2001 sportster le low compression but runs great

[Brh615]

I just ran my boat on the hose for a few minutes then tested both cylinders and they are both reading at 100 psi. The boat seems to run great, no hesitation at any rpm and has no problem running at 6800 rpm and about 47mph on the speedo. Should I be concerned about the psi or have it looked at before I damage something. The top end was rebuilt last season and has only 15hrs on the rebuild. New Pistons, rings, machines head and cylinders. Any info helps. Thanks

 

[Coastiejoe]

Use a different gauge, at 100 it can run, but not as strong as you describe. I bet a different gauge comes up with higher numbers.

 

[ORION '95 Speedster]

Low compression is not going to "damage" anything. If it runs good the way it is, don't fix it. If the compression eventually falls too far below 100, the boat will simply stop running. It may stay at 100lbs and run good for years.

 

[Brh615]

Well the gauge was one I rented from autozone and I tested the battery today and it was at 58% so I'm sure that a some to do with it. I'll keep running it as long as it running as well as it is. Thanks all

 

[ORION '95 Speedster]

58%? How did you manage to calculate that figure? I don't doubt the figure, I'm just interested in learning how to calculATE THat . :- + () =

 

[Brh615]

Digital battery charger. Reads voltage and battery charged percentage.

 

[Dr Honda]

Low compression is not going to "damage" anything. If it runs good the way it is, don't fix it. If the compression eventually falls too far below 100, the boat will simply stop running. It may stay at 100lbs and run good for years.

What the heck are you talking about !!!! Low compression will EAT an engine !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

AND... it WILL NOT hag out at 100 psi for years. it will quickly decay to a self-destruction mode.

The only time that MAY happen, is a low power, 4-stroke. (like a lawn mower) Where the heat is low... and the piston already gets oil from another source.


Basically... when you loose compression... something isn't sealing. If it's an old 2-stroke engine... that would be the rings. SO... since they aren't sealing... the hot combustion gasses are getting past them. In turn... it will burn off the fuel and oil that are protecting them... and then the skirts melt ! Not to mention... you will put hot (exhaust) gasses in the lower half... and that will displace the oxygen that the engine needs to run. (so it won't run right)

Now... if the engine was designed to run with low compression... and it's spec was 100 psi... then there isn't a leak.


But as Coastie said... a rotax engine at 100 psi probably won't run as strong as you are describing, and I would suspect that the gauge isn't reading properly. If it's truly at 100 psi static... I would think that it may not even start in the water.


As a final FYI... most of the time... below 115/120 psi on the 800 and smaller engines, is normally the point where they don't preform well. 145/150 psi is spec.

 

[ORION '95 Speedster]

The volt meter gives you a percentage? Weird, but handy I guess. As for the low compression, I disagree with everything that Dr. Honda said. No offense, but we are all entitled to our own opinions. Ya, 100lbs is not the best compression in the world, and it may continue to drop, but it might stay where its at and run just fine for a long time. Or it could die tomorrow, there are not certainties with engines. If it will start on land, why would it not start in the water or vise versa? Hypothetically speaking, if you were right about the top end melting itself due to low compression, then how do people wind up with 75psi compression readings all the time, rather than seized engines? If your melting the top end, its because you have a lean condition, no oil, a timing issue, or maybe the wrong plug heat range.

 

[Coastiejoe]

Starting on the trailer is MUCH easier as there is no load on the impeller. A low compression engine often will not start in the water as the impeller places a substantial load on the engine.

An engine can eat itself and not seize,, It happens often with the 2-strokes. So much so that the piston wears to the point that the top compression/oil ring can actually come off and then be on top of the piston. At that point, the piston beats it to death and it is pretty well trashed.

Even then, the engine still cranks or turns over but doesn't seize..

 

[ORION '95 Speedster]

Thanks for not banning my IP for disagreeing with you. Ya might be right about the water thing, but it would need to be right on the edge of not starting, or maybe a weak battery. I don't think that low compression causes piston or ring damage, I believe that its the other way around. Overheat the piston - which damages it, then the compression drops. As opposed to the compression dropping too low for some reason, resulting in piston damage.

 

[Coastiejoe]

I never mentioned "low compression". But the compression doe slower as the Pistons starts to eat itself for whatever reason, once it wears to much, the ring will pop off and come to the top.

If you do some reading here, you will find hundreds of posts saying "My ski starts fine on the trailer but won't start in the water..." Kind of posts.. First thing to do is a compression yes at that point. Most of them are at 120 psi or lower....

 

[ORION '95 Speedster]

I have had a ring come off before. It was a Kawasaki 750 Super Sport jet ski. The crank bearings failed, and the rods punched through the bottom of the engine case. The rod bearings, and the pieces of the engine case got sucked up into the top end, and the parts got smashed between the pistons and the heads, then the whole works get thrown into the expansion chamber. Basically, the motor chewed it self up, and sharted itself right out into the exhaust. The crank, case, rods, pistons, cylinders, heads, and spark plugs were all completely destroyed. The engine was not even good as a core.

 

[Brh615]

Update. Fully charged the battery and borrowed a compression tester. Ran the boat on the hose for a few minutes and retested. 120 outta the front and 125 outta the rear cylinder.

 

[Coastiejoe]

I figured it would be higher than the initial posting.. Pretty much had to based on how it was running for you..

 

[Dr Honda]

Thanks for not banning my IP for disagreeing with you. Ya might be right about the water thing, but it would need to be right on the edge of not starting, or maybe a weak battery. I don't think that low compression causes piston or ring damage, I believe that its the other way around. Overheat the piston - which damages it, then the compression drops. As opposed to the compression dropping too low for some reason, resulting in piston damage.

We don't ban for having an opinion... and at least you backed your reasoning. (but you are still wrong LOL) We ban for personal attacks.

There are a lot of engines out there that are designed with low compression, and run fine at 75~100 psi. But all the parts are working. But when you take an engine that has a static spec of 150, and it's down at 100... then it's on it's way out. AND, if you push it... it will eat itself.


And... I was just thinking about it... and I can't ever remember a seadoo/Rotax engine actually seizing. Normally... they will run really bad... or just lose all power, and you find one cyl sitting at 0 psi. (Generally the PTO piston)


Oh... as far as the comment of "Over heat the piston, and it cases the damage"... that's absolutely right. But, think about this... When the rings are going bad... that's what lets hot combustion gasses past... in turn, putting the heat on the skirts. AND... in turn, causing the damage. (snowball effect) Normally... the heat is held in the combustion chamber, and put on the crown. That heat is transported by the rings, into the cyl wall... OR... the in-flow of cool air/fuel is focused on the edges of the crown (in the squish band) and keeps the temps in check.

 

[Brh615]

It is the 951cc motor. What are these cylinders supposed to blow by own specs?

 

[AndyF]

I would say (from two stroke motorcycle experience) if you had new piston and rings 15 hours ago and you are running fully synthetic two stroke oil, it never had a chance to bed itself in to start with. After a rebuild I would run it on crap non synthetic oil for a tank of oil before switching back to synthetic. The oils these days are so good, rings don't get the chance to bed in to the cylinders which will give you higher psi.

 

[ORION '95 Speedster]

If you were blasting that much heat past the piston (enough to melt metal), wouldn't that ignite the near stoich fuel/air mixture that is in the crank? Your saying that the skirt is getting hotter than the top of the piston? That's the only way the skirt could melt from combustion gases and not melt the top of the piston. Doesn't make sense to me. I think people overheat the piston from running lean/lack of oil, and the piston expands from the heat, which decreases the clearance between the piston and the cylinder wall, resulting in massive friction, which increases the heat even more (but only on the skirt, not the top of the piston) and the surface of the skirt begins to rub off/melt.

 

[ORION '95 Speedster]

Update. Fully charged the battery and borrowed a compression tester. Ran the boat on the hose for a few minutes and retested. 120 outta the front and 125 outta the rear cylinder.

Your compression ratios look good. No need for concern or replacement of parts. Having compression readings of 110 / 110 would be far better than 110 / 150. Consistency is far more important than the actual reading, as long as they are at or above 100lbs.

 

[Dr Honda]

It is the 951cc motor. What are these cylinders supposed to blow by own specs?

OK...

That engine should be 135 PSI stock. So... 100 isn't as bad... but it's still significant.

 

[ORION '95 Speedster]

Agreed, higher figures would be ideal.