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Old 01-10-08, 07:10 PM   #1
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Question for the gearheads here

My neighbour's snowmobile, an 800cc two-stroke with a carb, recently had a new head and forged pistons installed, and the cylinders ported. This thing had ridiculously high compression, to the point that it was virtually impossible to pull over.

Well after the break-in period, it stopped dead on a lake. A piece was gone out of the piston, out of the edge so it was no longer round. I'm thinking it was filled with the wrong fuel, i.e too low of an octane, and the thing detonated. What do you guys think?
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Old 01-10-08, 07:15 PM   #2
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Should probably mention he's pretty hard on it, usually wide open or stopped.
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Old 01-10-08, 07:50 PM   #3
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My gut says that the carb was never rejetted and as a result the motor was running lean on the main. Chances are good that the metal missing off the piston is in the area of the exhaust port.
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Old 01-10-08, 07:55 PM   #4
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My gut says that the carb was never rejetted and as a result the motor was running lean on the main. Chances are good that the metal missing off the piston is in the area of the exhaust port.
Yep. Exhaust side. I would think if the carb was not rejetted, it would have detonated sooner, especially with the ridiculous-high compression, no? Correct me if I'm wrong. Is it possible they just got the wrong octane gas somewhere?
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Old 01-10-08, 08:13 PM   #5
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Does it run premix or have an oil injector system?

If premix, another guess was not enough oil in the fuel mix and if injected, the oil injector may have fouled or partially plugged. Too lean an air fuel mix will do that AND not enough oil/fuel mix will also do that w/ a 2 cycle.
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Old 01-10-08, 08:13 PM   #6
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Yep. Exhaust side. I would think if the carb was not rejetted, it would have detonated sooner, especially with the ridiculous-high compression, no? Correct me if I'm wrong. Is it possible they just got the wrong octane gas somewhere?
I think you could be correct. Without knowing the octane of the fuel in question, and the compression ratio, it's hard to say for sure.
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Old 01-10-08, 08:15 PM   #7
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It should run premium. 91 octane at least. That's all I know.

Tom, oil injected.

What's odd, and what I just found out, is that he ran for three hours on the same tank before it exploded. Clog in a carb jet?
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Old 01-10-08, 08:17 PM   #8
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Very, very possible!

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It should run premium. 91 octane at least. That's all I know.

Tom, oil injected.

What's odd, and what I just found out, is that he ran for three hours on the same tank before it exploded. Clog in a carb jet?
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Old 01-10-08, 08:20 PM   #9
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From what I understand, a high compression engine has very little tolerance for running lean.

Also, it wouldn't run on one cylinder. Shouldn't they compress seperately?
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Old 01-10-08, 08:33 PM   #10
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From what I understand, a high compression engine has very little tolerance for running lean.
correct

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Also, it wouldn't run on one cylinder. Shouldn't they compress seperately?
.
I have zero experience with snowmobiles, but the outboards I've worked on will not run on 1 cylinder.
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Old 01-10-08, 08:37 PM   #11
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The compression can make a ring hang up on a port , too...
Is the ring chunked ? The edge ripped off the top of the port ?

Is it an ATK motor
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Old 01-11-08, 08:21 AM   #12
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I say the snow genies cast a spell on it.
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Old 01-11-08, 08:02 PM   #13
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The compression can make a ring hang up on a port , too...
Is the ring chunked ? The edge ripped off the top of the port ?

Is it an ATK motor
Nah, just the piston screwed and the cylinder wall scored, presumably thanks to debris.
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Old 01-11-08, 08:05 PM   #14
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I have zero experience with snowmobiles, but the outboards I've worked on will not run on 1 cylinder.
Why is that? Do the cylinders have to compress together to run, or are they totally isolated?
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Old 01-12-08, 04:23 AM   #15
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Probably because the flywheel isn't heavy enough to support a 360 degree ignition cycle.
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Old 01-12-08, 01:40 PM   #16
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Why is that? Do the cylinders have to compress together to run, or are they totally isolated?
They're connected, and the higher compression makes it hard to run on just one cylinder. As would the other changes they probably made to increase the performance.
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Old 01-13-08, 10:43 AM   #17
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Probably because the flywheel isn't heavy enough to support a 360 degree ignition cycle.
No flywheel, Stace. Snowmobiles don't have transmissions, just clutches and sprockets.
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Old 01-13-08, 10:47 AM   #18
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Trust me... They do have a flywheel. Usually it's disguised as part of the (magneto) ignition system.

They have transmissions too. CVT's, more commonly referred to as torque converters.
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Old 01-13-08, 11:02 AM   #19
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Where would I find the flywheel?

And yeah, I guess the torque converter is a transmission...
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Old 01-13-08, 11:57 AM   #20
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The end of the crankshaft. As either part of that torque converter or as I mentioned the magneto/alternator.

What kind of sled... specifically?
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Old 01-13-08, 01:13 PM   #21
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2006/7 Ski Doo MX-Z X 800, on the Rev chassis. Has a higher-compression head and some clutch work.

Drive looks like this: Motor drives one clutch, that drives the belt, which makes a second clutch move, which drives the jackshaft (runs sideways across the engine, has the brake on it) to a sprocket, bathed in oil in the chaincase, which spins the chain, driving the driven sprocket, which drives the track. As near as I can tell, that's it. That's what I can see in my 600, and his looks pretty much the same.

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