How to unstick small 2-stroke motor?

[Liet]

I tend to value skills based on a hypothetical zombie apocalypse, and I'm confident in saying that you'd make a great addition to the team.

Seriously, it's better than my healthy interest in thermal grease.

Way to go man, nice to have knowlege on something practical!

 

[Black88GTA]

Originally posted by: Liet
I tend to value skills based on a hypothetical zombie apocalypse, and I'm confident in saying that you'd make a great addition to the team.

Seriously, it's better than my healthy interest in thermal grease.

Way to go man, nice to have knowlege on something practical!

Heh. I consider myself a backyard mechanic with limited knowledge / experience. I can do basic to intermediate diagnosis / repairs, but there are people on this forum (and posting in this thread) that have likely forgotten more things about this stuff than I will ever know. Two-stroke motors are relatively unfamiliar territory to me. But, I am always eager to learn all that I can, so that I have the knowledge handy when I do need it - if not the hands-on experience.

Honestly, most of what I do know I either learned from repair manuals, internet forums, or breaking shit. Protip: Learn on a Chevy (or in this case, a free leaf blower), not a Ferrari. Your wallet will thank you.

 

[geno]

Originally posted by: Liet
As someone completely clueless about all this, I'd like all the posters in this thread to know that you're geniuses. What, did you all take classes or something?

I find it takes just a little initiative and a bit of free time to learn a lot. When something breaks, don't be afraid to try to fix it yourself, you'll save a lot of money. With the internet at your fingertips, there's really nothing you can't fix which you would otherwise pay someone else more for

 

[jemcam]

I used to race 2 stroke yamaha go karts and have stuck many a piston in 2 stroke air cooled motors. What causes them to stick is the piston is aluminum and cylinder liner is steel. When they overheat, the aluminum from the piston transfers onto the cylinder leaving streaks and the piston gets tights and sticks. If you shut it down fast enough, it would still run when it cooled down, but you would have a slight loss of power.

I used to stick the piston because you can adjust the fuel mixture on the carb while driving. The mixture could be adjusted since the carb was right under your right forearm and there was an extension that allowed you to do so. Many drivers would lean the mixture on the last lap or so and it would give you a slight increase in power, but you had to watch your cylinder head temperature, IIRC, you weren't supposed to exceed 335 degrees or so for very long. However, it was not unusual to push it to almost 400 on the last lap. Sometimes it worked, sometimes it didn't. It was worth it (at least on the last lap if you're in second right behind the guy in front of you and you needed a little boost a the end of a straightaway or if some guy was on your ass and you needed a little distance before going into that final turn.

The times that I stuck a piston, I would pull the head and cylinder, hone out the cylinder with a battery powered drill, install a new piston and voila! I did this many times between heat races, it took me less than 20 minutes to do this, from the time I pulled out the tools until it was idling and breaking in the new piston. As long as I had all the parts on hand, and I always did, it was not a problem to do with basic hand tools and a battery powered drill.

IT'S NOT THE RINGS THAT STUCK, THEY ARE SPRING STEEL AND ARE THE LAST THING TO MELT DOWN!

The longevity of this motor all depends on how bad it stuck. The only way to tell for sure is to pull the head and look at the cylinder. If you have some minor streaking from aluminum transfer, you'll be fine. If you see huge gobs, it ain't gonna last.

PS, don't pull the head unless you have a new head gasket, which in the case of the motors I used to work on, it was just a one time use copper ring that looked kind of like a big thin washer.

Someone up above got it half right through. Although the rings will not "fuse" the aluminum transfer will sometimes smear over the rings and cause the rings to not be able to move freely which will result in a slight loss of compression because parts of the rings (usually closest to the exhaust port) will not make good contact to the cylinder walls.

Edit again: The bottom end will be fine. You don't need a new cylinder, just buy a hone and use ATF while honing it with your power drill. You only need to hone it long enough to remove all the aluminum from the cylinder. NO MORE. Install new piston and rings and you're set. Again, make sure to replace the head gasket.

 

[slag]

Originally posted by: Eli

Originally posted by: Black88GTA
I don't have another plug handy...I would buy another, but the one in there already looks pretty good. I did take a dremel with a wire brush and clean up the electrode some before reinstalling it. The gas is good - after the mistake was discovered, my coworker drained the regular gas, refilled it with the proper mixture, and continued running it on Saturday when it seized up. It has about 1/2 tank left, and the gas is ~3 days old.

I got a pretty good look at the top of the piston through the exhaust port / spark plug hole, and it looks relatively undamaged. I saw a bit of gouging on the side that passes over the exhaust port, but it doesn't look really severe. From what I can see of the cylinder walls, they look pretty smooth too. The piston froze up about 1/2 way down the bore, so I was able to get a look at the area where it stuck through the exhaust port, once the piston was out of the way. The part I can see looks OK... but I couldn't see all of it.

Unsticking it took very little effort, so I'm hoping that the damage was minimal (although there certainly was some damage done).

ANY gouging to the cylinder wall is "severe".

It should be as smooth as glass all the way around.

Can't compare 2 cycles with 4 cycles. They won't take half the abuse before dying.

A 4 cycle will run on as little as 50PSI of compression.

You would be lucky to get a blip out of a 2 cycle with 80PSI.

Most 2 strokes will run fine with a minimal amount of cylinder wall gouging. Notice I'm not talking about just scratches, but grooves cut in the cylinder wall either from piston seizure or rings breaking off. General rule of thumb, if you can start it, it will run.

Smooth as glass would be idea, but 2 strokes will take a hell of a lot of abuse before dying.

My hobby is jetskis and I've seen it all--water ingestion, improper warm up resulting in 4 corner seizure, lack of oil, fuel starvation, detonation, etc. You'd be surprised how well some of these 2 stroke engines run with deep grooves in the cylinders, half of the piston missing, etc. I also collect lawn boys which are single cylinder 2 strokes and they run as well also with grooves in their cylinder walls, half a piston ring missing, blown lower seal, etc.

If it were me, after unsticking the piston, spray some WD40 in there and take a piece of emory cloth to the cylinder to remove any high material. Wash it out well with kerosene when done and use a clean rag until its spotless. Run it @32:1 oil ratio with some good opti2 2 stroke oil for a few tankfulls and then go back to normal ratio.

 

[Eli]

You don't overbore small 2 cycle engines like that. Not only is it just not worth it, but the cylinders are usually coated with Nicosil or something else, which would require replating.

It sounds like parts are pretty cheap. If you really want to fix it, go ahead and get a new cylinder, piston and ring. It sounds like it would be less than 100 bucks.

Oh, and gasket. Put it all together and it should be as good as new.

Is it a full crankshaft engine, or half crank?

If it's a half crank, it's only designed to last ~500 hours anyway before needing a top end rebuild.

 

[Eli]

Originally posted by: slag

Originally posted by: Eli

Originally posted by: Black88GTA
I don't have another plug handy...I would buy another, but the one in there already looks pretty good. I did take a dremel with a wire brush and clean up the electrode some before reinstalling it. The gas is good - after the mistake was discovered, my coworker drained the regular gas, refilled it with the proper mixture, and continued running it on Saturday when it seized up. It has about 1/2 tank left, and the gas is ~3 days old.

I got a pretty good look at the top of the piston through the exhaust port / spark plug hole, and it looks relatively undamaged. I saw a bit of gouging on the side that passes over the exhaust port, but it doesn't look really severe. From what I can see of the cylinder walls, they look pretty smooth too. The piston froze up about 1/2 way down the bore, so I was able to get a look at the area where it stuck through the exhaust port, once the piston was out of the way. The part I can see looks OK... but I couldn't see all of it.

Unsticking it took very little effort, so I'm hoping that the damage was minimal (although there certainly was some damage done).

ANY gouging to the cylinder wall is "severe".

It should be as smooth as glass all the way around.

Can't compare 2 cycles with 4 cycles. They won't take half the abuse before dying.

A 4 cycle will run on as little as 50PSI of compression.

You would be lucky to get a blip out of a 2 cycle with 80PSI.

Most 2 strokes will run fine with a minimal amount of cylinder wall gouging. Notice I'm not talking about just scratches, but grooves cut in the cylinder wall either from piston seizure or rings breaking off. General rule of thumb, if you can start it, it will run.

Smooth as glass would be idea, but 2 strokes will take a hell of a lot of abuse before dying.

My hobby is jetskis and I've seen it all--water ingestion, improper warm up resulting in 4 corner seizure, lack of oil, fuel starvation, detonation, etc. You'd be surprised how well some of these 2 stroke engines run with deep grooves in the cylinders, half of the piston missing, etc. I also collect lawn boys which are single cylinder 2 strokes and they run as well also with grooves in their cylinder walls, half a piston ring missing, blown lower seal, etc.

If it were me, after unsticking the piston, spray some WD40 in there and take a piece of emory cloth to the cylinder to remove any high material. Wash it out well with kerosene when done and use a clean rag until its spotless. Run it @32:1 oil ratio with some good opti2 2 stroke oil for a few tankfulls and then go back to normal ratio.

Industrial designed 2-strokes like in jet skis and the old Lawnboy engines are much more robust than your typical leaf blower or weed eater 2 cycle engine.

They won't take anywhere near the abuse that even the cheapest 4 cycle lawnmower engine will take.

You will be lucky to get a weed eater or leaf blower 2 stroke to fire, let alone run, with any significant gouging of the cylinder.

 

[jemcam]

Most 2 strokes will run fine with a minimal amount of cylinder wall gouging. Notice I'm not talking about just scratches, but grooves cut in the cylinder wall either from piston seizure or rings breaking off. General rule of thumb, if you can start it, it will run.

Agreed. For utility purposes, you'll never notice the loss of power. For race motors, every HP counts.

Also, I'd like to add that when you looked into the exhaust port, you were looking at the opposite side of the cylinder, which is the intake side and is the coolest part of the cylinder/combustion chamber in a 2 stroke motor, so that is the very last part to get any heat damage. Most people don't realize that the fuel in a motor actually helps cool it, thus the reason a lean mixture makes the motor run hotter and faster.

FWIW, the proper fuel mixture should produce a slight bit of blue smoke when accelerating. if it's contstantly smoking, it's too rich, and if it's never smoking, it's too lean.

 

[jemcam]

Originally posted by: Eli
You don't overbore small 2 cycle engines like that. Not only is it just not worth it, but the cylinders are usually coated with Nicosil or something else, which would require replating.

It sounds like parts are pretty cheap. If you really want to fix it, go ahead and get a new cylinder, piston and ring. It sounds like it would be less than 100 bucks.

Oh, and gasket. Put it all together and it should be as good as new.

Is it a full crankshaft engine, or half crank?

If it's a half crank, it's only designed to last ~500 hours anyway before needing a top end rebuild.

Boring and honing are two different things. We're not talking about boring, just honing.

 

[slag]

Originally posted by: Eli

Originally posted by: slag

Originally posted by: Eli

Originally posted by: Black88GTA
I don't have another plug handy...I would buy another, but the one in there already looks pretty good. I did take a dremel with a wire brush and clean up the electrode some before reinstalling it. The gas is good - after the mistake was discovered, my coworker drained the regular gas, refilled it with the proper mixture, and continued running it on Saturday when it seized up. It has about 1/2 tank left, and the gas is ~3 days old.

I got a pretty good look at the top of the piston through the exhaust port / spark plug hole, and it looks relatively undamaged. I saw a bit of gouging on the side that passes over the exhaust port, but it doesn't look really severe. From what I can see of the cylinder walls, they look pretty smooth too. The piston froze up about 1/2 way down the bore, so I was able to get a look at the area where it stuck through the exhaust port, once the piston was out of the way. The part I can see looks OK... but I couldn't see all of it.

Unsticking it took very little effort, so I'm hoping that the damage was minimal (although there certainly was some damage done).

ANY gouging to the cylinder wall is "severe".

It should be as smooth as glass all the way around.

Can't compare 2 cycles with 4 cycles. They won't take half the abuse before dying.

A 4 cycle will run on as little as 50PSI of compression.

You would be lucky to get a blip out of a 2 cycle with 80PSI.

Most 2 strokes will run fine with a minimal amount of cylinder wall gouging. Notice I'm not talking about just scratches, but grooves cut in the cylinder wall either from piston seizure or rings breaking off. General rule of thumb, if you can start it, it will run.

Smooth as glass would be idea, but 2 strokes will take a hell of a lot of abuse before dying.

My hobby is jetskis and I've seen it all--water ingestion, improper warm up resulting in 4 corner seizure, lack of oil, fuel starvation, detonation, etc. You'd be surprised how well some of these 2 stroke engines run with deep grooves in the cylinders, half of the piston missing, etc. I also collect lawn boys which are single cylinder 2 strokes and they run as well also with grooves in their cylinder walls, half a piston ring missing, blown lower seal, etc.

If it were me, after unsticking the piston, spray some WD40 in there and take a piece of emory cloth to the cylinder to remove any high material. Wash it out well with kerosene when done and use a clean rag until its spotless. Run it @32:1 oil ratio with some good opti2 2 stroke oil for a few tankfulls and then go back to normal ratio.

Industrial designed 2-strokes like in jet skis and the old Lawnboy engines are much more robust than your typical leaf blower or weed eater 2 cycle engine.

They won't take anywhere near the abuse that even the cheapest 4 cycle lawnmower engine will take.

You will be lucky to get a weed eater or leaf blower 2 stroke to fire, let alone run, with any significant gouging of the cylinder.

His was a very light seizure. It will run again. There is no indication of any "significant gouging" anywhere.

A gouge in a 2 stroke is a gouge, regardless of whether its an industrial/commercial engine, or a cheap pot metal leaf blower. The same rules of combustion apply and while I agree with your statement that 2 strokes require more compression to run optimally, a simple piston stick should be no big deal to any 2 stroke engine provided it unsticks easily and there is minimal transfer of metal.

 

[Eli]

Originally posted by: slag

Originally posted by: Eli

Originally posted by: slag

Originally posted by: Eli

Originally posted by: Black88GTA
I don't have another plug handy...I would buy another, but the one in there already looks pretty good. I did take a dremel with a wire brush and clean up the electrode some before reinstalling it. The gas is good - after the mistake was discovered, my coworker drained the regular gas, refilled it with the proper mixture, and continued running it on Saturday when it seized up. It has about 1/2 tank left, and the gas is ~3 days old.

I got a pretty good look at the top of the piston through the exhaust port / spark plug hole, and it looks relatively undamaged. I saw a bit of gouging on the side that passes over the exhaust port, but it doesn't look really severe. From what I can see of the cylinder walls, they look pretty smooth too. The piston froze up about 1/2 way down the bore, so I was able to get a look at the area where it stuck through the exhaust port, once the piston was out of the way. The part I can see looks OK... but I couldn't see all of it.

Unsticking it took very little effort, so I'm hoping that the damage was minimal (although there certainly was some damage done).

ANY gouging to the cylinder wall is "severe".

It should be as smooth as glass all the way around.

Can't compare 2 cycles with 4 cycles. They won't take half the abuse before dying.

A 4 cycle will run on as little as 50PSI of compression.

You would be lucky to get a blip out of a 2 cycle with 80PSI.

Most 2 strokes will run fine with a minimal amount of cylinder wall gouging. Notice I'm not talking about just scratches, but grooves cut in the cylinder wall either from piston seizure or rings breaking off. General rule of thumb, if you can start it, it will run.

Smooth as glass would be idea, but 2 strokes will take a hell of a lot of abuse before dying.

My hobby is jetskis and I've seen it all--water ingestion, improper warm up resulting in 4 corner seizure, lack of oil, fuel starvation, detonation, etc. You'd be surprised how well some of these 2 stroke engines run with deep grooves in the cylinders, half of the piston missing, etc. I also collect lawn boys which are single cylinder 2 strokes and they run as well also with grooves in their cylinder walls, half a piston ring missing, blown lower seal, etc.

If it were me, after unsticking the piston, spray some WD40 in there and take a piece of emory cloth to the cylinder to remove any high material. Wash it out well with kerosene when done and use a clean rag until its spotless. Run it @32:1 oil ratio with some good opti2 2 stroke oil for a few tankfulls and then go back to normal ratio.

Industrial designed 2-strokes like in jet skis and the old Lawnboy engines are much more robust than your typical leaf blower or weed eater 2 cycle engine.

They won't take anywhere near the abuse that even the cheapest 4 cycle lawnmower engine will take.

You will be lucky to get a weed eater or leaf blower 2 stroke to fire, let alone run, with any significant gouging of the cylinder.

His was a very light seizure. It will run again. There is no indication of any "significant gouging" anywhere.

A gouge in a 2 stroke is a gouge, regardless of whether its an industrial/commercial engine, or a cheap pot metal leaf blower. The same rules of combustion apply and while I agree with your statement that 2 strokes require more compression to run optimally, a simple piston stick should be no big deal to any 2 stroke engine provided it unsticks easily and there is minimal transfer of metal.

lol

He will not get this engine to run again without a new top end.

 

[slag]

Originally posted by: Eli

Originally posted by: slag

Originally posted by: Eli

Originally posted by: slag

Originally posted by: Eli

Originally posted by: Black88GTA
I don't have another plug handy...I would buy another, but the one in there already looks pretty good. I did take a dremel with a wire brush and clean up the electrode some before reinstalling it. The gas is good - after the mistake was discovered, my coworker drained the regular gas, refilled it with the proper mixture, and continued running it on Saturday when it seized up. It has about 1/2 tank left, and the gas is ~3 days old.

I got a pretty good look at the top of the piston through the exhaust port / spark plug hole, and it looks relatively undamaged. I saw a bit of gouging on the side that passes over the exhaust port, but it doesn't look really severe. From what I can see of the cylinder walls, they look pretty smooth too. The piston froze up about 1/2 way down the bore, so I was able to get a look at the area where it stuck through the exhaust port, once the piston was out of the way. The part I can see looks OK... but I couldn't see all of it.

Unsticking it took very little effort, so I'm hoping that the damage was minimal (although there certainly was some damage done).

ANY gouging to the cylinder wall is "severe".

It should be as smooth as glass all the way around.

Can't compare 2 cycles with 4 cycles. They won't take half the abuse before dying.

A 4 cycle will run on as little as 50PSI of compression.

You would be lucky to get a blip out of a 2 cycle with 80PSI.

Most 2 strokes will run fine with a minimal amount of cylinder wall gouging. Notice I'm not talking about just scratches, but grooves cut in the cylinder wall either from piston seizure or rings breaking off. General rule of thumb, if you can start it, it will run.

Smooth as glass would be idea, but 2 strokes will take a hell of a lot of abuse before dying.

My hobby is jetskis and I've seen it all--water ingestion, improper warm up resulting in 4 corner seizure, lack of oil, fuel starvation, detonation, etc. You'd be surprised how well some of these 2 stroke engines run with deep grooves in the cylinders, half of the piston missing, etc. I also collect lawn boys which are single cylinder 2 strokes and they run as well also with grooves in their cylinder walls, half a piston ring missing, blown lower seal, etc.

If it were me, after unsticking the piston, spray some WD40 in there and take a piece of emory cloth to the cylinder to remove any high material. Wash it out well with kerosene when done and use a clean rag until its spotless. Run it @32:1 oil ratio with some good opti2 2 stroke oil for a few tankfulls and then go back to normal ratio.

Industrial designed 2-strokes like in jet skis and the old Lawnboy engines are much more robust than your typical leaf blower or weed eater 2 cycle engine.

They won't take anywhere near the abuse that even the cheapest 4 cycle lawnmower engine will take.

You will be lucky to get a weed eater or leaf blower 2 stroke to fire, let alone run, with any significant gouging of the cylinder.

His was a very light seizure. It will run again. There is no indication of any "significant gouging" anywhere.

A gouge in a 2 stroke is a gouge, regardless of whether its an industrial/commercial engine, or a cheap pot metal leaf blower. The same rules of combustion apply and while I agree with your statement that 2 strokes require more compression to run optimally, a simple piston stick should be no big deal to any 2 stroke engine provided it unsticks easily and there is minimal transfer of metal.

lol

He will not get this engine to run again without a new top end.

I'd be very surprised if it doesn't run with just a little tuning and cleaning up. I cannot comment on the OP's mechanical ability.

 

[Eli]

Originally posted by: slag

I'd be very surprised if it doesn't run with just a little tuning and cleaning up. I cannot comment on the OP's mechanical ability.

I wouldn't.

The engine seized for Christ's sake.

I wouldn't say I have extensive experience with small 2-cycles like this, but I do have quite a bit.

They don't take abuse like this well. It only takes one streak down the exhaust port side of the cylinder wall to kill bottom-end compression enough to make it very difficult, if not impossible, to start.

It isn't top end compression that is the issue. That's plenty for combustion. The issue is getting the fuel air mixture from the crankcase and into the cylinder when the piston moves down.

 

[JJ650]

Originally posted by: jemcam

Most 2 strokes will run fine with a minimal amount of cylinder wall gouging. Notice I'm not talking about just scratches, but grooves cut in the cylinder wall either from piston seizure or rings breaking off. General rule of thumb, if you can start it, it will run.

Agreed. For utility purposes, you'll never notice the loss of power. For race motors, every HP counts.

Also, I'd like to add that when you looked into the exhaust port, you were looking at the opposite side of the cylinder, which is the intake side and is the coolest part of the cylinder/combustion chamber in a 2 stroke motor, so that is the very last part to get any heat damage. Most people don't realize that the fuel in a motor actually helps cool it, thus the reason a lean mixture makes the motor run hotter and faster.

FWIW, the proper fuel mixture should produce a slight bit of blue smoke when accelerating. if it's contstantly smoking, it's too rich, and if it's never smoking, it's too lean.

Right on!!
I race nitro R/C truggis and Buggies and there is ALWAYS a kid out there (or adult noob) that thinks the smoke is bad for it. I don't care how many times you tell him that there SHOULD be smoke they keep leaning out the mix. No smoke = lean.
Of course they learn when the compression is shot on the motor, or they keep getting a lean/bog condition and they have crap for acceleration.
The nitro rc engines don't have rings for compression. The engine sleeve is pinched to size and is an aluminum, brass, chrome (ABC) layered sleeve. Once the compression is gone...that's it. You CAN get it repinched, but the sleeve will be out of round.
It's always entertaining to see the guy out there with a brand new truck, with ZERO brake in time on the engine and leaning it out to 270 degrees + on a $400. The con-rod either snaps or he'll never get it restarted due to no compression. That's even after you try to help them out.

 

[SuperjetMatt]

It isn't top end compression that is the issue. That's plenty for combustion. The issue is getting the fuel air mixture from the crankcase and into the cylinder when the piston moves down.

Don't think so.
Crankcase compression ratio is FAR lower than topend compression ratio.
At sealevel, you'll see about 120-150 psi in the topend and 5-15 psi in the bottom end.

Even with severe gouging, getting a/f mixture thru transfers and boosts is not a problem.
Topend compression ratio MUST be at least 1:8 in order for the motor to work right. That is quickly lost through gouging.
Has nothing to do with the bottom end.

 

[Eli]

Originally posted by: SuperjetMatt

It isn't top end compression that is the issue. That's plenty for combustion. The issue is getting the fuel air mixture from the crankcase and into the cylinder when the piston moves down.

Don't think so.
Crankcase compression ratio is FAR lower than topend compression ratio.
At sealevel, you'll see about 120-150 psi in the topend and 5-15 psi in the bottom end.

Even with severe gouging, getting a/f mixture thru transfers and boosts is not a problem.
Topend compression ratio MUST be at least 1:8 in order for the motor to work right. That is quickly lost through gouging.
Has nothing to do with the bottom end.

Ah.

I could be wrong. That's just what I've always assumed.

Why will a regular 4-cycle engine run on ~50PSI(or less!) when you would be lucky to get a weed-eater style 2-stroke started with 90PSI?

The main difference is the fuel/air mixture transfer method. Combustion dynamics should be pretty much the same.. in fact, I would say it's better in the typical 2-stroke, with their small hemispherical combustion chambers.

Shrug.

 

[SuperjetMatt]

Ah.

I could be wrong. That's just what I've always assumed.

Why will a regular 4-cycle engine run on ~50PSI(or less!) when you would be lucky to get a weed-eater style 2-stroke started with 90PSI?

The main difference is the fuel/air mixture transfer method.

Neither 2 nor 4 stroke will run on 50 psi cranking compression.The primary compression ratio must be above 6-7 for the engine to run (has to do with thermodynamics and engine efficiency). Primary compression is in the combustion chamber, secondary compression is in the crank case (2 strokes only, obviously). A ratio of about 8-10 is about 110-160 psi (depends on air density)

However.....the 4-stroke has no holes in the cylinder above the piston save for the valves. So it has consistent compresson.
The 2-stroke has a huge hole in the form of the exhaust port. This takes away from cranking compression, unless you can crank it really fast. An electric start takes care of that. With a hand-pull start, this gets a bit more difficult. The slower you pull, the more air/fuel mix goes out the exhaust port, the less compression there is.
This is the primary reason why a psi compression number isn't all that meaningful for 2-strokes. Compression ratios (swept piston volume / combustion chamber volume) is much more reliable.
If you do want to go with psi numbers, dynamic compression (compression once it runs) is better, because speed doesn't influence it (nearly as much) and the exhaust pipe pulse gets figured in.

 

[Eli]

You sound like you know what you're talking about, but I don't know where you're getting this stuff. If it's from a book, you're ignoring the real world.. Which I am guilty of sometimes.

But none the less, a 4 cycle engine will happily run on 50PSI as long as the rest of the engine is mechanically sound. Sometimes they're even very easy to start.

They obviously lack power, but it will run.

I have seen engines run with as little as 40PSI of compression. Of course, these older small engines typically have compression ratios of ~6:1 to begin with...

They will also turn over at just a couple of hundred RPM...

Edit: I suppose I shouldn't be so broad. I am sure not all 4 cycle engines would run on 50PSI of compression.

Realize that the lack of compression must not be from a severe mechanical problem in my scenario. I'm just talking about severely worn engines.

 

[SuperjetMatt]

Touche, I should have been more careful with the term "cranking compression".
In the case of severely worn engines, you might indeed see something like 50psi during a compression test.
The rings and possibly valve seats are worn, letting some a/f mix go by at slow engine speeds. When the engine runs (and gets more oil in the cylinder & head) those leaks seal up a bit. Combine that with the higher running speed, and compression is not 50 psi anymore, but far higher.
So I am pretty confident that they're not "running" on 50 psi, but rather starting on 50 psi.

Next time you see that (I realize it's not something you run into every week, LOL), try pouring a little bit of oil in the spark plug hole. Should bring the compression right up.

EDIT: Turning over at 200RPM is a hell of a lot faster than I've seen anyone hand-start em.

 

[Eli]

Originally posted by: SuperjetMatt
Touche, I should have been more careful with the term "cranking compression".
In the case of severely worn engines, you might indeed see something like 50psi during a compression test.
The rings and possibly valve seats are worn, letting some a/f mix go by at slow engine speeds. When the engine runs (and gets more oil in the cylinder & head) those leaks seal up a bit. Combine that with the higher running speed, and compression is not 50 psi anymore, but far higher.
So I am pretty confident that they're not "running" on 50 psi, but rather starting on 50 psi.

Next time you see that (I realize it's not something you run into every week, LOL), try pouring a little bit of oil in the spark plug hole. Should bring the compression right up.

EDIT: Turning over at 200RPM is a hell of a lot faster than I've seen anyone hand-start em.

Yeah, fair enough. I understand what you're saying.

It's actually not hard to hit 500RPM when cranking an engine though. Of course you really have to rip on it, but you can do it.

Digital RPM gauges are awesome.

I have an old Wisconsin engine with an external magneto that will start at practically 0 RPM. It's actually really cool. The magneto fires regardless of RPM, you can turn it over by hand very slowly, and it will click and the plug will fire.

Anyway, if you spin the engine backwards by hand until you feel compression, then spin it forward, again by hand, so it goes through the intake stroke.. this is probably 50RPM.. lol..

There was no compression release on these engines, so it's very obvious when you hit the compression stroke. By hand, you can only turn it over very slowly through the compression stroke, as the compression leaks past the rings and such. But when you hit BTDC and the magneto fires, it will go Kerrrplunk..... kerplop.... plop... plop.. plop, yanking the PTO out of your hand.

I should make a video or something. It's hard to explain.

 

[SuperjetMatt]

That's pretty neat to have magneto that old that fires that well regardless of RPM. Nice engineering there.
Usually the power to the ignition coil comes from the magneto, so at lower speeds you'd have less power in the spark.
In newer systems, the power comes directly from the battery (if there is one).

 

[Eli]

Originally posted by: SuperjetMatt
That's pretty neat to have magneto that old that fires that well regardless of RPM. Nice engineering there.
Usually the power to the ignition coil comes from the magneto, so at lower speeds you'd have less power in the spark.
In newer systems, the power comes directly from the battery (if there is one).

Yeah. It's pretty neat. There is no battery. The magneto armature is driven by a gear on the crankshaft.

It' makes a loud CLANK noise when it fires at low RPM. I guess the points cam must be on some sort've spring loaded deal. Even at very low cranking RPM, you hear no noise from it. But when you turn it over slow, you'd almost think something was wrong.

I've lived in a place with no garage for the last 9 months. I now have a double garage.

It is time to break out all my engines! w00t.

 

[SuperjetMatt]

I just moved from a triple garage to a non-garage house.

I like 2-strokes. I should be getting my pro-shop built 754cc engine for my Superjet in the mail next week. Should be around 110-120 HP.
If only it weren't so cold.

 

[Crucial]

It's been a week. Any progress?

 

[Black88GTA]

Originally posted by: Crucial
It's been a week. Any progress?

No, nothing yet...I never was able to get it to catch and stay running. I was getting ready to tear it down last weekend, but my roommate decided to have some friends from out of town come and visit at the last minute. Since my workbench is also our beer pong table (and it was in constant use last weekend) I had to put it on hold. Plus, the room reeks of gasoline every time I work on this thing.

I'm going to try to get some work done on it this weekend...hopefully I'll have the area to myself. I haven't been able to order any parts yet, because I still don't know exactly what I need to get. Will know more once I get the engine apart again and can inspect it more closely.