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Discussion Starter #1
This is my first post, new to the forum, but have been reading its excellent info for a year. Friends zr 580 lost power and just would't go, also it was hard to start. I had this problem once before when my exhaust broke and burned a hole in the mag side piston of my 340.
Did a compression test mag side low, looked at the spark plug, a hint of silver color so I pulled off the head, just as expected rings burned up along the exhaust port. But what puzzled me was the hole right under the spark plug, not through the piston but 1/8 - 1/4 in" deep in an oval shape. The plugs that he runs are ngk 9's, I don't believe they are to hot, if they were 7's or lower I might question it. The gas was horrible in it I tink it probably sat for 2 years by the smell. Could this hole be from bad detonation, Knocking? The reason for the burned up rings?
He got the sled last year from someone for some work he did, about $200 worth of work. It has 380 main jets, and on the inside of the clutch shield for our altitude it calls for 400's, this might be the reason for the ring meltdown, but would this be the reason for the hole in the middle. The pto side looks goods and has good piston wash, dark carbon in the middle and clean by the ports. The bad mag side is clean as a whistle with a hint of aluminum.
Any thoughts would be appreciated.
Thanks, Jason
 

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you got it right the motor is way too lean, thats what burned down the piston, and the gas probably made it worse, what altitude is he running it at know?
 

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A burn down like that always comes from either too little fuel, or (more than likely) too much air. As to where the culprit is, you'd have to do some checking. Find out what the specs are for the jetting, and check all those. Check for any rips in the carb boots or flanges. Since your going to have to do a rebuild, you'll be replacing all the gaskets...do the crank seals as well.

Be sure to clean the carbs and all the jets really well too. A clogged jet can cause a lean condition.
 

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Discussion Starter #4
Mattgross, we are running at below at Mi altitudes 0-1200 ft?
Tfin70, I will be checking the carb boots and flanges. The jetting chart calls for 400 mains at 0-20 degrees, it has 380's. It was probably rejetted by the previous owner, and unaware to my friend. One cold night is probably all it would take.
Thanks for the input!
Jason
 

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Classic lean out. Spark plugs don't lie, white plugs, too lean, silver, way too lean, cylinder walls and piston filings.
Your plugs should be a nice cardboard brown, jet till you get that.
I did the same thing a few weeks ago, but was having so much fun on the first ride of the season forgot to get off and check plugs.
Skol, K

Oh yeah...welcome to TUSF.
 

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it has the wrong jets, and if i read you first thing corectly you said it calls for 400 in the mountain, so you should be running larger then that at sea level or where you are i bet it isa from 420-450 or more, my mxz calls for 180 in mounts and 240 at 1200 feet, so you jetting is way off
 

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The gas didn't help at all, but I would guess it was jetted to lean. Somewhat puzzled why the pto side didn't look a little lean to though. I would suspect something in the carbs or oil delivery. I would check it thouroghly before restarting after rebuild and then check it often so you don't burn it up again. I think somethings up with this one.
 

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I vote for the bad gas for two reasons, one it would have had water in it from sitting and droplets can freeze in the small orifices of the carb and two, it would have been way too low on octane causing some problems. also if it was jetting wouldnt the other side show some signs too.
 

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Does this motor run differant mains pto to mag? Sounds lean, she ran hot ,starts to detonate, eats into piston etc etc..
Good seal on that side?, for what ever reason she;s sucking too much air.
 
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