281xp with 288xp Cylinder Compression Problem

[YATYAS]

Hello Everyone, I would appreciate some help trying to get this problem solved.
I have an old 281xp that has been sitting around for over 20 years that I decided to get running again. After giving it a few shots of fuel down the carb it fired up after 2 pulls. The original jug and piston was pretty worn but had no major damage and was at 130psi. I decided to order a new Hyway 288xp jug and piston for it since it needed a new top end anyway.
After putting the new 288xp top end on I'm only getting around 95psi out of it. I plugged the decompression, no change, installed new oem decompression valve, still no change so I pulled it off and checked the ring end gap. The gap is at .012 and the old 281 is at .026 but has higher compression. What I also find weird is that on the new jug as the ring gets further up to the top the gap starts to open up. I've checked the base gaskets and both are the same so I'm wondering if the squish band on the new cylinder was milled correctly and if it has too much squish. But why would the ring open up as it gets closer to the top? I have less than a tank of gas through the new top end and it seems to run fine other than low compression. Any ideas? Bad cylinder or ring? I'm at a loss on what the issue might be.
Thanks

 

[YATYAS]

Update.

I checked the squish in the old 281xp jug and it was .039 and in the new 288xp it was .041 so I'm still not sure why it has so much lower compression than the 281. I know the ring hasn't seated yet but I can't believe it would be the issue.
I've attached some pictures of the saw and the jugs.

 

[Ronie]

Update.

I checked the squish in the old 281xp jug and it was .039 and in the new 288xp it was .041 so I'm still not sure why it has so much lower compression than the 281. I know the ring hasn't seated yet but I can't believe it would be the issue.
I've attached some pictures of the saw and the jugs.

Your right, 95psi seems very low. I don't think the saw would be running good with the compression that low. Maybe your compression gauge isn't reading right?

 

[Ronie]

If your interested here's a link to the Hyway cylinder I put on my saw. https://www.arboristsite.com/community/threads/hyway-288-cylinder.335539/

 

[YATYAS]

Your right, 95psi seems very low. I don't think the saw would be running good with the compression that low. Maybe your compression gauge isn't reading right?

Hello Ronie,
I used the same compression gauge on both top ends and it's what I use on all the dirt bikes so I feel pretty comfortable with it. There is also a huge difference between the two when you pull them. With the 281 I have to set it in the floor and pull hard and with the 288 I can just leave it on the bench and pull. You also can see and hear the difference between the two when they are idling.

 

[homemade]

Poor machining in after market jugs can cause ring gap to change thought out the stroke. I usually see it fat at the bottom from guys “over-honing” and not getting full stroke on the hone.

And with the bad compression, the port timing could be off.

 

[Ronie]

Hopefully one of more experienced members will be able to help you out.

 

[MacAttack]

Are a 281 and 288 the same stroke but just a slightly bigger bore on the 288?

 

[YATYAS]

Are a 281 and 288 the same stroke but just a slightly bigger bore on the 288?

That's what I understand.

 

[YATYAS]

Update:
Well after speaking with the people I bought the cylinder from they were at a lost also on what the issue might be. I exchanged the piston and cylinder for a Meteor this time and the ring end gap was a constant .006 and the squish stayed the same. I idled about a half tank and bucked some firewood with a full tank. Saw ran great and seemed to pull the 28" bar fine, although I wasn't bucking anything big and I had it running pretty rich for the first tank. So after that I checked the compression and it was only 85psi. Using the same tester on the old top end with the same squish and a .026 ring end gap it read 130psi. All I can think of is that the ring needs to seat so I'll run some more tanks and let you know. What I find odd is that when I do a top end on one of the kids 2 stroke dirt bikes you can immediately see a feel the increased compression, I would think that it would be the same for this saw.

 

[ELECT6845]

Are you checking it hot?

 

[YATYAS]

Are you checking it hot?

I've checked it with the cylinder is warm.

 

[Ronie]

I think that's still low for a warm cylinder. Does your compression gauge have a valve at the end of the hose where it screws into the spark plug hole? If not, the combination of checking it hot and no valve cold be the reason it's low. Also are you pulling it over until the needle stops moving or only a couple of times.

 

[YATYAS]

I think that's still low for a warm cylinder. Does your compression gauge have a valve at the end of the hose where it screws into the spark plug hole? If not, the combination of checking it hot and no valve cold be the reason it's low. Also are you pulling it over until the needle stops moving or only a couple of times.

The part that screws into the cylinder has a schrader valve on the end that goes into the cylinder and then there's the button you push to relieve the pressure on the gauge. I just checked the saw cold and it was at 90 psi and I pull it till the needle stops and then go a few more times just to make sure. I've used the same gauge and same technique on the old top end and came up with 130 psi. You can also feel the low compression in the rope when you start it.

 

[Ronie]

I would try and return that cylinder. Maybe there's a spot that's allowing some of your pressure to escape past the ring.

 

[JohnT34]

Hi the issue here for me is that you state the saw ran great and then you checked the compression and it was only 95 psi
I think the saw wouldn’t run at that compression it certainly wouldn’t be easy to start if even possible at all
Secondly you state that the original cylinder had 130 psi when checked but that you had to put it on the ground to pull it over indicating it had decent compression. I imagine if it only had 130psi it wouldn’t be too hard to pull over
To resolve this issue I suggest the following
1. Get a new pressure tester I am not convinced it’s accurate
2. reuse original piston with new caber rings or put in new meteor piston
3. Use the original cylinder but remove the base gasket and seal with 1184 or dirko




Sent from my iPhone using Tapatalk

 

[YATYAS]

Hi the issue here for me is that you state the saw ran great and then you checked the compression and it was only 95 psi
I think the saw wouldn’t run at that compression it certainly wouldn’t be easy to start if even possible at all
Secondly you state that the original cylinder had 130 psi when checked but that you had to put it on the ground to pull it over indicating it had decent compression. I imagine if it only had 130psi it wouldn’t be too hard to pull over
To resolve this issue I suggest the following
1. Get a new pressure tester I am not convinced it’s accurate
2. reuse original piston with new caber rings or put in new meteor piston
3. Use the original cylinder but remove the base gasket and seal with 1184 or dirko




Sent from my iPhone using Tapatalk

Hello,
When I exchanged the cylinder for a Meteor it came with a Meteor piston and Caber ring. The only reason I checked the compression to begin with was because after replacing the first top end it had noticeably less compression than the old cylinder that you could feel when starting it and is still low with the second new cylinder. I'm sure my compression gauge isn't 100 percent accurate but just using it for comparison between the two it shows a big difference that you can also feel. I will borrow one from work and see what it says. I'll also try to put a video up with the saw running and try to get one with it in the cut.
Thanks

 

[mbrick]

Don't have a ton of experience myself, but I am surprised it runs a 28" bar well at 95psi. Can you borrow a gauge from someone else for a sanity check? Is the o-ring where the gauge meets the cylinder head ok?

On my project 576xp with a pretty worn P&C it read 140psi.
When I first reassembled with a new OEM P&C (with some oil) and base gasket, it was 145psi before being ran. I wouldn't expect it to have to 'bed in' for long to reach a good compression number. I think it should be min 125psi but likely 150+?

How many pulls before the pressure reading stabilizes? 8-10 or so?