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Decompression valve question.

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I've got a kx500 with a decompression valve installed on the cylinder. Not through the head like some of the other CR and kx500s. It's similar to the ones used on the old Maico 2 Strokes. I love the thing to be honest and it works great. Like kicking over a 125. but I haven't gotten an answer to this question. I'm getting compression results around 100 PSI. Which is on the low side obviously and might indicate a top end rebuild. On the other hand the bike has great power. Poles wheelies with the throttle role in the first three gears. So the bike doesn't seem too tired, although most KX owners claim it'll pull wheelies and all gears with a roll of the throttle. Although I am a larger guy at 235. I and some others have guessed that the decompression valve might be giving me lower results than what I would see while the bike is running. Long story short I'm hoping to hear from somebody who's used one of these valves or possibly used to have a Maico and had done compression test on them. What were your results? Should I expect the same or very similar compression test numbers? Regardless of the bikes pretty darn good power I don't want to run the top and into the ground or risk grenade in the engine if it should be replaced. And before I tear into it and spend time and money to take a look for myself I thought ask the form first.

 

Thanks

 

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The Maico one is a port in the cylinder wall about 2.5" down. Once the piston passes it, there is only compression.  I guess you could pull it and plug the hole if it is a common thread size.

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That's how mine is as well. I guess I wasn't clear but the whole is plugged with the decompression valve itself. I'm getting readings it's not just open to Atmosphere. My question is, should the reading be lower than what would normally be expected of a regular compression test even though the hole is sealed I would guess having some of the compressed air go into that cavity would have to lower the compression at least a little bit.
So just to sum it up. Everything is working fine and the decompression valve works fine as well. It's sealed off. My only question is should I expect lower compression test values for the simple fact that there is a de-comp valve installed, or are my "bad" readings of 100psi low regardless?

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My missunderstand. I was thinking you suspected your valve as possibly leaking causing a low test when I suggested plugging it.

I hope you find someone who knows what good psi is with these.

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Yeah me too. Got to be somebody on here running the old makos. Maybe I'll have to go into the Exotic for him as I didn't see Maico brand Under the types of bikes on thumpertalk. Or maybe I'll sign up on a Maico forum

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A leak down test will determine engine condition pretty conclusively, but this is not generally necessary on a two stroke.  For a 500, if its got 100 hours on it, put a top end in.  If you don't know how many hours it has on it, put a top end in it.  If it has less than 100 hours, your top end has not yet reached the service interval.

Engine integrity tests are more useful on automotive engines where the condition can vary tremendously regardless of mileage.  A 500 will make great power right up until the piston fatigue fails from excessive piston slap and explodes at high RPM.  Compression only tests ring seal, which can be quite good despite a tired piston and worn bore.

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Definitely.
I ordered a top-end rebuild kit. Even though the guy I bought the bike from said it was fairly fresh. For 150 bucks I'm just going to replace it. It's been through at least half its life cycle since I got the bike even if it was pretty much new. I'll see what kind of compression I get after the rebuild and I'll know then what should be expected even with the decompression valve. Anybody that reads this in the future may benefit from the information

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Is your decompression device a button style like a chainsaw? Should not effect compression readings unless it is leaking. 

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