had to install a new low speed orifice plug. The stock plug was a 30 and I went up to a 35.

The results are awesome! It idles around the dock better than ever and transitions from idle to full throttle without hesitation. It's pretty cool to have a $1,200 boat that works so well (Not to mention a 32 year old motor). I hope it continues to run like this for a while now that I have it dialed in.

GB

 

[mobiletec]

Re: 2-cycle Timing Advance (Small Book but Interesting)

I have the same motor. What is your timing pick-up point? Thanks

 

[tashasdaddy]

Re: 2-cycle Timing Advance (Small Book but Interesting)

that is all part of the LINC N SINC!!!!!! get the OEM manual, for your specific motor.