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Discussion Starter · #1 ·
Took my freshly restored 1981 Trailcat out last Saturday for a run.
I forgot that when in final assembly a month ago, I had put in a smaller jet and was going to check the plugs after a mile or so to see if she was lean, then tune from there.

Well she ran just great, way snappier than ever, since I've had it.
It was so much fun, even on my old beast, just cruising along.
But after about five miles I found out that she was lean, it lost a little power then I knew exactly what was wrong so I killed it.
The fan side plug was a nice light brown, maybe a touch lean, but the clutch side plug was completely white.............D'Oh!!

Oh well, it was a beautiful day for a 5 mile walk on the prairies.
You know your stressed at work when blowing up your machine is still a good day.
Plus my 500XC is sitting cozy in the car-hold waiting for more snow, and for me to get a day off.

My first piston replacement, pretty sure I know what I am doing, but any tips would help.

 

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Did it seize? I have a buddy that ran a 700 Polaris in a mini sprint car. Engine leaned out but didn't seize. He jetted up and dumped some 10-30 in the cylender and pulled it over a bunch. Long stoy short it smoked like crazy from the oil, but he won the main event that night.


You don't have to believe everything you think!
 

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It sounds like seize. Just to be sure, take the carb off and look in the port to see if the rings melted into the piston. When you change it here are a few tips. Hone the cylinder out, change the pin, rings and bearing, and don't try to go cheap and reuse the circlips. Last but not least line up the rings when reassembling and coat the cylinder with lubricant. Good luck.
 

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Discussion Starter · #4 ·
No...didn't seize, but there is a loss of compression on the hot side cylinder.
No hole in the top of the piston either, so probably one on the exhaust side, or blown rings.

Can't hone these aluminum pots, so I really hope that there is no damage to them, otherwise I'll have to sleeve. That is why I killed it so quick, and was happy to walk.

Thank God for my anal retentive father who insisted on a winch at the front of our custom/home-built 2 up trailer...made loading by myself a breeze.

 

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How far down did you jet it from stock? Also was that single or dual carbed?

Sorry to hear about the leaner, I have 3 rebuilt older engines I get to test out first snowfall.
 

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D'OH.......that sux.....at least it didnt happen in the prime of the season to ur XC......now u have something to work on if it is a shitty winter again


When's Winter Coming Back?​
 

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if you leaned both carbs out both spark plugs should be the same, seems you've got a bad crank seal or an air boot leak! What probably happened is the exhaust bridge expanded into the piston, leaving the piston somewhat scored and a little on the cylinder. If it's not bad you shouldn't have to replace anything.

Win if you can...
Loose if you must...
But NEVER stop trying!
 

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that sled should have a single carb? so im kinda wondering why only one cyl. got so hot, unless the fan kept the other one cold, nevertheless, i just looked in my old arctic cat repair manual, id slip new pistons and rings and just lightly hone the cyls in a 60degree cross hatch pattern if they arent too bad, do a compression check this will let you know how your doing,also they want you to run the jetting to the stock size "way rich" if its a 440 it should be like 300 main, also they want you to run additional oil to the injection, and mix your gas 50:1 too! which should keep things well lubed on fresh break-in, id also run the heads in a figure 8 pattern with some 400 grit paper to make sure the heads are decked well, if ya do all that id say after proper break-in you ought to be just fine, now on my old cat 440 i run 280 "the 300 is stock for break-in "main jet i could go to a 270 and still be ok but i like running a bit rich, and i never foul a plug, also i almost forgot be sure to check that the oil injector system is working, this could also been your problem also, get an old cat manual they are filled with engine rebuild pages that should make things pretty easy, good luck, these old cats are like a timex they keep on ticking, let me know what you find on dissasembly.

if its not a CAT, then it must be a dog!
 

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with chromed cliynders not much you can do,hope there not scored,340 or 500.jet it back up.when you say it was, way snappier, theres a old saying,they always run better then ever just before they blow,
 

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<BLOCKQUOTE id=quote><font size=1 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>they always run better then ever just before they blow[/quote]

oh yes they do...with my mx bike I'm always running it just on the edge, and does it ever make some nice power then


You need to pressure test the bottom end, it sure does seem that you've got a faulty main seal.

Win if you can...
Loose if you must...
But NEVER stop trying!
 

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Discussion Starter · #11 ·
Thanks for the info guys.....it is single carbed with a mikuni round, stock is butterfly, so stock jetting means nothing. It had a 280 orignally but put in a 250 and expected to go up from there had I not been having so much fun.

Is this the only sled that runs cooler on the fan side??...it has always been like that. If it had duals I would jet them diferently, but that's probably a no no.

I just need time to get it apart and see what needs to be done.

I have heard that if I try to hone these things, the hone will just skip over the high spots, and gouge a valley on the back side of each bump. Is this what anyone else has heard?

Thanks again.

I'd be lost without TUSF

 

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It's pretty common to have the pto side fail first on a fan cooled machine. It sees warmer air than the other side. You might get away with just a new piston and honing the cylender because it sounds like you caught it pretty quick. Pull the jug and take a look. Be careful not to drop anything into the crank case though. Good luck.

You don't have to believe everything you think!
 

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Discussion Starter · #14 ·
She was just fully rebuilt about 3 years ago, put on maybe 400 miles on the old girl since I got my 500 XC........so I hope those bearings and seals are OK...I'd rather not have to tear it all down and have to buy a full gasket kit.


 

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I doubt the crank seal is bad. You can check if you want, but I would rebuild the cyl. and check it after it's running again because you are pretty sure you leaned it out by jetting.

You don't have to believe everything you think!
 

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Well that sucks. But at least you know why it went and that. Good luck with the rebuild I would give a word of advice but I know as much about fixing engines as an eggplant would.

Trees don't make the greatest brakes.
 

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holy heck !!!, a 250 main, im suprised it went that long, a 270 is the leanist jet for that engine, your jetting is what definetily caused your meltdown, no doubt about it. heres a word of advise although it may be a bit late for you, maybe someone else can use it,start with your stock "rich" jetting then move down one size at a time 300 to a 295, do a plug chop if your still rich go to a 290 and so on, use your air screw to fine tune, small adjustments at a time then check, i know its a pain changing those jets that many times but it saves you in the long run.

if its not a CAT, then it must be a dog!
 

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Discussion Starter · #18 ·
Hey Vetboy......rich to lean is not as fun as lean to rich, keeps you on your toes
.

I know your dead right, should have done it that way.

BTW. How do you know 270 is the leanest, it's a different carb than stock, I thought that was out the window.

Anouther thing with temperature jet up for cold, jet down for warm....right?


 

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<BLOCKQUOTE id=quote><font size=1 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>Originally posted by 500fasEnuf:

Anouther thing with temperature jet up for cold, jet down for warm....right?

[/quote]

Exactly.
Larger main jets for colder, more dense air. Smaller jets when it gets really warm out. Check your plug color if your not sure which way to go.
 

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Discussion Starter · #20 ·
Thanks Mega......I thought so.

 
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