Compression

[Roachie33]

I have a 2001 seedo gtx di with 120 psi in cylinder closest to front and 90psi in cylinder closet to rear. I tested several times same or similar result. I added a little SAE 30 to both heads through plug opening and both cylnders tested at 130psi. I waited approx 2 hours and retested and they both tested 125Psi. Is it normal tohave to add some oil to get a good compression reading or is something wrong?

 

[DanSquires]

Something is wrong.
Adding oil actually gives you a false, high reading. Based on compression numbers, sounds like you're just ready for a rebuild.

 

[Sportster-2001-951C-Stock]

90 psi is pretty low, mine blows ~122 in both and still runs fine.

When was the last time it was run? The reason I ask is b/c there may be some slight rust on the cylinder sleeves interfering with ring seal and if it's not too bad will wipe off in a couple of minutes of running. Usually if there's some rust you will hear a scraping sound from the plug holes (plugs removed) while turning it dry, by hand. Adding oil will muffle that sound so it may not be detectable.

At this point I'd turn it over by hand and feel for tight spots (plugs out). If it feels smooth and hasn't been run in some time then fire it up and recheck compression.

I get 4-stroke motors out of our used bone yard all the time that blow low on one cylinder then perk right up to normal after a few minutes of running and I always check compression and oil the cylinders a bit before startup.

If the compression doesn't come back up, it's likely to seize b/c the piston will overheat and blow-by will destroy the oil film on the cylinder wall. Or it may have already seized, wiped aluminum onto the sleeve, or the rings may be worn too much.

These motors tend to grenade when they do seize, putting windows in the crankcase. This gets ugly and adds expense, so always a good idea to monitor compression.

 

[Roachie33]

The last time run for a length of time was the last week in Sept, but I have started a few times in my driveway to test compression and for other issues.
It was rebuilt in 2012 so I wonder if maybe some new rings would do the trick. Does the whole engine have to come out to do rings?

I also have a jerking issues when get to about 35-40mph. Would the low compression cause this as well? Just trying decide if I should consider a rebuild. Up to 35 mph it runs good

 

[Sportster-2001-951C-Stock]

These engines have a history of needing rebuild before 250 hours, reportedly. Since you've been running it lately, light rust in the cylinder interfering with ring seal probably isn't the issue.

I believe you should pull the RAVE valves and have a look at the piston skirts, examine them for scoring while rotating the engine by hand (PLUGS OUT) watch the skirts using a light through the open RAVE ports.

Some thought on what can go wrong, in no particular order:

But it sounds like you might be in need of a rebuild b/c 90psi is too low and you're at risk of grenading the motor if the low compression is due to poor ring sealing issues. Worn or poor sealing rings causes excessive blow-by and those hot gasses of combustion combined with excessive clearance between piston and cylinder wall will lead to torching the oil film and the piston.

If the piston breaks and flies apart, the rod will flop around in the case, probably break and otherwise do lots of damage to the engine cases.

You might get by with just a top-end rebuild but keep in mind, the bottom end may also be tired so there's some risk involved.

As far as the jerking goes, this might be a symptom of low combustion pressure as the cylinder will probably miss fire but this symptom may actually be the root cause of low compression as well.

The DI has both, a knock sensor and ability to detect misfire, so I expect detonation (knock) and misfire should cause MPEM to throw a code and/or go into limp mode.

Anyway, it's never a good idea to go for any long rides when a 2-stroke isn't running properly, damage is occurring when they aren't running correctly and running should be brief only for testing purposes.

You can do a top-end with the engine in the hull. Depending on what you find maybe you can get by with new pistons, rings and matching cylinder assembly all sized to correct dimension.

If the crankcase has been peppered with bits of disintegrated piston, the crank and rod bearings will probably be ruined by the contamination.

Perhaps the misfire is being caused by fuel system issues such as low fuel pressure or a clogged injector and this has caused cylinder damage.