[RockZ]

A buddy has a 98 Mach 1. Last year he had the head cut to 0.065" and a thicker base gasket installed (I believe he also had the carbs bored out as well). At the end of last season he took out the center cylinder (he didn't put on many miles last year). He took it to the Doo dealer and they believed it went because the guy that had done the modifications put it together with silicone (gas eats silicone).

He had the dealer put in three new pistons along with a new center cylinder. The cases were also opened up and the crank checked and found ok. It seemed to be running good this year and this past Sat. we went out for a ride. About five minutes into the ride the center cylinder went again (he was doing around 100 mph when it happened). Pulled the plugs and the center one was greyish white again while the other two were a dark tan. On the way over to my house he was driving it in the same manner and did a plug check when he got to my place. At that time all of them were a dark tan. This was the fourth time he had checked the plugs this year and every time they looked fine. Why would the center cylinder suddenly decide to run lean and go down? If I recall correctly it went when he let off the gas at 100.

 

[mr670]

I would have a good look at the carbs, check the small fuel filter that is behind the needle and seat.

 

[MightyMach]

Is the airbox gutted?  I have heard that when this is done, it can cause wierd turbulance in the airflow and possibly lean out one of the carbs. Especially at high speeds where they are trying to suck as much air as they can get.

 

[skidoo4me]

A riding buddy of mine had a 99 mach that blew the centre cylinder with only 700 miles on it. He was having trouble blowing plugs and the dealer told him he thought it was bad gas so he took it out for a ride with fresh gas and that is when it blew. It ended up the stator had gone bad which was causing a miss fire. Bombardier came good for a new motor (5200 cdn.) and he since has 4000 miles on it without a problem. It might be worth checking it out because I have heard of problems with stators on late model ski-doos. Good Luck!

 

[RockZ]

I'll have to ask him if the airbox is gutted. Last year just before the motor went it was acting strange where the rpms were erratic. The dealer replaced the CDI box initially thinking it was the cause of the erratic rpm and subsequent burn down (until he discovered that it was put together with silicone after the head mod). The center carb boot was also replaced when it was rebuilt as it had started to crack underneath (not sure if it was cracked enough to cause the first burndown). As for the stator I'm not sure if it was ever replaced. Could a bad stator cause a burndown of one cylinder (when mine went on my 98 Zed it just quit at about 80 mph but it never harmed the motor)? As for the small fuel filter, is that part #35 on the HLSM.com microfiche (http://216.37.204.202/Skidoo_OEM/skidoo.asp?Type=13&A=273&B=10)?

 

[skidoo4me]

When the stator went bad on my buddy's 99 it sounded like it started to run on two cylinders. We changed all three plugs but when we started it it didn't sound good. It ended up it had burned a hole in the top of the piston. He then proceeded to drive it about 2 miles to the nearest road which caused the piston to totally disintigrate and it threw a rod through the back of the cylinder. The plug from the middle cylinder was covered in what looked like metal flakes when we changed all the plugs. This may not be your problem but it's just a suggestion.

 

[RockZ]

Thanks skidoo4me, the more input as to the possibilities the better. These modern day electrical systems are starting to scare me. It sure seems like a lot of people are experiencing similar problems???

 

[bobber]

Center cyl will run hotter due to coolant flow. Another thing to check is cooling system. Is the center carb main jet larger to provide extra cooling fuel?

 

[POWERHAULIC]

remove the small fuel filters in the inlets. they were the cause of a lot of unexplained burn downs. I believe there is a bulletin to remove them

 

[RockZ]

I believe his jetting is 300 across the board (the Mach 1s didn't call for staggered jetting).

 

[speed is everything]

i would jet up at least 1 size in the center just for insurance. i would also check the oil line to that carb.i would also suggest not gutting the air box on the 99.happy sledding.

 

[RockZ]

Thanks to all that replied. I'll pass this info on to him and hopefully he can find what is causing this. Speaking of jetting, another buddy that was riding with us said his stock jetting in his 98 FIII 700 is 300, 290, 300. I find it weird that his center cylinder is one size leaner, the Mach 1 is even across the board and my 98 Mach Z is 310,320,310 (center is one size richer). What would be the reason for this?

 

[maluchm]

I wondered the same thing about the jetting between these
sleds!?! Is it the exhaust?

RockZ - Stock mains for the 98 FIII 700 are 310,300,310!
Between the 98 FIII 700 and the 98 Mach 1 the needle and
needle jets are different between the two sleds. Also the
RAVE springs!

 

[xc 600]

What does the burned down piston look like. Is it the four scuff marks in the corners sayin the piston was square two on exhaust two on intake.  Or is it a hole in the center of the piston which would mean detenation and you need to run better gas higher octane.  Or the cylinder fired at the wrong time meaning electrical trouble.  Or did the ring turn and hook the exhaust port taking off the top of the piston or at least a little piece.  anyway what does it look like.

 

[theHIGHLANDER]

The F III jetting is probably due to the single pipe. The center would "see" more returned fuel from the exhaust...path of least resistance.

 

[RockZ]

I never got to see the piston. I believe it was scuffed but didn't have a hole in it. I talked to him yesterday to pass on the info I recieved and he informed me that it is up and running again (replaced the center piston and cylinder). He is now thinking that he may have been jetted a little lean for the oversize carbs (they are bored to 39.5 mm). I mentioned the possibility of an air leak and he said that he had noticed that the castings were a little rough looking but another buddy had ran these same carbs on his 98 for two seasons without a problem (he was selling his sled so he swapped him for his stock carbs). What I didn't know was that the original owner of the carbs was running 350s in them whereas he was running 320s (he originally told me he was running 300s but made a mistake). This is why he believes he may have been too lean (not too sure why the burn on the other two cylindeers looked good if that's the case?). Now that the story is unfolding a little more I have to wonder what was he thinking when he dropped down three jet sizes from what was in them???Needless to say he has the 350s back in so hopefully that solves his problem.

maluchm, I stand corrected on the FIII jetting. I had asked my buddy with the FIII 7 what his stock jetting was and he thought I meant what is in it right now (he is jetted down one size). That's the problem, when your running the big dog and your buddies are running smaller sleds that run almost as quick you get all kinds of answers when you ask them what they did to get it to go like that. The most common one is " I didn't do anything" yeah right&#33

! I think he may have learned a liitle lesson - lets hope so anyhow!