Irratic timing on a 90Hp SportJet. HELP!

[WillyBWright]

Sea Rayder jet boat with an early 90Hp SportJet (trade-in). It runs awful! I put a timing light on it and it never flashes in the same place twice and most of the time it's off the scale. The linkages are tight and moving appropriately, so the trigger unit isn't moving wildly or anything like that. I recall that the Flywheel magnet in the center of the flywheel had some issues with loosening up which would cause irratic timing, but the bonding is still sound. I can't get the magnet to move at all.<br /><br />What else could cause this? The switchbox? The Sales Department never wants to spend any money on their trade-ins so I don't want to just start throwing parts at it. It has a Red Stator upgrade and I'm getting 230 DVA at the kill circuit out of the switchbox. Anybody else had this problem? I've never seen a switchbox cause wildly irratic timing, so I tend to think it's not the problem. Any ideas???

 

[Gus Mortimer]

Re: Irratic timing on a 90Hp SportJet. HELP!

I dont know anything about your boat but is it posable the magnet is broken or cracked causing two sets of poles on that one magnet?

 

[kenimpzoom]

Re: Irratic timing on a 90Hp SportJet. HELP!

Check all the wiring, maybe somehow wires are causeing a cross fire. A trigger is about 90 bucks. Switchbox is cheaper from seilormarine.<br /><br />Also, check on my homepage for info.<br /><br />Ken

 

[WillyBWright]

Re: Irratic timing on a 90Hp SportJet. HELP!

I called the factory and they said pretty much the same thing. I looked at the magnet and can't see any cracks, but it has a stainless steel sleeve around it and I can't see much of the magnet.<br /><br />I tested the switchbox, especially the Bias circuit. Readings were smack dab in the middle of the spec ranges, both while cranking and running. I was also told in the past to look for 15,000 ohms between the bias and ground ... right on the button. Funny thing is, the manual said if the readings check okay and the problem remains, replace the switchbox. That don't seem to make much sense. I'm betting on the multiple N/S poles from cracks in the magnet that I just can't see.<br /><br />I ordered a flywheel. I'll let you know. Thanks!

 

[2springers26]

Re: Irratic timing on a 90Hp SportJet. HELP!

I've worked n a few sport jets BUt mostly W/ a v6... oN a v6 we have the idle stabilizer.. wich will mess up the timming & make it irratic... I don't remember about the 90...

 

[WillyBWright]

Re: Irratic timing on a 90Hp SportJet. HELP!

I put the new flywheel and it solved half of the problem. Timing is still all over the place, but now it's at least on the scale. I ordered a new switchbox and that should take care of the rest. Apparently it had Both problems. These don't have idle stabilizers, but they do have rev limiters and this one was causing bad misfiring to boot. Got good and steady spark with it disconnected. I replaced that last week.<br /><br />This little SOB had a complete ignition system failure. The stator had already been replaced. The ignition switch had a choke circuit problem. The rev limiter was causing bad misfiring, even when cranking. The flywheel must have a crack or two in the trigger magnet. And even though the switchbox tested perfectly good, it is not. About the only thing working correctly was the lanyard switch. I never ran into that before and I hope I never do again!<br /><br />Mercury Igntiion Systems...as long as they make 'em, I'll have plenty of work. Call it Job Security!

 

[Typhoon90]

Re: Irratic timing on a 90Hp SportJet. HELP!

I know it's an old thread but...
I got exaclty the same problem as your irratic timing.
2 Magnets under my flywheel are broken.
Where did you get your flywheel?
My mercury dealer tells that is not available anymore
So im' looking to buy it on www.stemtostern.com but i can't see my part number.
I got 817494a18 and www.stemtostern.com got 817494a2
I asked em but i ain't got no reply yet.