blown motorized bike carnage!

for the benefit of all I hope that anyone who has had a blown engine will share all details such as
engine oil used
at what ratio
size engine
any modifications to engine (compression increase, porting, carburetor, exhaust pipe)

-25:1 oil ratio
-66cc (generic but not bgf)
-boosted compression by shaving my original head
-carburetor set to one notch richer on slide pin
1 hole drilled through my old-style exhaust pipe baffle tube for a very slight increase in flow due to the compression increase.
-boost bottle
-wallmart brand 2 stroke oil (yeah I know shame on me)

rings were pretty worn down. minimal carbon for such mileage. ran it good and rich, just ran it pretty hard in the rpm's. lasted many miles for what I put it through. rings failed due to the fact that they are simple iron rings. I do SBF & SBC builds and always use moly coated rings(what all cars run today). those iron rings only last 80 thousand on a car tops and they aren't running between 3500 to 5500 rpm none stop.

wristpin wear was very very little despite the deformed wristpin. could not feel any up and down play in the connecting rod.

ran darn good right up until its final wheelie. was actually still running on that junk, I just shut it down because I knew it was had and did not want shavings going into crankcase.

bottom end is still very solid. 35 dollars in parts waiting at home to completely redo the top end. will make a vid on that when I do it so all can see how that's done.


there I do believe I have explained myself.
 
you mentioned worn out rings but not what actually caused you to want to shut it down and then rebuild it
 
you mentioned worn out rings but not what actually caused you to want to shut it down and then rebuild it

well there is one problem here then. watch the video from my original post. that should really help you to understand in detail. I think that is what you are missing here sir.


I take an in depth look at the aftermath.


I was riding a wheelie and it started to loose power and make a metal on metal noise as it ran. was backfiring and popping with minimal compression.
 
OK I just watched the video but am still not sure what loose metal thing was responsible for the damage.
You suspected the rings since one was broken. Looking at what is left of them can you see that one small piece of it is missing? If so then that would be the prime suspect.
 
OK I just watched the video but am still not sure what loose metal thing was responsible for the damage.
You suspected the rings since one was broken. Looking at what is left of them can you see that one small piece of it is missing? If so then that would be the prime suspect.

okay I understand now. when the rings where, they do not just wear away from the side, but the piston also wears in the slots where the rings run in. when this happens the rings develop some play and can rock back and forth. on these two strokes with port valves eventually the rings will catch a port lip when they develop play like this and then that's the end.

ring to piston play and ring too wall play was enough to do this. I believe this is what happened and it broke the ring that caught.

this is why when engines are built there is usually a piston to ring clearance.

are you jaguar CDI btw?
 
yes I make the Jaguar CDI

Are you aware that the excessively advanced timing on these engines greatly increases the combustion pressure (before it can do any good) which seeps behind the rings and causes excess outward ring pressure on the cylinder?

so do you think the cheap oil had something to do with the rings getting too much vertical play?
 
don't increase compression without a Jaguar CDI

With the excessively advanced ignition of the standard CDI the peak pressure happens too early which is not that horrible on a low compression standard engine but when you shave the head for more compression it takes the whole shebang into the danger zone as you have found out the hard way. That excessive combustion pressure
1) wears out conrod and crank bearings too soon
2) increases engine temperature which can cause detonation and engine failure if there is a lean fuel mixture
3) causes too much outward ring pressure onto the cylinder which increases possibility of the ring catching on a port and causing unrepairable damage

On my site I warn against increasing compression without a Jaguar CDI but some people think that's just promotional talk.
 
well this seems to validate some of your claim. iv always believed the ignition advance on these engines to be too retarded and that's why they cant pull very high rpms.

what degree of initial advance are these bikes set to?(which is also the total I imagine as there is no progressive advance correct?)
 
on the aspect of compression vs the failure, I note that there will always be quicker ring wear and blow-by conditions with heightened compression along with less bearing life. it makes for a much more potent explosion. so naturally the rod bearings take a beating.

the biggest problem I could see with the motorized bike is that the cdi will not be at the right advance and if too far advanced(which leads to detonation) can really do a number on these cast aluminum pistons and bearings as the piston will be fighting the compression fire(too much advance)

if its retarded that's fine, but too much advance with too low grade of fuel(higher octane can withstand higher combustion pressures) I can see these not living long at all.
 
click on my signature link and go to the CDI page to see the ignition curve of the standard ignition as well as that of the Jaguar CDI
 
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