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Discussion Starter #1 (Edited)
I'm stumped, looking for some help here. Some background information for you: Last fall, I found a 2000 Xplorer 250. Thought it was a cool machine, and something that my son could learn to ride on, and could do some chores around the yard. Went and looked at it, made an offer, and took the machine home. With it being the end of the season, I took it out one time, noticed some things I wanted to work on, but ultimately winterized it and parked it until a few weeks ago.

It initially was not back shifting, so I tore the secondary apart. The buttons were worn, and the helix was scarred, so in went a new helix and buttons. Backshifting fixed. Set the clutch with the Red spring on 1 on the moveable sheave, and 2 on the helix. Did not spend any time with the primary, but did notice that it has the blue spring in it. Before final reassembly, I started it in neutral and the clutches seemed to function fine, though it did seem to take a while to get the primary to slide all the way back out completely.

I live at 4500' and took it for a ride at 6200' at 40*F on Memorial Day and the machine was very lethargic. Engine fires up quickly, only needing the choke at or below freezing for 10-20 seconds. On the trip the machine would move under its own power, but struggled getting it on the pipe, and would bog at 2/3 throttle to WOT in high range. At times it was really struggling, so I put it in low range, and it seemed to cruise around better, and would actually get into the power band, but still didn't seem to run as good as my '98 Trail Boss. Chalked it up to be jetting. Tore the carb apart tonight, and its got a 120 main, and 40 pilot which looks to match factory jetting specs for the elevation.

Pulled the plug to do a compression test, the plug is black, and has a lot of carbon build up on it. Did the compression test and got between 95-110 PSI depending on 1 of 3 gauges used.

Not sure where to go next. Would low compression manifest itself more at elevation? What else should I look for?
 

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I would say yes elevation would effect compression. 95 is on the low side for a 2 stroke anyway. They usually say they need 100 to run right. But you said it starts ok which is a sign that it has enough. Low compression would cause a hard starting issue. Your black plug says it is running rich. Did you try it with the air box lid open?
 

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Low compression will let a two stroke start and idle fine, but lack power - I had an old Suzuki 100 with 90 PSI compression - it would start on the first kick and idle fine, but you could push it faster than it would run - it was kinda OK in low gear, but in any other gear it run, but lose speed - put some new rings in it and immediately noticed the idle was about 2000 RPM higher - ran fine after that - rings can do funny things - I think the compression should be about 120 to 150 psi and altitude will drop compression, by about 3% for every 1000 feet above sea level. Here's a chart to figure how to compensate for altitude reduction of compression: 150 psi at sea level will test 125 at 6000 feet

1000 .9711
2000 .9428
3000 .9151
4000 .8881
5000 .8617
6000 .8359
7000 .8106
8000 .7860
 

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Low compression will let a two stroke start and idle fine, but lack power - I had an old Suzuki 100 with 90 PSI compression - it would start on the first kick and idle fine, but you could push it faster than it would run - it was kinda OK in low gear, but in any other gear it run, but lose speed - put some new rings in it and immediately noticed the idle was about 2000 RPM higher - ran fine after that - rings can do funny things - I think the compression should be about 120 to 150 psi and altitude will drop compression, by about 3% for every 1000 feet above sea level. Here's a chart to figure how to compensate for altitude reduction of compression: 150 psi at sea level will test 125 at 6000 feet

1000 .9711
2000 .9428
3000 .9151
4000 .8881
5000 .8617
6000 .8359
7000 .8106
8000 .7860
Hi Latebird,
I do not mean to hijack the thread but I need some clarification. My presumption here is that there is nothing you can reasonably do to increase compression beyond stock. In other words, if the max compression is 150psi at sea level, then the most you can get in compression at 6000ft elevation is 125 psi. Yes, I do understand you could gain compression with a longer rod or different crank but I do not consider that type of change to be reasonable.

In addition, if stock compression is 150 psi but there is enough wear in the engine so compression is only 115 psi and the engine runs fine at sea level, it would probably not run great at an elevation of 6000 ft because the elevation based compression is only 96 psi.

Note: The values used in this example are pulled from the air for discussion purposes only.

Is this a fairly accurate way to look at it?

Another question, if the air is "thinner" (less oxygen) I would also guess having less compression and less available oxygen would also be a factor. The engine would run richer at elevation and this would also contribute to not running as well as it would at sea level.

Is this true as well?

Thanks
 

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@pigseye - that is exactly right - as you go up in altitude the air gets thinner and the engine runs rich so jetting down will restore power and performance - also (especially with a two stroke) compression rises with an increase of engine speed, but there are ways to raise the compression without major engine work. The cylinder head or top of the cylinder may be shaved or the combustion chamber welded up and reshaped. Personally I prefer shaving the cylinder head.

Back in my days of racing two strokes, I had an assortment of cylinders and heads - each was marked as to the preferred place of use - timber, motocross, hare scrambles, flat track and divided into categories - short track, long track, groomed, natural terrain and elevations - most were for mid-west, but I had a couple for Colorado and Tennessee and Nevada. Damn, that was a long time ago; 1970 to 76.

If a vehicle is going to be used primarily at an altitude above 4000 feet, the performance may be improved by having the cylinder head shaved about .050" to permanently raise compression. If the vehicle was going to be part time at sea level to 2000' feet above, the compression could be dropped by installing two head gaskets (you don't to remove and replace base gaskets because that will affect port timing) or having one cylinder head for below 4000' and another for above 4000' or you could just leave the shaved head on the engine and use 87 octane fuel above 4000' and 91 to 93 octane below 4000'.

Regardless, the jetting may have to be changed from seal level to high altitude.
 

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@pigseye - that is exactly right - as you go up in altitude the air gets thinner and the engine runs rich so jetting down will restore power and performance - also (especially with a two stroke) compression rises with an increase of engine speed, but there are ways to raise the compression without major engine work. The cylinder head or top of the cylinder may be shaved or the combustion chamber welded up and reshaped. Personally I prefer shaving the cylinder head.

Back in my days of racing two strokes, I had an assortment of cylinders and heads - each was marked as to the preferred place of use - timber, motocross, hare scrambles, flat track and divided into categories - short track, long track, groomed, natural terrain and elevations - most were for mid-west, but I had a couple for Colorado and Tennessee and Nevada. Damn, that was a long time ago; 1970 to 76.

If a vehicle is going to be used primarily at an altitude above 4000 feet, the performance may be improved by having the cylinder head shaved about .050" to permanently raise compression. If the vehicle was going to be part time at sea level to 2000' feet above, the compression could be dropped by installing two head gaskets (you don't to remove and replace base gaskets because that will affect port timing) or having one cylinder head for below 4000' and another for above 4000' or you could just leave the shaved head on the engine and use 87 octane fuel above 4000' and 91 to 93 octane below 4000'.

Regardless, the jetting may have to be changed from seal level to high altitude.
Hey Latebird,
This is an awesome explanation with relevant experience. And duh! shaving the head is really not that big a deal to raise compression for consistent use at elevation. Using different octane fuels at different levels is a great idea too.

Thanks so much!
 
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