785 project - restoration work

[xlint89]

I've never bench tested an engine. Leak down test is a must though.

[casey67]

Yeah at idle, no big deal.
Under a load would be worse. Rev and decel would be getting much worse.

Some guys just freak out if it's not to the letter of the service manual.

[pascoea]

You should know that on ALL 2-strokes will run lean when the exhaust is not in place.

I think that was the meat of their concern, that it would be really lean without the exhaust. I understand you wouldn't want to run it long, but I just can't see it getting hot enough in 20 seconds at idle to hole a piston or something. Barely got warm to the touch.

[Myself]

15-20 seconds dry is no issue at all. You should know that on ALL 2-strokes will run lean when the exhaust is not in place. Good job on the coil!

[pascoea]

Quick update, and a question...

I re-wound the pulser coil. I wanted to give it a whirl, basically to just to see if I could do it. It was certainly an interesting adventure, lol, but it appears to have worked. Good solid spark, and it fired right up on the bench. Guess we'll see how the longevity goes.

Which leads me to my question: Do you guys bench test your rebuilds before you install them? And, if so, how much of it do you put together? I don't consider myself anything more than a shade-tree mechanic, but I've been working on various forms of small engines for since I was in High School, and I can't see any harm in running it on the bench. But I posted a video of it running to FB and was getting some crap for running it "dry" without the exhaust on it. I'm curious what possible damage could be incurred by running it for 15-20 seconds in that manner? Running it on 50:1 pre-mix with no exhaust, power valves installed but not connected to the actuator.

I'm not intending to disparage the dude's advice, just always wanting to learn more. I appreciate it, as always.

[pascoea]

Thanks for the input gents. The more I was thinking about it, not having everything hooked up is probably not the best way to test. I'm going to get everything hooked up tonight and re-test.

I didn't test voltages on the other coils, as the resistance values were all within spec, but I'll give that a go as well. The prior owner didn't have any info on what electrical gremlins they were chasing down when they shelved it, so probably best to just check them all.

[K447]

That sounds low but I'm not 100% certain what it should be. How do the cranking voltages compare to the other windings in my thread? If the rest are all similar then I would assume the stator windings are too weak to create enough voltage.

If multiple stator coil voltages measure weak (and other factors such as slow engine cranking have been ruled out), sometimes the ring magnet inside the flywheel can become weakened. If you have another flywheel available you can compare the magnet strengths using a screwdriver tip, or just install the other flywheel and see if the results improve.

Stator windings can fail if the insulation between windings fails and effectively shorts those windings together. The insulation can fail if the winding wires rub against each other or against the armature structure with engine vibration. I would expect when this type of failure occurs it would affect one stator winding more than the others, as whichever winding fails first will prevent the engine from running, therefore no further vibration to further degrade the other windings.

[Myself]

That sounds low but I'm not 100% certain what it should be. How do the cranking voltages compare to the other windings in my thread? If the rest are all similar then I would assume the stator windings are too weak to create enough voltage.

[pascoea]

I don't ever condemn a stator on ohms alone UNLESS it's dead shorted to ground. Does it still produce a good AC voltage number while cranking? Around 20 VAC?
Maybe read through this thread..Troubleshooting Fuji 650 750 780 with no/intermittent spark - Greenhulk Personal Watercraft Performance Forums
I've been ALL OVER the Fuji electrical systems!

Hey Myself, read through your thread (the starter, really? Lol. Don't mean to laugh at your misery, but that sounded like a bear to chase down.) and tested the voltage on the pulser. It hits just north of 6vac while cranking. So the dumb question... how much of the CDI to I need to have hooked up to consider it a valid test? I connected the pulser, and a couple other random wires between the CDI and Stator, basically everything I could without involving the junction board. I have to assume that with the lower resistance, some of the windings are shorted together, and that's not letting the voltage get high enough. Maybe tomorrow I can get the rest of the jbox hooked up and give it a try again, I wouldn't think that would make a difference, but who knows.

If I wanted to get that coil re-wound do you have any recommendations on who I could send it to? I emailed a couple local motor shops, but they pretty much told me to kick rocks.

[xlint89]

Can I get some input on the stator? All of the readings are within the +/- 10% except the Pulser Coil. Supposed to be 90 (assuming the Pro stators are the same as the others) but it's testing at 22 ohms. Would that be enough difference for you guys to condemn the stator to a life of landfill duty?

I looked thru ebay for a replacement, but unsurprisingly came up empty. Any thoughts on where to source one? And I thought I had heard that the 750 or 780 stator/flywheel would be an acceptable alternative, is that accurate?

I agree with the previous replies. Keep it. Ohms can change due to ambient temp.

But to answer your Q, you can use a 1994-95 750 and any 780 magneto and flywheel as a replacement. (you cannot use a 785 flywheel with a 750 or 780 magneto. You need to use the matching magneto and flywheel)

[pascoea]

I don't ever condemn a stator on ohms alone UNLESS it's dead shorted to ground. Does it still produce a good AC voltage number while cranking? Around 20 VAC?
Maybe read through this thread..Troubleshooting Fuji 650 750 780 with no/intermittent spark - Greenhulk Personal Watercraft Performance Forums
I've been ALL OVER the Fuji electrical systems!

Thanks for the link. I'll take a look through that. (I tried going thru a similar link from the "Polaris knowledge" site, but they are all still busted since the GH update.)

I'll have to check the cranking voltage once I get this back together, the engine is still sitting naked on my bench. I assume I want the CDI hooked up when I do that test?

[pascoea]

Don't ever condemn a stator. They can be rewound and a ton of things if they aren't right. Though it's hard to find someone who will do it and it's usually easer to get a new one.

Hmm. That's an interesting idea. I poked around a bit, this is one of the wrapped coils, so it wouldn't be SUPER hard to re-do.

[sdlvx]

Don't ever condemn a stator. They can be rewound and a ton of things if they aren't right. Though it's hard to find someone who will do it and it's usually easer to get a new one.

[Myself]

I don't ever condemn a stator on ohms alone UNLESS it's dead shorted to ground. Does it still produce a good AC voltage number while cranking? Around 20 VAC?
Maybe read through this thread..Troubleshooting Fuji 650 750 780 with no/intermittent spark - Greenhulk Personal Watercraft Performance Forums
I've been ALL OVER the Fuji electrical systems!

[pascoea]

Can I get some input on the stator? All of the readings are within the +/- 10% except the Pulser Coil. Supposed to be 90 (assuming the Pro stators are the same as the others) but it's testing at 22 ohms. Would that be enough difference for you guys to condemn the stator to a life of landfill duty?

I looked thru ebay for a replacement, but unsurprisingly came up empty. Any thoughts on where to source one? And I thought I had heard that the 750 or 780 stator/flywheel would be an acceptable alternative, is that accurate?