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Discussion Starter #1
I'm not sure why this happened, but while flying across a lake yesterday throttle pinned at mach 2.5 I suddenly went from running at the ideal 7800 RPM to bogging down to about 5000 and sounded like I was running out of gas. Machine stayed running but sounded a little different. I managed to get it back to the cabin and checked the plugs.

Clutch side was a perfect chocolate brown. Mag side was another issue. Electrode on the plug had those tell tale little droplets of molten aluminum splattered around. I figure I smoked a piston, but what troubles me is why?

The machine is a '98 700RMK. We were running in temps around 10-20*F around 2000' elevation. Engine is pretty much stock with an SLP boost bottle and stock jets. I moved the needle clips one position up (from center) earlier this season to clean up the idle a bit which (I thought) should only affect the low end. Only mod I can think of that screwed me is that I took the plastic baffle out of the air box.

The machine never seized so I am hoping I didn't screw up the cylinder. It still runs OK but I can tell something isn't right. A buddy of mine with EGT's said he had been running kinda high temps all day.

Any ideas what might have been the weak link? :confused:
 

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Also check for the usual air leaks or plugged up carbs. But most likely it was the shelf removal.
 

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Blew my mag side a few weeks ago. Those dropplets mean trouble. Probably leaned it out and melted it down good. I'd say pull it and see if you can fix it with new pistons and a little overbore. Other wise it gets costly. Mine had a 754 big bore and now I am looking at arround a grand to replace everything. Hope you didnt do it that bad. As far as still run. On the twin 700 the mag side usually heats up first and gets hottest. You probably melted the piston down and the cylinder stoped firing and cooled it down while still spinning at 5000 rpms. If you blew through the cylinder I would take it all out ASAP as you probably have a lot of metal in your crank and you dont wanna mess that up too. Good luck
Carl
 

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Absolutely, you can't increase your airflow without boosting your main jet unless you were already running rich. Your sled's ACCS does not know if you made intake changes and will just continue delivering the same amount of fuel as ever. You were running lean for sure at WOT. Raising the needle was the right idea to fix up your midrange and some low end, but at WOT only the main jet matters, and with a modified airbox the stock main jet is too small.
 

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Discussion Starter #6
ACCS didn't show up on the RMK's till '99 so mine is the old style (pre-ACCS). So how many jet sizes should I adjust to compensate for the extra air flow? I'm running stock now.
 

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Discussion Starter #7
Well....I finally got a little time to tear the beast apart and found the problem. I did burn a piston on the mag side. The piston was burnt at the exhaust port side but didn't go below the rings. I figured it was the air box mod that was the problem, but further investigation proved differently.

I just happened to examine the Y pipe kinda close while contemplating what to do with the piston when I found a small 1" crack on the inside of the "Y" on the mag side. Everything is making sense now. The crack was letting in a little too much air right at the exhaust port which melted the piston.

Bummer part is, the exhaust port on the jug cracked as well :bash: so now I'm into it for a piston and a jug ($$$). I'm not sure if the sled is worth putting $1k into at this point. If I can find some used components I may put it back together, but otherwise I may just part it out.
 

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i don't think the y crack burned your pistion,like 4 cycle engines it was a result of a lean condition that took out your piston first then cracked the manifold.The crack is an after affect.
 

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How bad is the crack? J.B. weld may be worth a try if its not too bad if your not willing to spend the money on a new jug.
 

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Discussion Starter #10
Well, I got a deal on the parts (about $450 for everything - used jug, two new pistons, full gasket kit and used Y pipe) so I will be putting it back together this weekend. Hopefully it will go for another 4k miles!
 

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Re-jet that mag side next fatter, or you may be looking at the same exact thing. Better safe than sorry ----proflow
 
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