Persistent Double-Fire on #5 - any ideas?

[gmacrae]

UPDATE EDIT: This turned out to be just a hyperactive timing light giving false readings, but because the motor wasn't running right I kinda discounted the idea to start with...

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So this is a pretty long story but I'll try keep it short...

Motor is an old Laser 220 with mainly new electrical parts on it, internals in good order.

The ECU died a few months back, I ended up getting a Brucato ACU (2nd hand, Tony remapped for me). Plugged it in, ran awesome!...for about 30 minutes. One switchbox died (both switchboxes were 3 years old, clear potted, possibly chinese junk).

Replaced both switchboxes (and key switch in case it was the cause) with 2 sierra/prufrex units and 2 cheap ebay knock-offs, timing light showing double fire on #5 - firing 3+5 at times, then 2+4 (going by my TDC numbers written on the flywheel).

Swap switchboxes around, both brands, swap coils/leads around, double-fire stays on #5

Trigger resistances and DVA voltages all in spec and even according to CDI's manual (triggers are CDI, about 8 years old).

Should I stop checking things and just drive the damn boat? I'm wondering if my inductive timing light is bullsh**ting me somehow... The thing is, I've read in here some switchboxes double-fire with ACU's, and this is the bank that's also wired back to the computer.... just reeeaally dont want to cook #5!

Thanks for any advice guys

Gene

[powerabout]

check the flywheel for cracked magnets?
make sure you hold the timing light cable clear of any other wires when checking
make sure there is no voltage on the kill circuit coming from the key switch

[gmacrae]

check the flywheel for cracked magnets?
make sure you hold the timing light cable clear of any other wires when checking
make sure there is no voltage on the kill circuit coming from the key switch

Haven't checked the flywheel... the trigger magnets are sealed in the centre of the flywheel though right? Yeah I've been making sure I hold the timing light pickup lead away from it all. There was .16v on that kill wire before I replaced the key switch, now its about .05v

[ChrisCarsonMarine]

First, let’s revisit the ACU/double fire issue regarding switch boxes. I had an issue with the Brucato ACU and Mallory switch boxes several years back. I had two cylinders that were running very rich after a full rebuild and the installation of an ACU. During testing I found the motor ran fine with the original mercury Box, but ran very rich on the center cylinders with The ACU . After sending the ACU back to Tony and having it checked out OK, I started replacing electrical parts on the motor trying to find the cause. Make note that at this point indexing the flywheel and checking for the double fire found nothing, and only after changing the Mallory switch boxes did the problem resolve . I finally realized that the Mallory switch boxes or at least one of them was double firing but did it so quickly that it could not be noticed with a timing light, however, the ACU saw the double fire and fired the injectors twice on the center cylinders, flooding them. This took all day to figure out.
So, looking back over this, I’m not so sure you actually have a double fire situation taking place, or you would be flooding the involved cylinders with double firing of the injectors, and the motor certainly would not run well. Give a call if you would like to discuss this, Chris

[gmacrae]

Thanks Chris!

I'd like to think this is a non-issue, but the motor isn't running well enough to be honest. When I first plugged in the ACU (with milled heads and your reeds/stuffers) it idled so smooth and clean, no bucking etc, I couldn't believe it! (i guess the old ECU has been affecting it for quite a while)... Then the switchboxes died, now it hasn't run like that since. Compressions are all within 5psi. It starts fine and will idle ok but definitely bucks quite a bit...might try putting up a vid of it running a bit later

I might know of a dead motor with good stock merc 'boxes that I could borrow - do you think I should give that a shot? Or try to track down an oldschool plug-in timing light (not inductive type)? Take the boat for a good run and compare the plugs? (pretty wary of running it fast right now)

[gmacrae]

to close this thread off - I borrowed another timing light, and it didn't show any double fire... My light's hyperactive pickup seems to have been faking it... sorry to screw ya round guys, thanks... Motor still ain't running right tho, separate issue, new thread:

https://www.screamandfly.com/showthr...-very-lean-ACU