Hello folks, I know this is not an OEM saw from Europe. I don't expect top performance. I had another log load dropped off two weeks ago and before I started chopping it up I wanted to eliminate the decompression buttons on both my dolmar 6400 and my clone 372xp. I took compression readings before and after on both saws to see if the plugs made a difference. No difference, the decomp buttons were fine. What I did notice was that the well used dolmar had a cold compression of 195 psi and the clone saw only 125 psi. Both start and run great but the dolmar has always been a bit tough to pull. The 372 had zero scoring or anything on the cylinder as viewed through the spark plug hole. Just factory hatching and nice looking oprt openings all around. Good color on piston top and always run 40:1 with red armor. I tune for 12000 rpm no load. Is this a normal compression reading? I did tape the throttle wide open during the compression test. Sure, it would be fun to replace the piston with a two ring pop up piston and maybe no base gasket but the thing pulls the 28" bar just fine. It has cut about 7 cords of wood so it should be broken in. Maybe? I thought I would see closer to 150 psi stock. Any thoughts?
How do you get along without the decomp on your 6400? If I forget to push mine in it will hardly pull over!!
Big monkey arms I guess. At least my right arm! My experience with decomps was poor. It would flood the saw and always made things start harder so I never used them. Then I bought a two pack of plugs to solve that problem. I have a keep it simple, dependable, durable philosophy for as much as I can control. So I wanted to remove that failure point. I'm finding that even OEM husky 372xp saws were known for pretty low compression from the factory.
I’ve heard that the Chinese rings are super soft. They break in fast, but wear fast as well. If it were me I’d put a Caber ring in it; they’re inexpensive. Also a base gasket delete when putting the cylinder back on as well
125 PSI would be on the low end of normal IME...BUT, it depends on the specs...the piston/ring(s)/cylinder may not be worn, it may just be spec'd as a low compression engine...lots of things play into that...piston/wall clearance, ring end gap, overall compression ratio (cylinder volume at BDC (or when the last port closes off) vs combustion chamber volume at TDC) Can't disagree with this...I did a base gasket delete on my farmertec MS660 build (after checking clearances) and it's stout! I hate trying to start it without using the decomp! Broke the pull cord more than once!
Would putting oil into the cylinder and rechecking compression tell me anything? I don't even know for sure if this piston has one or two rings. Definitely wouldn't want to be breaking cords.
That would probably give you a false compression reading, much higher than normal. I’ve put together a few saws where I had doused the cylinder walls with a liberal amount of 2 stroke oil, then had a bear of a time pulling them over at first.
Adding oil will likely make the psi go up for the test, but that doesn't necessarily tell you anything. You could pull the muffler off for a peak at the piston/ring(s) and cylinder walls too...can see more than just looking down through the spark plug hole...
Sure...hot or cold isn't a deal breaker either way...just make sure to have wide open throttle...then pull until the reading stops increasing by any meaningful amount.
I am usually wanting to see at least 145psi out of a 2T engine. 150-160 is better. How does it run, once started? How much squish on cylinder?
Runs great. Seems to make enough power. Not as snappy to rev as the dolmar at 195 psi but that could be carburetion. Starts and idles smooth. Revs right out to 12000. I've never actually measured squish on a cylinder before. Can I or should a I do it on this assembled engine? I do have some digital calipers.
I'm for sure not and expert...you need a length of solder...6". Pull your spark plug and insert the solder. Cycle the flywheel with intent to smash the solder between top of piston and side top of cylinder wall. Remove solder, measure thickness of smashed solder. This is the way I learned to do it which is probably the redneck way. My way is sortof close to what the AI says. I didn't pull the jug to do it. Worked for me once in past. Saw came away with 165lbs. (wasn't really low to begin but I was tinkerin') You just gotta make sure your cylinder won't slap top of jug. I'm betting that you can pull your cylinder gasket and bring that compression up a bit. (Hondabond sealant instead of gasket) The pros shave the bottom of the jug to accomplish same.
Yup, that's how I do it...bend the solder so you can push the tip over against the cylinder wall before turning the crank to and past TDC. Some (most) combustion chambers have less piston/head clearance over by the cylinder wall
Here's a website I've used in the past, for dirt bike stuff but it also applies to chainsaws. A little hard to measure squish as they suggest when the head is not removable. They show a picture, about 2/3 down, of how to bend your solder to get the most accurate measurement but it probably won't work on a chainsaw. Squish in 2-stroke engines
64-7900 saws are known to seep fuel through the inlet needle and into the case. If that fuel ends up on top of the piston you can end up getting abnormally high compression readings. Seems to me it should be about 160–170 but it’s been a while since I’ve checked the 6400. You might run it for a minute to be certain it isn’t flooded and then try testing the compression. Don’t need to hold the throttle open in a two-stroke. If the throttle is closed, it’ll suck air through the muffler to get a proper reading
I forgot about about the piston rock thing...I measured mine "to the side" in other words, the solder parallel with the wrist pin, that should minimize how much the piston can rock, giving you a false reading...at least it works on a new/tight engine, in my mind anyways. IIRC I took my squish down to .020" or so...which is about the minimum on a 660, from what I've been told...so far so good anyways.
I'm sure your safe. I think I got my ktm 300 almost down to .020". Maybe it was closer to .030". I'd have to check notes to know for sure.
