Loss of compression?

[Jdubb94]

So I’ve already searched and haven’t been able to find anything on this so I figured I would ask you guys. Right before I hit top dead center on the compression stroke should it naturally decompress right before hitting top dead center? Or does that mean I’m losing compression? If I’m rotating the engine counter clockwise and I start to feel compression and the intake valve is open fully, will it naturally fall closed if I keep rotating just a tad or do I have a problem? I’m going to do a compression test Friday I just don’t have the kit yet.

[Jdubb94]

So basically after I feel the compression blow my finger off the spark plug hole I slightly rotate towards top dead center but it pretty much rotates on its own and I don’t even have to put pressure on this socket for the valve to close after I feel the compression if that makes sense. I tried to post a video but it wouldn’t let me upload

[Skyd]

Well if I'm reading this right, the reason the compression goes away is because it's pushing out of the spark plug hole that your finger is covering. But maybe I'm not understanding what you mean.


If you do the same thing with the spark plug in do you get the same result?

[Jdubb94]

Yes I get the same result with the spark plug in. It is after the compression I just didn’t know if the valve closed on its own like that normally. Like right after I feel the compression come out of the spark plug hole, without putting much pressure to turn the engine to TDC by hand it pretty much just rolls over to shut intake valve on its own like I don’t even have to really turn it myself. I’m really sorry if I’m not being clear I’m very new to all of this I just wish it would let me upload the video.

[JerryHawk250]

On the compression stroke both valves should be closed. Once you go past TDC the exhaust valve will open.

[Jdubb94]

Yes sir I’m 100% on how to find TDC just the rotating on its own after release of pressure/compression Is all I’m confused about. May be nothing but with my lack of experience I figured I’d try to see if anyone might have an answer. If I ever figure out how to post a video I will do it

[Skyd]

I'm guessing there's nothing to be concerned with here but I'll pose a couple of questions that are related. First, are you sure you're on the compression stroke and not on the intake stroke? During the compression stroke the valves should be closed. So during the intake stroke the air/fuel mixture is drawn in, then as the piston moves back up it compresses the mixture which is then ignited by the spark (slightly after TDC? I always forget if its after or before) anyway, then you have your exhaust stroke where the "waste" is removed and then we start all over.


So if you're sure you're on the compression stroke, my next question would be are your valves adjusted? because if they are too tight, then the valves may not be seating all the way allowing for air to escape and loss of compression.


Someone better at explaining and understanding this should be along soon, HAHA. But hopefully I gave you a couple things to ponder and check. If it were compression loss, it has to go somewhere, either out the valves, the spark plug hole, or leak down past the rings which I'm sort of doubting here.


EDIT: Looks like Jerry said it much more succinctly while I was responding.

[Skyd]

Originally Posted by Jdubb94

Yes sir I’m 100% on how to find TDC just the rotating on its own after release of pressure/compression Is all I’m confused about. May be nothing but with my lack of experience I figured I’d try to see if anyone might have an answer. If I ever figure out how to post a video I will do it

This is me not actually knowing but is it not just the weight of the crank shaft pulling the piston down on it's travels? It's a little hard for me to picture what you mean without actually doing it to my bike, LOL. I'm at work so I can check later.

[Jdubb94]

Yea that’s exactly what it feels like actually But like I said my lack of knowledge leads me to ask these potentially stupid questions all of the time lol also I have adjusted the valves about three different times because I keep seeing different specs so I went with what specs MegaDan said to do and just stopped messing with the clearances so they should be good.

[Skyd]

Originally Posted by Jdubb94

Yea that’s exactly what it feels like actually But like I said my lack of knowledge leads me to ask these potentially stupid questions all of the time lol also I have adjusted the valves about three different times because I keep seeing different specs so I went with what specs MegaDan said to do and just stopped messing with the clearances so they should be good.

Ya he's a great source of info, alot of the confusion comes with using metric verses SAE numbers. Glad you've got that sorted out and ya I think that might be what you're feeling but I'd have to do it on my engine to make sure the description matches...haha. I'm a visual/hands on person.


Also once you've reached maximum "squish" inside the cylinder, it's gonna wanna push back...think about a bike pump on an inflated tire.

[Jdubb94]

Genuinely appreciate the help/support y’all are kickass. Likewise I can read it Word for Word 100 times and still not understand but I can see it with my eyes and feel it with my hands once and I’ll remember. Hate that sometimes lol

[gwowzer]

It is just the weight pulling it out of the alignment. Mine does it too. At TDC it has the tendency to fall a little. I normally just support the tool I am using to rotate in a manner in which it won't spin the flywheel so I can do my valve adjustments.
I've found a 1/4 inch drive with the necessary adaptors to go up to the correct socket works great. Not a lot of weight to spin it at that point.

[TominMO]

Jdubb, nothing to worry about. My KPX does the same thing when I try to adjust the valves. You can't use a ratchet on it, as that will let it spin past where you want to hold the flywheel at TDC. If you use a wrench on the flywheel nut to prevent it from spinning, all good. In fact what I use is a socket, and a socket adaptor or cap with a six-point head, that lets me turn it with a wrench, letting me control flywheel spin.

One of these. Socket on one end, wrench on the other.
https://www.harborfreight.com/3-piec...aps-67011.html

[bigdano711]

Socket on a breaker bar works for me!