anatomy of a burndown

[Tzed250]

EGT

Ben, I don't know about sleds, but 1300 sound awful high for an EGT. A friend of mine bought Doug Braunek's 1990 TZ250 (the bike Doug used to win the AMA 250GP championship) and it came with all of Doug's set up notes. That bikes jetting was perfect when the EGT was right at 1100. Let me know what you think.

John

 

[Fish]

Before this goes on, read his{beginners} last post, then reread
his other posts here, then say what you think.

 

[Crofter]

Ben, if you said low octane gas was the most likely cause I would agree with you, but when you make the leap to saying it is absolutely the only possible cause to explain the event, you lose me. Is there any pit bull in you? Lol!

Frank

 

[bwalker]

Ben, I don't know about sleds, but 1300 sound awful high for an EGT. A friend of mine bought Doug Braunek's 1990 TZ250 (the bike Doug used to win the AMA 250GP championship) and it came with all of Doug's set up notes. That bikes jetting was perfect when the EGT was right at 1100. Let me know what you think

Keep in mind that this is a twin cylinder machine with a single pipe. Also the temp is influeanced by how close the probe is to the cylinder. The egt's may be higher due to this, but I honestly dont know. I mounted the probe per my engine guys instructions and set the alarm at his suggested 1350. That being said I have taken it over 1350 when hill climbing on several occasions with no negative results. I was running nutec race fue doped with additional mtbe. With the trail jetting that was in it egts are not a issue. even prolonged runs usually wont trip the alarm

Ben, if you said low octane gas was the most likely cause I would agree with you, but when you make the leap to saying it is absolutely the only possible cause to explain the event, you lose me

I just like any human can be wrong. In fact i have been wrong many times. In this case there are only so many things that can cause this damage. I have narrowed it down to fuel through process of elimination. Could I bee wrong? Sure. Do I believe I am after doing the test I have done? No. Too bad my sled doesnt have a downloadable detonation counter like some Honda gp bikes do. Then I could be 100% certain.

 

[My2cents]

When you put the sled back together, monitor the heads with a Raytek thermo very often once you warm it up. Do not rely on your existing EGT.Stay very close to you garage as well. I have a feeling you are going to see the cylinder in question become very hot once again.

 

[bwalker]

Based on what. Did I mention it has a probe for coolant temp on the racepak also? It wasnt a coolant leak. Get over it. Coolant leaking into a combustion chamber will cool the chamber not melt aluminum.

 

[eyolf]

OK, Fisher, I'll bite; I've noticed that beginner seems to have multiple personalities from time to time. Sometimes he's a polite, intelligient sort, sometimes a dork.

I'm like that too. Just ask my wife.

 

[dbabcock]

???? beginner. What is your diagnosis of this little dittie? 15 miles after a fix. A Ski Doo TRA primary and it shears off the spline of the primary clutch.

 

[bwalker]

Hey Doug. Have you ever heard of a clutch puller breaking? I broke mine trying to remove the clutch from my XC. I didnt even bother trying to get it out. Just gave it to my engine guy which wasnt too happy about it.

 

[Kneejerk Bombas]

Don't forget to change the gas in the sled after you get it fixed up.

How about testing the gas? If it is bad, perhaps the gas station has some liability.

 

[Crofter]

Originally posted by bwalker
Based on what. Did I mention it has a probe for coolant temp on the racepak also? It wasnt a coolant leak. Get over it. Coolant leaking into a combustion chamber will cool the chamber not melt aluminum.

Ben leakage is a two way street. Granted the pressure in the coolant is maybe 15 psi max. it is not too hard to keep it out of the combustion chamber. The real bogeyman is that the pressures in the combustion chamber are plus 500 psi. under load. Combustion gasses are VERY interested in escaping it takes very little leakage to cause bubbles to collect around the head and cause localized hot spots that can initiate detonation. If it is a gross leak the signs are obvious and it will empty the coolant, but small ones can go almost undetected. 1350 F exhaust temp is too hot.. I know visually what colour that relates to. You got away with it this long but you were working very close to the ragged edge. You were courting a hot honey but they can scratch you up sometimes. If you weren't a bear for punishment Ben I bet you would put me on your ignore list. I'm just being the Devil"s Advocate here. Ben probably is right about the fuel , but we have dragged up a lot of other things to be wary of and how they can sneak up on you if you dont keep a safety margin.

Frank

 

[bwalker]

Crofter, as I mentioned in a earlier post. I leaked down tested the motor and it passed. All the gaskets where intact. It DID NOT HAVE A COOLANT LEAK. as for 1350 egt being too high. That all depends on where the measurement is taken. These twin cylinder single pipe sleds run very high pipe temps. The probe is mounted on the pipe itself, not on the y manifold. 1350 is not that hot when measured in this location. Move the probes closer and the egt would go down. When I first built this motor I also did plug chops and used a bore scope to read piston wash and it was quit safe at this temp so egt simply wasnt a issue. Also when the thing fragged it wasnt running hot at all as I hit the replay button on my racpak right after it puked.

 

[bwalker]

Don't forget to change the gas in the sled after you get it fixed up

Done. Its in my fiances cavalier as we speak. As far as testing goes. Know any body with a knock engine? I would love to get it tested, but I think the cost would be prohibative.

 

[Fish]

beginner

Eyewolf,
just trying to let you all know that this guy has made a career
of disrupting forums, and usually gets away with it.

 

[dbabcock]

Ben,
Found the clutch problem finally. It turns out that when I told them to replace the primary, they only replaced the shell and not the moveable cup where the spline female is. Put a complete new primary on it today and it's fine.

Be that as it may, I was with my buddy who manages the local Ski Doo dealer here today, and I mentioned what you had said about the 900 REV in the snowcross chassis next year. He said he'd heard it too. I asked him what the difference was between the regular REV and the snowcross and he showed me the new Blair Morgan REV 800 that they just got in. It has an extended tunnel with a conventional snowflap rather than that funky articulated one they put on all the other REV's. I like the looks of it better and it may make studding the track much easier (std. REV's require that the seat be removed for this). Haven't gotten to ride it yet, but that's neither here nor there because I bought it on the spot mostly because you said it was so good. It better be.

 

[bwalker]

. It has an extended tunnel with a conventional snowflap rather than that funky articulated one they put on all the other REV's. I like the looks of it better and it may make studding the track much easier (std. REV's require that the seat be removed for this). Haven't gotten

I agree Doug. The articulated snowflap is butt ugly. Also the windshield on the snowcrosser is like a standard shield unlike the phazeresque one the rev has.
Speeking of clutches. Watch the spider buttons and the weight bushings on your cat 900's clutch. Replace them as soon as they got sloppy or you will have a fragged clutch on your hands.

I bought it on the spot mostly because you said it was so good. It better be.

Doug, I am a diehard polaris guy(have had 8) and I can honestly say the REV has me changing my loyalties. Its that good. It does take abit of saddle time to get used to the riding position coming off a poo. I will have one next year for sure. BTW The only thing I wish was differant was that it had a poo clutch.

 

[dbabcock]

Yeah, I've heard that about the 900 clutch. I'll keep on top of it for sure. I also agree with you about the TRA. Too much belt squeeze at the high end and I also prefer a roller secondary. Oh well, I'll ride the unit next weekend and I'll let you know how it goes. The only real bad side I can see to owning this model is that I think that Blair is a pompous jackass. Can't wait until next month to see if they offer a 900 for snow check. I'll unload the '00 MXz 500 I got for my son three years ago and get that too. Just the thing for a 15 year old. Sort of like putting a shotgun in the hands of an infant.

 

[bwalker]

The only real bad side I can see to owning this model is that I think that Blair is a pompous jackass.

It seems most succesful motorsports guys are.

 

[kev]

what temp was the engine during the leakdown test?

A friend of mine has a Yamaha Banshee quad that he races, it is a twin cylinder and he had exactly the same problem you have had. He did a leakdown test and it passed with flying colors, trouble is, he did it with a cold engine. Come to find out, when the engine got to operating temperature the head would warp ever so slightly allowing it to suck in air and lean out just that one cylinder. Funny thing is, he burnt down his motor 3 times, he replaced almost the entire bottom end every time but the head looked perfect so he reused it. He finally figured it out the last time it happened, he was making a run and before he made it down the track it sounded like a Huey helicopter landing for less than a second then POOF! Racepak or EGT monitors never saw it coming it happened so fast. Just an idea, I hope you find the problem.

 

[bwalker]

trouble is, he did it with a cold engine

I tested it warm. Set the whole motor on top of our air compressor at work for about 8 ours. This comp puts out enough heat to keep opur ware house warm and is probaly 6wx20lx5t. It was plenty warm. Also of note. This motor doesnt use head gaskets. It uses o rings. With o rings its easy to see if there where a failure. A head gasket leak on a liquid cooled two stroke usually wont burn it down any way, same with a base gasket leak. Its the crank seal leaks, and reed valve leaks that do that.