High compression engine ping?

[lazylightning@mail.r]

I removed metal from the head and the cylinder deck and widened the transfers and ports to the max. When it's cool out, it runs like a bat out of ****. At idle, it's pretty rough and sounds like a diesel. No vibrations at any higher rpm noticed. Today I took the head off and noticed little dings in the surface of the combustion chamber in the head. The larger ones are all located at the outer edge of the chamber and the once smmothed (polished) hemisphere is slightly course everywhere, evenly, except for the outer edge where there are quite noticeable gouges in the surface. Then I examined the cylinder walls. Smooth as a baby's ***. Absolutely mirror smooth. Not a scratch anywhere in the cylinder wall. Then I noticed that the piston top suface is identical to the hemispherical chamber in the head. The same deep gouges at the outer edge and smooth - roughness on the general majority of the surface in the center. Wow!
Is this the result of detonation from having gotten the compression ratio up pretty high. I use 95 octane gas....not sure what I'd add to make 107....lol!)))
Well, after removing the cylinder I have noticed a single fairly deep gouge in the wall of the cylinder. The piston doesnt seem to wobble more than before, so I doubt its the wrist bearing, but I'll take a look at that too. I noticed some metal flakes and pieces stuck to the upper wall of the cranckase on the intake side. It's still not clear to me where that much metal came from. Wondering now if a recent conflict at work might be the source of the problem. Bike is parked/locked unattended behind hangar during work hours. I guess I'll have to have that crankcase apart now to clean it out.

 

[Fabian]

Just a matter of time before your big end connecting rod bearing tears itself to into itty bitty little pieces.
Those little dings i the piston are a sure sign that bits of metal are removing themselves from the bottom of the engine and making it to the top of the engine whereby they're squashed in between the piston and cylinder head.

It's quite possible that your engine sounds like an old clattery diesel because the needle rollers on the big end connecting rod bearing are breaking up, mostly from the the incorrect ignition advance curve of the standard CDI, but also a good chance the engine has had a period of high revs combined with incorrect jetting which has induced detonation, further exacerbating the problems associated with the standard CDI...

Time to replace the crankshaft (because it's f*&^ed) , then fit a Jaguar CDI and work on the carburation to optimise the jetting.

 

[lazylightning@mail.r]

Hi Fabian,

Thanks for the tips. I noticed a loss of power the day before and when it happened, I couldn't start it. I thought it might be siezed because the engine didnt turn over when I would try to pedal start it and it was hard to pedal. I moves easily in the cylinder when the head is removed and there is minimal scratching in the cylinder. I think it just happened and didnt have time to do maximum damage. Hopefuly it was just the upper, wrist - piston rod bearing. It was stock. I will go ahead and change the lower bearings too. I hope the crankshaft is fine, it came really nicely balanced. Actually the port heights were just perfect too. I think there are a number of factories in China making different models and they probably analyze our bike forums to get ideas for improvements that they seem to have been making.
I'm using a stock NTTC carb that I drilled out to .8 and soldered the end of the needle to make it symetrical in width to the end. So no jetting options here. I'm also using a stock muffler and cdi. My piston skirt was trimmed 3mm higher than it should have been, so I lost power on the low end, but maybe gained on the upper. Here's a video of how it sounds. I start to open it up about 2:30 into the vid: http://www.youtube.com/watch?v=jE580ppFwng

Paul

 

[lazylightning@mail.r]

http://www.youtube.com/watch?v=sWBthIbQvIU

This one might be more interesting if you have time. I made it in May, here in Moscow at the northern edge where I live.

All my bearings were stock too. And I'm running a 35 tooth sprocket on a 26 X 2.35 wheel.
My biggest problem is the rear wheel. So if you have any ideas where I can get an absolutely indestructable rear metal mag rim that doesnt bend. I would be interested. Considering fabricating my own with a large hub and super fat homemade spokes from some kind of instrumental steel/ tempered and all.

 

[Fabian]

lazylightning, did you get my second post showing the photos of connecting rod and bearing damage?

Maybe i've posted it somewhere else but it seems to have been deleted from the thread?

Fabian

 

[lazylightning@mail.r]

Hi Fabian,

I didn't see the post with the photos. Well, I got the engine apart and wiped out all the metal bits I could and washed them from my finger tips into a glass of hot soapy water. I collected quite a bit of steel(magnet attractable)....turnings. It's flakes and bits of steel that are character to lathe turnings. The bearings seem absolutely fine, though I haven't opened any of them yet. It seems to me so far, judging by the kind off metal bits that someone added them to my intake. We have a lathe shop in our hangar at work. At the begining of the week I raised a stink about the painter spraying his paint at the entrance of our unventilated hangar and not waiting for the forklift driver to leave so we could close the doors. I yelled at the forklift driver too cause he's also a sunny beach and doesnt wait for the old grandaddy with the paint sprayer to finish just a couple quick passes and just opens the doors and lets the poison fumes clouds in. I tried to just ignore it the week before when it happened but it made me really sick for two days. So when it happened again I started accusing them of poisoning us and not giving a duck about our health. The main engineer had a minor stroke a couple of weeks ago and hasn't been back yet and I reminded them of that and they didn't like it.
I'm going to go pull the piston off, and if the upper bearing is okay, then I don't think there would be reason to think that the lower bearing is bad. They both "feel" symetrical and turn easily and smoothly as if nothing is wrong...and judging by the large amount of bits of steel in the crank case, I really doubt they came from inside my engine....

............one word : Drama

 

[lazylightning@mail.r]

Examining the connecting rod connection to the crankshaft more closely, I've noticed some of the needles are at angles and not perpendicular. Some seem to be missing. So maybe they were just squashed into flat shaving like pieces.
The main question for me, since I'm into racing and off roading, how do I pry that crankshaft apart so I can replace those lower rod bearings with some good high performance racing bearings. Even if I have to get a new crankshaft, I can't go without good high compression and high rev's. So I'll have to take the new crankshaft apart and replace those bad bearings with good ones.... And what about high performance connecting rods? Do they ever break?

 

[Fabian]

I'm going to post the link a second time, just in case it was my error in posting the link incorrectly. I'll also send it to you in a PM:

http://www.motoredbikes.com/showthread.php?23865-Failure-of-Happy-Time-style-2-stroke-Big-End-Connecting-Rod-Bearing&highlight=connecting+rod%2Bfabian

Back then i didn't know that the standard CDI was the cause of detonation issues, so i compensated by going heavily rich on the air/fuel mixture.

 

[Fabian]

When you have a look at the photos you will see why an ignition curve suited to a 2-stroke engine is absolutely necessary.

The Jaguar CDI has been designed to suit 2-stroke ignition curve and overcomes the (virtually linear) 4-stroke advance curve of the standard CDI.


Upon first hearing those barely audible big end connecting rod rattles, the needle rollers have already been hammered with flat spots as the hardening material has worn away; getting blown through the transfer ports and into the combustion chamber.
Next step in the process is the needle rollers resisting to roll over the crank pin surface, just waiting to get hammered again in the same spot.
From that point the needle roller just takes a general hammering on all sides, being compressed to a smaller diameter but also lengthening, whereby it starts to bite into the crankshaft, furthermore resisting any rolling action.

Now it gets terminal and unbearably rattly as the connecting rod has free play to completely move away from the needle rollers, allowing it a period of acceleration when the spark plug fires, then slamming back down on the needle rollers with serious force; braking away bit sized pieces and getting blown through the transfer ports, either jamming in between the piston and cylinder or getting thrown about in between the piston and cylinder head; causing visible dents.

If the small end bearing in the connecting rod looks ok, there's a good chance the needle rollers in the big end have begun to break up.

 

[Fabian]

This action is also another reason why you don't over rev a 2-stroke

As the cranshaft rotates through 360 degrees, the needle rollers will reverse direction as the connecting rod angle changes with respect to the rotation of the crank pin.
The needle rollers can spin only so fast before they can't reverse their spin motion over the crankpin surface, momentarily coming to a rotational stop whilst still sliding.
This puts microscopic flat spots on the hardening layer and over time the hardening starts to break up - impact of continual detonation will also cause this layer to break up.