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Discussion Starter · #1 ·
My '91 Mach 1 left me stranded on the trail last weekend. It bogged and then the engine quit. It would barely run on one cylinder. After I got it home, I performed a compression test. The plugs looked good but the mag side had 20 psi compression.

At first, I thought I accidentially interchanged the carbs last fall so I checked them out this evening. The mag side had a 420 main jet and the PTO side had a 400 jet which is correct.

I then checked the needles just in case I switched the carb tops. The mag side had a 7DH2 needle with the circlip on the 3rd notch from the top. The PTO side had a 7DH2 needle with the circlip on the 2nd position from the top. According to the Ronnies ski-doo parts finder website, the mag side should have a DH3 needle with the circlip on the third notch from the top.

I have owned this machine since it was 3 years old and put approximately 3800 miles on it. I do all my own service work and have not changed needles or jets. It's hard to believe that this could cause the problem after all those miles - I would have expected it to surface years ago.... I am going to remove the head tomorrow night to inspect the cylinders, etc. but would appreciate any assistance.

Thanks!
 

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my daughter rides a 91 mach I've had it since new as far as the jets go there is 2
different size jets because of the rotation of the rotary valve. did i read the post
right you only have 20 lbs compression on one side sounds like you dropped a piston
 

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Discussion Starter · #3 ·
mtn-z said:
my daughter rides a 91 mach I've had it since new as far as the jets go there is 2
different size jets because of the rotation of the rotary valve. did i read the post
right you only have 20 lbs compression on one side sounds like you dropped a piston
[snapback]292725[/snapback]​
I plan on pulling the head tonight to inspect the damage. I need to find out why it burned down so I don't have a repeat event. It has been a really good sled for many years. I know the jets are different due to the rotary valve intake - mag side is richer than the PTO side but the needles were what was making me wonder - the mag side should have a DH3 needle with the circlip in the third notch and the PTO side should have a DH2 needle with the circlip in the second notch. My sled has DH2 needles in both and the circlips are in the proper positions. I have not touched these since I've owned the machine and I bought it when it was three years old and had 1900 miles on it. It now has 5800 miles on the odometer.
 

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Without being there and seeing your situation it would be hard to offer "what Happened", advise.

However, here is a link to a piston guide that might help you diagnose for yourself.
http://www.poetonaptec.co.uk/technica/pistonguide.htm

If your carbs are squeekey clean and the main jets are in the right spot, I would say the needle sizes is probalbly not the problem. Further the posistion of the e clip is for really, really fine tuning and problably did'nt lead to this failure.

Make sure you rebuild the whole engine, seeing how its down anyways. New crank seals, new gaskets and matching pistons.

Hope it helps.
 

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Discussion Starter · #6 ·
GoingMach1 said:
shouldn't the pto side be the richer side cause of the heat. but if the mag side went any way, i don't konw.


nice name i'll change if u want
[snapback]293248[/snapback]​
The dealer confirmed that the needles and jets are the correct ones. I figured that anyway since I've never messed with them in all the years I have owned the sled. The mag side is jetted richer on this sled. I have seen some newer ski-doos that had the PTO side jetted richer.

Tonight I am going to pull the head and examine the extent of the damage....I'll post back my findings. Thanks for the replies!
 

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Discussion Starter · #7 ·
Willie said:
GoingMach1 said:
shouldn't the pto side be the richer side cause of the heat. but if the mag side went any way, i don't konw.


nice name i'll change if u want
[snapback]293248[/snapback]​
The dealer confirmed that the needles and jets are the correct ones. I figured that anyway since I've never messed with them in all the years I have owned the sled. The mag side is jetted richer on this sled. I have seen some newer ski-doos that had the PTO side jetted richer.

Tonight I am going to pull the head and examine the extent of the damage....I'll post back my findings. Thanks for the replies!
[snapback]293609[/snapback]​
I drained the coolant and removed the head. The Mag side piston looks similar to the "ash trash" picture in the link posted by goingmach_1 except it was on the outer 1/2" of the piston. The website posted by goingmach_1 states: "The main causes of this problem are too lean carb jetting, too hot spark plug range, too far advanced ignition timing, too much compression for the fuel's octane, or a general overheating problem."

The carb jetting is at the factory specs; the plugs are the BR9EYA which are the correct heat range; the fuel was regular unleaded gas that I've used since I bought the machine in 1994; the machine did not overheat (at least by the temp gage on the dash).

The only other possible root cause (per the website) would be too far advanced ignition timing. I can check that after I repair the damage but can't help but question that since it's been at the factory setting for years.

I'm thinking about checking the oil pump output but can't help but wonder if I'm overlooking something simple that caused the problem.... When I checked my carbs, they were clean and the airbox was installed properly. I am afraid to run the sled after it is repaired if I am unable to find the root cause of the meltdown as I know it will just melt down again...
 

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Is there any debris on top of the piston. Like things impindging onto the piston?

It looks like some one threw little pices of metal shrapnel in there.
 

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Discussion Starter · #9 ·
goingmach_1 said:
Is there any debris on top of the piston. Like things impindging onto the piston?

It looks like some one threw little pices of metal shrapnel in there.
[snapback]294873[/snapback]​
No debris on top of the piston. It was starting to destruct and the head has a similar appearance.
 

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Inspect your carb intake adaptors for cracks...
 

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Discussion Starter · #11 ·
Willie said:
goingmach_1 said:
Is there any debris on top of the piston. Like things impindging onto the piston?

It looks like some one threw little pices of metal shrapnel in there.
[snapback]294873[/snapback]​
No debris on top of the piston. It was starting to destruct and the head has a similar appearance.
[snapback]294966[/snapback]​
I'll check that out this evening. I will also inspect the integrity of the airbox boots as well. Thanks for the help. If anyone can think of anything else to check, please post. I will continue to post my findings.
 

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Discussion Starter · #12 ·
Willie said:
Willie said:
goingmach_1 said:
Is there any debris on top of the piston. Like things impindging onto the piston?

It looks like some one threw little pices of metal shrapnel in there.
[snapback]294873[/snapback]​
No debris on top of the piston. It was starting to destruct and the head has a similar appearance.
[snapback]294966[/snapback]​
I'll check that out this evening. I will also inspect the integrity of the airbox boots as well. Thanks for the help. If anyone can think of anything else to check, please post. I will continue to post my findings.
[snapback]295138[/snapback]​
I inspected the carb boots. Both were in new condition - no cracks, defects. I then inspected the airbox boots and they were in good condition as well. I did not see any leakage from the cylinder gaskets either. I removed the recoil starter and could not see any evidence of the seal leaking.

I noticed some oily residue from the RAVE valves but doubt that would have caused the problem.

My next step is to remove the engine and completely disassemble it. I want to flush out any debris. I just don't know what caused this problem....
 

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Discussion Starter · #13 ·
I am going to block off the carb intakes and the exhause and pressurize each cylinder with 3 to 4 lbs of air and then go over every seal/gasket searching for leaks. I think that will uncover a leaky crank seal if present.
 

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Discussion Starter · #14 ·
I pulled the engine and pressurized the mag side cylinder to 5 psi. I checked the crank seals with soap water and they were OK. I soaped the entire length of the case half and the base of the cylinders and everything appeared to be OK and not leaking.

I'm going to check the fuel pump for the mag side and the impulse line for leaks but other than that, I'm out of ideas.....I will check the crank bearings when I disassemble the engine but it still strange that the mag cylinder melted down with no apparant root cause.

If anyone has any other suggestions on what to check, please post.

Thanks!!!
 

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Discussion Starter · #15 ·
I completely disassembled the engine. The crank bearings appear to be bad - they exhibit a rough feel/make noise when spun. The Mag side is rougher than the rest so I think the crank bearing may have gone bad causing air to leak past the seal causing a lean condition and the subsequent meltdown of the mag side. Shouldn't crank bearings be smooth like idler wheel bearings? Maybe they make noise because they are dry from the disassembly??? I think I might bring the crank into my dealer and have him verify that the bearings are indeed bad.

A remanufactured crank is $452 and that combined with the other parts required to rebuild the engine comes to a total of $940......too much money to put into that old sled. Appearance-wise, the machine is in fantastic shape.

I'm thinking I will part it out. It is really sad to do that but I'd rather put the $940 towards a newer Ski-Doo. I wish I could afford an '04 MXZ 600 HO but I think I need to settle for something in the 2001-2003 range...

Anybody interested in any parts off of it, please post. We can work something out.
 

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Discussion Starter · #16 ·
I bought a 2003 MXZ-600 yesterday! Immaculate condition and 1200 miles.

I stopped by the Ski-Doo dealer to have them look at my Mach-1 crank and noticed lots of used machines - typical for this time of year. I started talking to the salesman about the '03 MXZ and he said that they were asking $4300 for it. I asked him how low he would go and he said $3500. I noticed it needed carbides on the left side and the hy-fax would be due for replacement soon. He said that could be worked out.

After BS-ing for awhile, the salesman asked if I'd like to submit an offer. I told him $3000 out the door and they fix the carbides and hy-fax. They asked if they supplied the hy-fax and carbides (for me to install) would I accept. I said yes and it was a done deal!!!

What an awesome sled. I need to go to the U.P. to try it out. I'm definitely going to part out the Mach-1....look on ebay.
 
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