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Discussion Starter #1
96 STX, getting worst by the ride, fix something and assume its good to go, runs good for about 5 minutes then becomes a ski-poo turd. Need help desperately, we're baffled.

Here are the symptoms, categorized into two plug gap settings:
.018
Starts nice and idles very nice after a few primes and some throttle play
Sled tops out at 5500 rpm
top speed 40mph
runs for 5 minutes until ONE plug fouls, other plug burns tan and looks nice, which plug it is is variable which completely blows my mind.
Quickly jerks the ski's up on launch then bogs and runs terrible
After 5-10 minutes of running it dies.
Won't start again until the wet plug is pulled out and wiped off and dried.
Doesn't run long enough for a gas mileage analysis.

.030
Starts nice, idle's perfectly almost instantly, maybe one prime no throttle playing
Sled tops out at 6500, 7200 if you play with the throttle
Will pick the ski's up on take off, runs fast low rpms... then dies.
top speed of 50-55mph
Will run for about 10 minutes until it starts to sound like its flooding, play with the throttle and it will run to 5-6 will die if left to idle on its own, play with throttle to keep it running, not enough power to climb a small snow bank at this point, starts smoking and running very rich
Dies as soon as you stop playing with throttle
One plug will be tan, the other will be gummed up with oil and gas and black as night. Again which plug it is varies from time to time.
Gets slightly worst mileage than a ZRT600.

At both settings it will run fine to idle for a few minutes (5 or so) and then die and not start again after a full throttle run, if its started and left to idle it seems to be able to idle as long as you let it. At both settings with no load (drive belt taken off) it will rev to 7800 but doesn't sound right to me when it does... but thats probably just me. Hasn't been full throttle or run for more then a couple minutes with no belt attached, so it doesn't really eliminate anything, just wanted to see if it WAS the clutches, but now we know its running very rich and fouling a random plug... Before it would just not hit top rpms, but would run nice, was very reliable, now you dont turn it off for fear of it never starting again.

So far its had 3 new sets of plugs, rave valves cleaned (the guilotines appear to have been replaced as they dont have top stamped anywhere on them), airbox has been checked, carbs are in sync, throttle is opening full, exhaust does not seem to be blocked in any way, clutches seem to be functioning fine, pistons are not scoured, testing compression tommorrow. A mechanic i talked to thought the STX's had a little fuel filter on the top of hte carb where the fuel lines go in, but they dont i checked. I dont know where the fuel filter is.
We baught a coil to change the old one out, but then when we gave it a test only one plug wasn't firing, so we switched htat plug out about 5 times (apparently ALL our spare plugs were fouled too) and they both fired so we decided it wasn't the coil.

Now here are some of my ideas.
1) Bad oil, the rave valves are filled with oil after only a short 10 minute run and there's oil on plug threads and on one of the plugs. It has some REALLY crappy oil in it. I'm guessing the oil isn't burning and its gumming up the plugs, BUT that doesn't explain why it doesn't hit top rpms and can run fine at idle when not opened up.
2) Carb settings, although it idles nice and doesnt seem to run rich initially
3) Oil injection settings (doesn't seem to be using too much oil i think)
4) Rotary valve problems, since its only one plug at a time (i dont know where this valve is, or really what it does, or how to clean it)
5) Fuel filter maybe, if i knew where that was.
6)That box that appears to divert gas on the bottom of the tunnel.

Ok and here's another question, if it is theoretically the same plug/cylinder that keeps fouling what to check then. My son had it out when it fouled the other side and i had it when it fouled clutch side, its POSSIBLE that one of us is crazy, but this is HIGHLY unlikely, i'm almost positive last time i did my own personal test it it was one or the other and varied from stall to stall.

I'm a heavy equipment mechanic but i dont know sleds. ESPECIALLY the newer ones.
 

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Sounds like you are taking a good systematic approacg at it.

1, I suggest to drain all the crappy oil, get Castrol or Doo oil. I personally run Doo Mineral for 15 years, no problem with it, ever.
That will rule the oil out. Crappy oil can cause VERY weird things. Bad oil can take the power out of the gas, causing the inability to rev

2, I wold confirm the setting of your oil injector pump. There is 2 marks on the lever, and one on the case. The case mark should be between the 2 arm marks. I line mine up just above the lower mark. To much oil will foul a plug FAST. Have a bud who insists on running his Yammi MX bike 32:1. He goes through 2 plugs in a days riding, allways. I tell him to lean the oil, he never listens.

3, the box that splits the fuel, is the fuel pump. That might be going bad too. A low flowing pump will starve the engine of fuel, but the oil will still be pumping into the engine cause they are 2 separate pumping systems.

4, could be weak coils. You say that it's worse at a lower plug gap. Lower gap is easier for the coil to "jump" the gap, but is acctualy harder on the coil, becaause a smaller gap can draw more current from the coil. Larger gap, the coil has to build more voltage to jump the gap, and in essence uses less current. This will cause less heat in the coil, letting it acctually produce spark longer.


Just some ramblings off the top of my head to check. Sleds are not to much different than HD engines. Fuel, air, spark. They all need those three things. Fouling plugs is usually oil or weak spark, in 2 and 4 stroke engines
 

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Discussion Starter #3
Sounds like you are taking a good systematic approacg at it.

1, I suggest to drain all the crappy oil, get Castrol or Doo oil. I personally run Doo Mineral for 15 years, no problem with it, ever.
That will rule the oil out. Crappy oil can cause VERY weird things. Bad oil can take the power out of the gas, causing the inability to rev

2, I wold confirm the setting of your oil injector pump. There is 2 marks on the lever, and one on the case. The case mark should be between the 2 arm marks. I line mine up just above the lower mark. To much oil will foul a plug FAST. Have a bud who insists on running his Yammi MX bike 32:1. He goes through 2 plugs in a days riding, allways. I tell him to lean the oil, he never listens.

3, the box that splits the fuel, is the fuel pump. That might be going bad too. A low flowing pump will starve the engine of fuel, but the oil will still be pumping into the engine cause they are 2 separate pumping systems.

4, could be weak coils. You say that it's worse at a lower plug gap. Lower gap is easier for the coil to "jump" the gap, but is acctualy harder on the coil, becaause a smaller gap can draw more current from the coil. Larger gap, the coil has to build more voltage to jump the gap, and in essence uses less current. This will cause less heat in the coil, letting it acctually produce spark longer.
Just some ramblings off the top of my head to check. Sleds are not to much different than HD engines. Fuel, air, spark. They all need those three things. Fouling plugs is usually oil or weak spark, in 2 and 4 stroke engines[/b]
Thanks for the reply, I figured the box was the fuel pump but wasn't sure.

First thing tommorrow we'll check the compression, if thats good then we're going to put the new coil in tommorrow anyways (it was only 50 dollars and then that will be eliminated for sure). What you said made alot of sense, and it could definately be the coil. Also will be draining all the oil and putting in some bombardier oil tommorrow. The more I think about it the more I think its the oil or a lack of gas at high rpms resulting in too much oil going in.

I'll post tommorrow with results if we get a chance to take it for a test run. Fingers crossed that its the coil or oil.
 

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Discussion Starter #4
Alright checked compression:
145 both sides.

Changed the coil, checked oil injector adjustment (perfectly set), adjusted RAVE valves a turn and a half tighter (now fully snug back to factory spec) and put in some good synthetic oil thats made for power valves, fired it up and it sounded SO much nicer, quicker throttle response as well. Sled no longer vibrates at idle. (i assumed this was normal for a double, but i guess not) Engine doesn't shake nearly as much any more. Barely touching the throttle hits 3500, im VERY confident it will hit top rpm now. Also didn't want to run it for more than 5 minutes, we only put a liter of oil in.

Unable to test top rpms since im in the middle of the city with anti-snowmobile rich neighbours. They get to rip on their quads and dirtbikes but i can't test out my sled. Even though im sure at 10:30pm they'll be snowblowing their driveways. Where's the justice?

Initial thoughts are that its definately fixed, my very first assumption of it being the coil was probably right. Going to stick with the ridiculously expensive synthetic oil for a tank just to be sure, then go to some cheaper bombardier non-synthetic stuff. Yes, the name brand oil is cheaper than the stuff i put in, we got a surprise blizzard tonight and i didn't want to travel out to the bombardier dealership so i hit up the canadian tire store.

Going to stick with the larger gapped plugs until all the old oil in the crankcase is blown out. Hate to have yet another day ruined because of a little oil.

Thanks for the help banditpowdercoat, probably wouldn't have replaced the coil since we had spark to both plugs. We weren't even going to do it tonight to be honest but finally decided we might as well (mostly because of the blizzard and neither of us wanting to travel to the parts place to return it)

I'm very excited to go sledding this week, the way it sounded at idle was just beautiful.
 
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