Demolitron
Seaman Apprentice
- Joined
- Sep 17, 2007
- Messages
- 32
Hello,
I have a 1974 Evinrude 115 HP Starflite Outboard, model 115493B. I have a question regarding the overall concept of outboard timing.
As a baseline I summarize my existing knowledge of timing. In the automotive world, timing is advanced based on engine RPM and load to allow the fuel/air enough time to burn and reach peak cylinder pressure at around 12-15 degrees ATDC. As the load increases fuel/air density increases is slows down flame front propagation and therefore it takes longer to burn and so timing is advanced to start burning sooner. And likewise, as the RPM goes up there is less angular time to burn so you start burning sooner by advancing the timing.
From what I can tell on my outboard it looks like this, please correct me if I'm wrong or have my setup tuned poorly.
At the idle throttle position, warm-up lever down, the spark advance arm is all the way against the low end stop, carburetor butterflies are closed, and measured timing is just around TDC.
As the throttle control is moved forward the cam follower makes contact with the CAM and the butterflies start to open just as the cam follower center is lined up with the marking on the cam. I have a large mark, then a "start" mark, and then another mark much lower down on the cam. At that point the spark advance arm is forward some and measured timing is at approx 5 degrees BTDC. Here I am assuming that the spark is being advanced but the throttle is just now barley starting to open. On my tune-up sheet it lists this as throttle pickup timing at 5 degrees.
Then as the throttle control is moved forward more the butterflies continue to open, and the spark advance arm moves forward until it hits the high spark advance stop. Now, at this point the throttle butterflies are about 1/3 of the way open. So I have 21 degrees of spark advance at 1/3 of the butterfly opening.
From this point the spark advance is pinned against the high advance setscrew and the throttle butterflies continue to open until they are 100% open.
My biggest question is this... Is this what is supposed to happen? Secondarily, can someone shed some light onto the how and why an Outboard's timing seems so much different than any other engine I've worked on?
Thank you all for your time and sharing your knowledge with me.
I have a 1974 Evinrude 115 HP Starflite Outboard, model 115493B. I have a question regarding the overall concept of outboard timing.
As a baseline I summarize my existing knowledge of timing. In the automotive world, timing is advanced based on engine RPM and load to allow the fuel/air enough time to burn and reach peak cylinder pressure at around 12-15 degrees ATDC. As the load increases fuel/air density increases is slows down flame front propagation and therefore it takes longer to burn and so timing is advanced to start burning sooner. And likewise, as the RPM goes up there is less angular time to burn so you start burning sooner by advancing the timing.
From what I can tell on my outboard it looks like this, please correct me if I'm wrong or have my setup tuned poorly.
At the idle throttle position, warm-up lever down, the spark advance arm is all the way against the low end stop, carburetor butterflies are closed, and measured timing is just around TDC.
As the throttle control is moved forward the cam follower makes contact with the CAM and the butterflies start to open just as the cam follower center is lined up with the marking on the cam. I have a large mark, then a "start" mark, and then another mark much lower down on the cam. At that point the spark advance arm is forward some and measured timing is at approx 5 degrees BTDC. Here I am assuming that the spark is being advanced but the throttle is just now barley starting to open. On my tune-up sheet it lists this as throttle pickup timing at 5 degrees.
Then as the throttle control is moved forward more the butterflies continue to open, and the spark advance arm moves forward until it hits the high spark advance stop. Now, at this point the throttle butterflies are about 1/3 of the way open. So I have 21 degrees of spark advance at 1/3 of the butterfly opening.
From this point the spark advance is pinned against the high advance setscrew and the throttle butterflies continue to open until they are 100% open.
My biggest question is this... Is this what is supposed to happen? Secondarily, can someone shed some light onto the how and why an Outboard's timing seems so much different than any other engine I've worked on?
Thank you all for your time and sharing your knowledge with me.
