1988 XC250 revival 2016

Here is the leak down tester I rigged up when I was having trouble with my 79 , you may have to tweak the long stem with a 90 or something to fit it in a newer bike .

I can't remember but I think you only want 6 or 7 psi to check with so use a hand pump not a compressor or I think you could damage your crank seals.
There is a small allowance of leak down on a 2 stroke , maybe some one will chime in and let us know.

Would a leak in the crank seal area cause low compression though? A leak down test is on the list of things to do this weekend for sure!

Apparently so, I found this while searching the topic .
A Two Stroke Engine must have compression above and below the piston rings. We all know about taking engine compression readings from the spark plug hole but a lot of people seem to forget that a two stroke engine must have a sealed bottom end that will hold both compression and vacuum.
http://www.dansmc.com/vacuum_testing.htm

Yes but it wouldn't be the same amount of compression as what's building on the compression stroke in the head would it? I'd imagine since it's much more volume it would be a lower pressure

correct
http://www.amrca.com/tech/tuners.pdf

This is like a 2 stroker's bible... most of us could learn a thing or two here.
I hope this helps.

Either way having done the bottom end I should have tested for leak down before now. Shame shame

Bad reeds?

All I've found on reeds, (haven't read that bible get tho) says that reeds shouldn't affect compression even when bad, am I mistaken about that?

Bike runs flawlessly, just showing low compression

Your bike only draws air when the skirt opens the intake port, in other words the air is only drawn into the crankcase by vacuum, then once the piston starts it's down stroke it closes the ports and compresses the air in the crankcase until the top of the piston opens the transfer ports allowing the compressed mixture in the crankcase to escape under pressure to above the piston to be compressed, so if you are unable to compress the crankcase gasses enough (bad seal or gasket), then a lesser charge is transferred to the combustion chamber to be compressed by the piston on it's up-stroke resulting in a lower compression reading. Sealing (pressure and vacuum) is critical in two strokes in both the crankcase and the bore above the ports. I hope this makes sense?

Correct Kyle, reeds are in effect a "one way valve", air can be drawn through in one direction but should almost completely seal any gasses from travelling in the opposite direction thereby not having any real influence on compression.
If you didn't have reeds, and many early two strokes don't, the engine will still draw gasses by the vacuum created in the crankcase..

I think you're on the right track, check your compression tester first, then a leak down test should tell you if there are any sealing issues which could cause low vacuum/compression.
Tony

I know you had it bored, but did you measure ring gap on the new rings? I have had a bad bore job done. Also I have x-tra base gaskets. They are from different venders. They are different thickness. How did you check compression? Are you holding the throttle wide open? Kliem Research recommends a "rolling" compression test. Compression gage screwed in while rolling down hill.

correct, reeds do not affect compression..that is why you do compression tests wot. klemm likely suggests a "rolling" compression test to eliminate weak kicking.
you will learn your statement "an engine is an engine is an engine" is false, especially comparing 2 and 4 stroke. but this is how you learn and we will help with whatever
you do have the same compression above and below the rings. the crankcase is an important part of the two stroke's engine operation..ajcmbrown sums it up pretty much.

I did not. I suppose I can't rule out a bad bore job just yet then. Funny you mention rolling tests, I did it once and it read around 185-190 but totally didn't think it was valid due to the testing method.

i asked earlier if you held it wot..go back and do it again!

All this over a bad reading , lol . Gotta learn one way or the other usually the hard way to be meaningful.
I hope this is all it was, better to be safe than sorry tho.

I had the throttle wide open. I didn't measure ring gap is what I meant by I didn't.

I checked compression by kicking it over and over until the needle stopped moving while WOT.

I really wish it was something stupid simple like that but I dunno. I'll grab a rubber expanding plug and pipe fitting tomorrow to put air to the engine. We have a ton of different sizes for when we repair diesel fuel tanks.

Sorry for the confusion!

Jeff B makes a good point, what thickness base gasket did you use? Too thick can lower compression.
You can measure combustion chamber volume to calculate compression ratios.

Kyle don't get too hung up over your compression test, if its running well, starting easily and good compression on the kickstart forget the test. I have ridden competition 2 strokes for over 40 years and never had the need to carry out a test. Now ducking beneath the parapet!!

Yeah, if it ain't broke..........don't "fix" it.

I'd love to leave it alone but it's just how I am!

I did notice there were some vertical lines in the bore when I pulled it apart. None of these lines could catch a finger nail so I don't think it's gouging per se but still enough for me to likely pull the top end this weekend and have it checked by the engine shop sometime next week.