Friend's 250 engine compression "went from 110 to 30"

[Monkeytofu]

And I have no idea what that means. She said that her mechanic told her that meant her bike is dead and the engine is blown.

Can anyone explain to me what this means? Does that mean the body of the engine is broken and can't produce enough pressure to keep working?

I'm not going to try and diagnose what exactly caused the problem on her bike, but I'm interested in learning what engine compression means/ various causes that could "kill" an engine like that if anyone has useful links or advice.

[choneofakind]

From my experience, one thing that will "kill" compression is having the spark plug threads strip. Air will leak out, the bike will run poorly, and eventually, the plug will blow out while riding.

Compression is made when the air in the cylinder is forced to fit inside a smaller space when the piston goes up. Basically measuring compression tells you how well things inside the engine are sealing. In simple terms...

In my case, the threading on my head for the spark plugs leaked air, and the engine was hard to start, and it idled kinda funny. Also, when the plug blew out, that side of the engine did not run at all.

[csmith12]

In simple terms.... If there isn't enough compression to force the controlled explosion inside either of engine cylinders, it will not run at all or run under a reduced amount of power output.

Reasons for loss of compression (educated guesses here)

Major Causes
Piston rings smoked
Broken piston arm
Hole in piston
Cracked Head
Cracked Case/piston sleeve

Semi Major Causes
Bent/Burnt Valve
Spark plug threads/Hole
Cam chain (break leaving valve open)

[Racer x]

My first guess is valves . Adjustment or maybe a burned/leaking one.

[Lychee]

Originally Posted by csmith12

In simple terms.... If there isn't enough compression to force the controlled explosion inside either of engine cylinders, it will not run at all or run under a reduced amount of power output.

Reasons for loss of compression (educated guesses here)

Major Causes
Piston rings smoked
Broken piston arm
Hole in piston
Cracked Head
Cracked Case/piston sleeve (possibly a rod sticking out of side of engine)
Broken Valve

Semi Major Causes
Bent/Burnt Valve
Spark plug threads/Hole
Cam chain broken or Valve clearance VERY low ( leaving valve open)
Head Gasket Failure

Added some
4 stroke engine process is: suck, squish, boom, blow.
No compression means the gas cannot be squished. No squish, no boom, engine quits.

[alex.s]

what it means is you now have a sweet parts engine that you can donate to your local racers for a reasonable cost!

[bdavison]

I cant believe how many times everyones response to a motor malfunction is just to toss the motor and get a new one.

FIX the thing. Sheesh, it ain't that hard.
Let me know if you need the service manual. Ill send it to you just out of sympathy for these engines that just need a little TLC.

[Monkeytofu]

Originally Posted by bdavison

I cant believe how many times everyones response to a motor malfunction is just to toss the motor and get a new one.

FIX the thing. Sheesh, it ain't that hard.
Let me know if you need the service manual. Ill send it to you just out of sympathy for these engines that just need a little TLC.

I'm not going to try and convince her anymore than I have. Some people just don't have the drive

Thanks for explaining it to me, hopefully this won't ever happen to my bike.

[choneofakind]

If she's not going to repair the engine, at least see if she'll give it to you. You can repair it, bore it, port it, and install it in your bike.

Then sell her your stock engine.

[Monkeytofu]

Originally Posted by choneofakind

If she's not going to repair the engine, at least see if she'll give it to you. You can repair it, bore it, port it, and install it in your bike.

Then sell her your stock engine.

I might list her bike up here for people looking to buy parts to get into contact with her. It's all in great condition other than the unknown engine problem.

I know next to nothing about boring engines out or working on the internals of one; maybe next summer if I'm still in the states or during the winter break if I feel comfortable going beyond valves

[FrugalNinja250]

In nearly all cases, the labor and parts cost of a major repair inside the motor far exceeds the cost of replacing the motor with a good used one. The main things that can cause a sudden (as opposed to a gradual) loss of compression are a burned or stuck valve, hole melted in piston, stripped spark plug, or failed head gasket. All of these are fairly easy to diagnose in situ.

[bdavison]

Originally Posted by FrugalNinja250

In nearly all cases, the labor and parts cost of a major repair inside the motor far exceeds the cost of replacing the motor with a good used one. The main things that can cause a sudden (as opposed to a gradual) loss of compression are a burned or stuck valve, hole melted in piston, stripped spark plug, or failed head gasket. All of these are fairly easy to diagnose in situ.

I disagree with this.

Let's just take one of your examples.
A holed piston.
I can get a new piston online for under $40 bucks, and that includes new wrist pins, and circlips. Now show me a new/used motor for less than $40 bucks.
Or how about a failed head gasket? under $20 bucks online...

It pays to learn how to work on bikes, and can save you a bunch of money if you just fix it yourself. Its like valve jobs on the 250R. You pay someone to do it, its probably gonna cost $300+ Order a shim kit and do it yourself for $40.

The answer to every problem is not to "order a new engine", unless you just want to take the lazy route and dont mind throwing money away.

Working on these engines doesnt take a rocket scientist. All you need is a service manual, and it tells you step by step how to fix anything on the bike, and a willingness to get your hands dirty.

[Motofool]

I would add that it is hard to know the real condition of the "new" engine.

[choneofakind]

Replacing individual parts might be easier and cheaper, than getting a used engine, but not everyone is brave enough to crack open an engine like that. Takes a lot of work. And lots of work means it would cost a lot to pay someone else to do it.

[dirty nasty]

Originally Posted by FrugalNinja250

In nearly all cases, the labor and parts cost of a major repair inside the motor far exceeds the cost of replacing the motor with a good used one. .

that's why it's called DIY.

I could use a parts motor. I have a feeling with the way I keep going on this bike I'm going to be needing one in the future.

[FrugalNinja250]

Originally Posted by bdavison

I disagree with this.

Let's just take one of your examples.
A holed piston.
I can get a new piston online for under $40 bucks, and that includes new wrist pins, and circlips. Now show me a new/used motor for less than $40 bucks.
Or how about a failed head gasket? under $20 bucks online...

It pays to learn how to work on bikes, and can save you a bunch of money if you just fix it yourself. Its like valve jobs on the 250R. You pay someone to do it, its probably gonna cost $300+ Order a shim kit and do it yourself for $40.

The answer to every problem is not to "order a new engine", unless you just want to take the lazy route and dont mind throwing money away.

Working on these engines doesnt take a rocket scientist. All you need is a service manual, and it tells you step by step how to fix anything on the bike, and a willingness to get your hands dirty.

I guess you overlooked the part where I said "labor". Of course, if you can DIY major engine repairs then your options open up a bit, though if something major like the rotating assembly or major casting is damaged it will still be cheaper to buy a used engine. For instance, a new crank from RonAyers.com is $416.66 plus shipping. With that you should get new bottom end bearings (crank and rod), that seems to run about $80 for a full set, and don't forget all the seals and gaskets you need, that ought to run another $100 or so. And, that assumes the rods are fine. Putting $600 into just a bottom end rebuild on a motor (leaving the pistons, valves, etc, just as worn as before) that can be replaced for less doesn't seem like that good of an investment to me.

In any case, the OP apparently is paying a mechanic rather than attempting a DIY repair, for whatever reason that is appropriate for them. Maybe they live in an apartment that forbids working in a parking spot? Or maybe they don't have tools (and a basic set of tools can easily run hundreds, not counting any special tools needed for internal engine work).

In any case, a piston is $33.67, ring set $30.54, head gasket $22.73, and a set of head bolts (I don't know if they're TTY or not, but when in doubt, assume they are) is $34.51. That adds up to $121.45 plus shipping, and state sales tax if appropriate. One thing to consider on a high mileage engine is that one cylinder with new rings and one with used will create a compression imbalance that will make the engine a little rough and hard to sync, generally if replacing pistons on such a motor you should do both. Also, this assumes not honing the cylinders to help the new rings to seat properly. To do this repair right you should pull the cylinder block and get the cylinders honed, that requires either a special hone tool or paying someone to do it, plus more gaskets and seals.

Of course, a half-assed repair will be far cheaper, but one gets what one pays for when cutting corners...