Max Advance timing question

[smplesoft]

1982 Mercury 70HP 3-Cylinder 2-Stroke SN# 6164457

This engine has a sticker on it that states:

MAX Advance timing is 23* BTDC @ 5500rpm. or 27* cranking speed of 300rpm.

However this engine cranks at 600 rpm. What would be the correct max advance ignition timing be cranking @ 600rpm?

 

[CharlieB]

Re: Max Advance timing question

Could you take a close-up picture of that sticker and post it?

I've seen Service Manual specify setting the cranking WOT Timing 2 degrees light of the running WOT Timing.

I'm really like to see that sticker to make SURE that is really a 7 and not a 1

 

[daveswaves]

Re: Max Advance timing question

Could you take a close-up picture of that sticker and post it?

I've seen Service Manual specify setting the cranking WOT Timing 2 degrees light of the running WOT Timing.

I'm really like to see that sticker to make SURE that is really a 7 and not a 1

I have seen the same sticker on a few 3 cyl mercs, its probably correct. 600 rpm seems high for cranking speed, I would doubt the tach frankly. To be safe the conventional wisdom on the older mercs is to drop the max timing 2 degrees to accomodate todays fuel.

 

[smplesoft]

Re: Max Advance timing question

Well here is a pic.

 

[daveswaves]

Re: Max Advance timing question

If it were mine I would set the timing at cranking speed at 25 degrees before top dead center. There is no mechanical advance that changes with RPM so the difference between 300 and 600 is not an issue. The timing is done with the engine NOT RUNNING. During operation the timing is retarded electronically and at WOT it would be at 21 degrees, safe for todays fuel.

 

[smplesoft]

Re: Max Advance timing question

What is the thinking behind the 2 degree timing change to compensate for todays ethenol enriched fuel?

 

[j_martin]

Re: Max Advance timing question

Alcohol doesn't make any difference in octane........oh wait. 87 octane oxygenated is really about 75 octane with 10% 130 octane alcohol in it. What happens when a small amount of water gets in it and it phase seperates?

I would time it very conservative if I were running that chit. If you can get non-oxygenated (no alcohol) fuel, usually 92 or 93 octane, then time it more aggressively.

At any rate, if I were pushing it a bit, I'd be doing a couple of WOT power offs and checking the plugs for proper burn.

hope it helps
John

 

[daveswaves]

Re: Max Advance timing question

What is the thinking behind the 2 degree timing change to compensate for todays ethenol enriched fuel?

Basically yes, also in the 70's Octane was calculated by a different formula and lead was the key octane enhancer or knock preventative.
Today it is almost impossible to get non alcohol fuel and the 2 degree drop in advance is a safety measure. I can post pics of inline pistons that melted down due to lean conditions and detonation. They are not pretty.

 

[smplesoft]

Re: Max Advance timing question

Thanks for the info.