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Discussion Starter #1
Some food for thought on the compression loss with the 800s. Some people are saying its a lean spot within a certain RPM that is causing the rings to flake. I know of a couple of people myself who are having this problem but they are saying the rings are flaking on the exhaust side. We have come to the conclusion that it is not a lean spot but something to do with the port timing or the pipe. Basically what is happening is that withing a certain RPM the charge is forced back through the exhaust port too early which exposes the rings to eccessive amounts of heat. That is why the rings are flaking on the exhaust side. If anyone out there is into high performance outboards you may have heard of this happening on the Merc Bridgeports. This could be way off but it makes too much sense and it has happened before in other engines.
 

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So your saying that maybe a diffrent design of pipe might fix the problem. Or they would have to change the port location?
 

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The lean spot would cause higher temps on the exhaust side.

I'm not saying your theory isn't correct - in fact it could very well be.
I'm just saying that the flaking on the exhaust side still could be
caused by the lean condition.
 

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great point. i have a shim kit on mine so i'll know at the end of the year if your right. i would think that would be enough to change the port timing.
 

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Is this something that the piston mod would help with? I have heard some people say that after the motor is put back together it has a much throaty-er sound.
 

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Discussion Starter #7
I am not sure how to fix it. The guys I talked to say its the port timing, the pipe thing was something I threw in and I know nothing compared to who I spoke to. Both the people I know who have had the problem have the new pistons. I would say its likely the port timing has to be changed.
 

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It seems that the moly flaking from the rings is caused by a slight detenation problem, it can be cleared up with a cly head mod.
 

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You know,, I've been in this 800 twin thing right from the start. My rings asre completely trashed all the way around. There is no way that you could tell which way was forward if you didn't know where the ring gap was supposed to be.

If it's not a lean condition @ 7K RPM, then why has there been numerous mods for the 800 that directly address that range?

1) The very first mod, for the DPM sleds - move the needle jet height to correct a lean condition.
2) MPEM re-program - change the timing curve - retard the timing to push some of the heat out the pipe - helps to cool the combustion chamber. Masqueraded as a MPG fix. Did it help your milage? I doubt it. Did it hurt performance? You bet!
3) Now this year already. A new needle, although still not adjustable, it is thinner in the midrange area. (think that might be to fix a lean midrange problem?) Not according to Doo. Supposed to help milage also.
4) Another for this year. A softer spring for the RER secondary. We all knokw what that does. It will bring down the cruising RPM's to get your machine to run below the midrange lean spot. - Now this one I do see as a MPG improvement. It gets you to run at a lower RPM while cruising and it won't backshift as hard, allowing the machine to coast a little more. - BUT... it also brings the machines RPM's down out of the lean mid-range area to help keep the rings (uh...ring) cooler.
5) The new 03's have a hooded needle jet (just like in the pre flatsides for twins days). All of the 01's and 02's have a pressed in needle jet with no hood on it. If that wasn't a problem for consistent fuel delivery, then why the change back?

I'll be the first to admit that the lean midrange condition is not the only factor for ring degeneration and poor performance. Head and piston design and the fact that the timing curve is so hard to change from stock also lend a hand to the poor performance we see from the Ski-Doo Series 3 twins.

Myself, I don't think that the 'Merc Bridgeport Syndrome' is even a small part of a big problem. I think there are big improvements coming from Doo in the next couple of years, but we still have to deal with what was dealt for the last two.

You know what? At least they still run. Take a hard look at the F-7!!! :nervous:
 

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In my opinion i think alot of the poor heat transfer and
premature ring deteriation is caused by the stock head
and it's design.
I just installed a cudney head/shim kit along with new doo piston/rings.My intention is to ride it for apprx.1000km and then pull the rings to inspect them.This should tell whether or not it is the head or the piston.The rings i pulled out had almost all of the moly gone as well as being very pitted,but the sled still had lots of compression and ran fine. ???
 

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I'm guessing that if you're working with Bill on it, that you are going to use a 942 needle too. If it was just a head problem, you wouldn't have to change the needle. Absolutely nothing against Cudney, but some people don't want the problem to be what it is or they wouldn't sell the # of parts that they do. I have some tricks up my sleeve, too, but only more snow and saddle time will tell.
 

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I'm starting out with stock carb set up other than 480 mains.I've heared mixed reviews as to whether or not to use the 942 needles.I guess i'll find out soon enough,but i don't really want to spend half of the season trying to tune it.
 

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Thanks for the input idooski :) I'll give bill a call tomorrow or maybe he'll catch a glimpse of this thread and post some advice.
 

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Idooski is right about the Ski Doo so called MPG fixes all of them were intended to address the midrange lean condition and they still have it, if you think the 03 engines aren't going to have moly problems your wrong, we have already seen 6 engines with the moly out of the rings and two of them had less than 100 miles. We talk to people everyday from across Canada and the Northern US and the ring problem is starting to show up in most areas. We have used the 942 needles and they do help the lean midrange but still we have seen the moly gone on rings in engines using the needles so that is not a real fix either. Changing the head does help but it is not the cure either, but the rings do last longer when you change the head, one thing I can tell you and that is when we install our 700 pistons in a 700 we don't have any moly problems, they leak down a lot less than the Ski Doo pistons and last as far as engine seal about 10 times as long as the Ski Doo pistons and rings, the reason we don't have a moly problem, we don't use moly rings.
 

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Good post Bill. So after reading all of this and NOT having an 800 Twin. I think the answer is to run the sucker rich! Just a thought........
 

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That is the thing Fast, they come way to rich on the top end. They have to get enough fuel in the mid range to make em live. EGTs drop to the 900s WFO with factory jetting, but 1200+ in the mid range.

The 800s come with 500 or 520N
 
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