what would cause a 400 Scrambler to not start. Let's say it was out on the trail when a nasty tree jumped in front of the quad and oh, broke the top of the radiator. The quad was ridden home, stopping every so often to let the engine cool somewhat. There is plenty of spark, and the quad seems to be getting gas and the piston seems to be compressing (cause it will backfire every so often when trying to start.) The radiator is off, but it just won't kick over. Have I hosed the motor?



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Hawk - stock (for now) 99 Scrambler 400

 

Hawk,
Try this, pull the sparkplug out, and put your thumb over the sparkplug hole and crank it over, every time it(the piston) comes up it should blow your finger off the hole.. If not, you've more than likely shelled the piston.
If you're unsure about how much "blast" it should blow out the sparkplug hole, go buy a compression tester or take it to the local dealership and have them run a compression test on it for ya...
If you already own a compression tester, but not the adapter for the 2-stroke sparkplug threads, just take a plug and knock the center out of it and braze a quick connect fitting onto it...
Compression should be 150 p.s.i. or better.. (I think!!)
If you're firing in the exhaust (backfiring) something is definitely WRONG!!...
Don't ya just hate it when those pesky trees jump out infront of ya....
Good Luck!!

 

9 out of 10 times, a trip with no coolent will do great damage!

 

Not ever having trouble with trees on my quad,I can't tell you what the problem is.But from experience I can tell you they hide a "tree magnet", under the hood of snowmobiles.If a guy could ever find it,it would be equal to building the better mouse trap.

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THE FORMER CORNCOB2

 

I will try the thumb over the sparkplug hole tonight when I get home from work. If the piston is gone, what else would I have to do, besides replacing it? I know that the rings would be replaced with the piston, but do I also need cylinder work? Head work?

The engine does not make any strange noises when cranking, is this is good sign, or is it not an indicator?

As you may have guessed, I am not a mechanic. I can change oil, and do brakes, and other minor stuff, but when it comes to inside the engine, I am not very experienced. If I had a manual, I feel fairly confident that I could replace the piston, but i don't trust my diagnostic abilities when it comes to telling what else might need to be done.

Should I just bag it and have the dealer look at it?





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Hawk - stock (for now) 99 Scrambler 400

 

While I wish you well, Hawk, those who say operating a liquid-cooled engine without coolant involves risk appear correct.

Besides obvious damage, like the postulated piston problem, excess heat can ruin bearings and consequently rods/cranks, etc.

Running without coolant may be justified, say, when hurriedly leaving Jurassic Park after operating hours; otherwise, approach cautiously. Good luck!

Tree Farmer

 

Update from the woods....
Took the carb and reed valves off today and examined the piston and cylinder. They looked shiny and clean. Put 'er back together, put on the radiator, which I have puttied up with JB weld. Filled 'er with coolant, and cranked 'er up. Sounded like more pressure, so I took the (new) sparkplug out and tried the thumb test. Blew my thumb off the hole and spewed coolant all over. - Can you say blown head gasket?. It's in the shop now, I would guess for a few days. They will verify my evaluation of the cylinder and piston while they have the head off, but I am pretty sure that's all there is to it.

Thanks for all your help and good wishes..



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Hawk - stock (for now) 99 Scrambler 400

 

Hawk,
Ya know.... ITs gonna be apart... now would be perfect timing for a Weisco forged piston.. maybe a little dome work or cylinder porting... If you just happen to have the coins... its the perfect opportunity..
NOT to scare ya... but I have heard that the stock Polaris 2-stroke pistons are prone to cracking after adding other mods..(I guess with the other mods, riders tend to ride them even harder more often) I don't know if this is all 2-stroke Polaris pistons.. or just say the 250's.. or the 400's or just certain year models of certain models..
Check with Ritter Cycle Racing.. I think thats where I read that (about the stock pistons cracking).. From what i've seen and heard.. he is "Tha' MAN" with tweaking out 2-stroke Polaris machines.
Its just a thought....
Good Luck, and glad to hear you atleast know WHATS wrong with it now...

 

Yeah, Mudmachine, I thought about a Wiseco piston, but I was a little hesitant to ask the Polaris dealer to put it in while the quad is still under warranty. I did learn alot just by pulling the stuff apart to look at the cylinder, as soon as I can scrounge up 1) a manual and 2) the money and 3) the time, and 4) the guts, I think I'd like to try my hand at adding the Wiseco myself.

I figure I'll wait to see how much of a habit this hitting trees is going to be... maybe I'll need to become more of a mechanic.

I read the same thing about the piston at Ritter's site.

Thanks,


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Hawk - stock (for now) 99 Scrambler 400

 

Mud Machine
Which years were the pistons cracking in 250s? Were the pistons cracking only after the modifications were performed?