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Ive got '98 Mach Z 800 triple. Its not exactly stock. I put 3 brand new spark plugs in & start it. Seems to run fine for a few minutes then its like a cylinder drops off. I can take plug wire off 1 cylinder & it will almost die.put it back on and do same with middle cylinder, almost dies out. I pull wire off 3rd one and no change. Rev motor to 2100 rpms and slowly bring that plug wire to top of plug and u can hear cylinder engage & tach shoots up to 3300 rpms. But let motor idle and back to 2 cylinders. When idling poorly you would swear someone is sitting on sled turning headlight from dim to bright repeatedly. Also, dont know if this would be sole cause, but there is wetness around bottom gasket of this cylinder, but nowhere else. Any insight from u fellow Ski Doo guys would be so much appreciated!
 

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No change when u pull 3rd wire off..... May not be getting any fuel thats why its only firing on 2. when you pull one of the first 2 wires off it actually is running on 1 cylinder. I would pull carbs off and clean them

Make sure pilot jets are not gummed up. it will starve for fuel at idle, flip choke on to see if it comes alive if so clean pilots

Also a week plug will be a pain in the ass, I run BR 10 ES in my not so stock Mach Z
 

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I've actually had the exact symptoms as you but it was flooding out. If you do alot of reading on this forum you will see alot of guys run 45 pilots as opposed to he stock 50's. I did that and high output coils with b9es rather than br9es plugs as per the race manual and its never happened again
 

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Assuming your engine has decent compression and is in good shape, set your idle to ~2000 rpm, consider a carb cleaning including new needle & seats, check float height, switch to 45 pilot jets. Another must is to unscrew your plug wires from the coils and plug caps, cut about 1/4" from each end and screw them back on. The wires can be quite difficult to twist off at first but they will turn easily once they break free. Also, consider switching to BR9EYA plugs, they are more resistant to fouling.

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So which side is this 3rd cylinder on? Left side or clutch side is pto. Right side or muffler side is mag. Potentially different issues with each.

I would stick with a BR plug. The R is for resistance and it protects your ignition system from damage as I understand it.

I might add a compression check to the above advice and look inside that problem cylinder to see if it appears to be washed clean compared to the other 2.
 

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i have a 1999 and a 1998 mach z running 50 pilots and they do not foul plugs
 

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i have a 1999 and a 1998 mach z running 50 pilots and they do not foul plugs
Some do some don't with 50's I've got one that does and one that doesn't. Both engines in good shape. Both work great with 45 pilots. My assumption is that Rotax engineered these triples very close to the rich side of things.

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