Timing Question

[Justinde]

Hi Brains Trust,
The below was posted elsewhere, and claims Frank Acampora’s incorrect with his tutorial video on timing and carb sync. I used the video and his sticky, and it works a treat, but someone thinks it’s wrong. Any advice would be appreciated.
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Sure, if you think. Riddle me this , how does the plug fire ahead of the trigger signal in degrees from TDC, answer it doesn't. He's wrong, you need quotes from the book on how to set it? It's the lead time in degrees the piston travels during the trigger event through the ign system to the plug end. At high rpm the piston position is further along towards TDC than at static or cranking speed. The distance and time in milliseconds the signal takes from trigger to plug remains constant. The piston speed is not.
His 28° static will achieve 26° from TDC at a much lower wide open throttle RPM. He's 6° retarded from where it should be at high rpm
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[jerryjerry05]

FrankA got his info from the factory manual and service bulletins.
Been doing it this way since 1986.
Someone will disagree about everything.

 

[The Force power]

Hi Brains Trust,
The below was posted elsewhere, and claims Frank Acampora’s incorrect with his tutorial video on timing and carb sync. I used the video and his sticky, and it works a treat, but someone thinks it’s wrong. Any advice would be appreciated.
———————————————————-


Sure, if you think. Riddle me this , how does the plug fire ahead of the trigger signal in degrees from TDC, answer it doesn't. He's wrong, you need quotes from the book on how to set it? It's the lead time in degrees the piston travels during the trigger event through the ign system to the plug end. At high rpm the piston position is further along towards TDC than at static or cranking speed. The distance and time in milliseconds the signal takes from trigger to plug remains constant. The piston speed is not.
His 28° static will achieve 26° from TDC at a much lower wide open throttle RPM. He's 6° retarded from where it should be at high rpm
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"The distance and time in milliseconds the signal takes from trigger to plug remains constant. The piston speed is not."

I THINK.......the timing will be affected because when at 5000 RPMs the Flywheel is also spinning faster like the pistons.

 

[Redbarron%%]

Actually the rpm does make difference as as the speed increases the voltage build up is higher and faster getting to the trigger point of the SCRs in the CDI.
This build up is counteracted by the bias circuit that rectifies the trigger signal and inverts it to keep the trigger voltage from advancing too much.
For example if you replace the CDI box with CDM modules the bias circuit (which in the CDI is built in the CDI switchbox) the bias is not present and the timing will advance with the RPM (in addition to the mechanical advance). This can cause detonation and a burned piston or two.
In the case of the CDM modules the bias circuit is built into the new style trigger assembly.
I built a separate bias circuit when I modified my pre-1996 Sport Jet with the CDM system.
The trigger coil for the CDM is single ended and not push-pull like the CDI and the magnets in the inner timing ring is different as well.
So when the bias circuit is working the timing should be close to the static and the movement of the timing when running at high RPM should track - as long as the bias circuit is correct.
I think that the advance may be slightly less advanced at high RPM to make sure that it does not go too high, but currently my short term memory loss makes me less sure about that.

 

[The Force power]

Actually the rpm does make difference as as the speed increases the voltage build up is higher and faster getting to the trigger point of the SCRs in the CDI.
This build up is counteracted by the bias circuit that rectifies the trigger signal and inverts it to keep the trigger voltage from advancing too much.

@Redbarron%%
Quick question, so it does advance? by a degree or two?

 

[Redbarron%%]

It could conceivable go either way a little bit, but if I remember correctly id you set the timing at cranking speed at 32 it backs up to 30 running WOT, but I may remember that backwards so check to be sure.
That is for the 90/95/120 hp Sport Jet engines and is close for the other Force motors.
Check the specs for your motor.
What causes the advance is the fact that a higher speed breaking the magnetic lines of force of the magnetic field generates a higher voltage and since the SCRs fire at about 0.6 VDC that point occurs faster at higher rpms. What happens in the bias circuit is the voltage generated by the trigger coils is rectified and applied to the trigger circuit as negative bias to stabilize the timing.

 

[The Force power]

It could conceivable go either way a little bit, but if I remember correctly id you set the timing at cranking speed at 32 it backs up to 30 running

I think you got this backwards

 

[Redbarron%%]

It is important to verify the actual running timing (tied to the dock or with a safety pilot etc. as the components of the bias circuit may have changed over the life of the boxes and they might be bad or ?
Anyway here is an excerpt from the Sport Jet manual for the 90, 95 and 120 hp Force powered units.

From the Sport Jet manual:
IMPORTANT: The advance characteristics of this ignition system are such that the ignition timing will retard itself slightly at wide-open-throttle. Thus, setting maximum timing at 32 BTDC at cranking speed should result in retardation to 30 BTDC at 5300 RPM. Therefore, it is recommended that to be assured of maximum performance all timing adjustments made at cranking speed should be verified with the engine running.

 

[jerryjerry05]

An outboard tied to a dock and then trying to get wide open throttle??
Doesn't work unless you have the test wheel replacing the prop.
That's why the factory suggests static timing.

 

[Redbarron%%]

You are correct in that it will not reach the highest RPM, but you can see that the timing is tracking at higher than cranking speed. With the Sport Jet the engine will normally rev just about like it would moving under power. In that case you are just pumping water through the jet.
Best would be to verify the timing at WOT moving, but that may not be safe. Definitely not without a check pilot and smooth water.
Mostly to see that the bias circuit is working and if you can reach WOT then that the timing is correct at high RPM.
Things are a little different with the Sport Jet I know.
Thanks for the input!