Lost Compression StockMoto CR125

Evan York · Posted: Tue Dec 04, 2012 6:39 pm Post subject: Lost Compression StockMoto CR125

All,

I finally had a chance to fire up my stock moto. To my relief, it fired up effortlessly. I knew I needed to break in the motor since it was recently rebuilt by the original owner, so I went through what I thought was the proper break-in procedure per Swedtech's website. Initially, the motor felt tight and pulled hard. I was pleased.

I ran the kart for a decent bit after breaking it in, no more than 5 laps at a time, tying up any loose ends once I came off track. One of my longer sessions ended because the kart lost power and stalled. What I thought was simply a fuel supply issue turned into a compression issue.

There isn't a hole in the piston, and no leaky gasket. I took the pipe off and took a look inside the exhaust flange, and it was evident that the ring was damaged. I was told that in some instances, the ring will "stick" to the piston. I examined the spark plug, and it was a touch lean (my mistake for not putting a larger jet in - I just ran what came with the motor). The head had just a touch of detonation around the edge, but still usable (it is a 1995 motor - just something to train with during the off season). I ordered a new top-end kit for the motor incase I did need to replace the piston.

Why did this happen, and how do I prevent it from happening again? Is it as simple as a jetting?

Thanks in advance for any insight!
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Regards,

Evan York

www.evan-york.com

@EvanYork

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"That which you manifest is before you" - Enzo the dog

Ace Rossi · Posted: Tue Dec 04, 2012 9:25 pm Post subject:

My first question would be, what gas and oil were you running and how were you mixing it?

Evan York · Posted: Tue Dec 04, 2012 9:30 pm Post subject:

I was using 98 octane race fuel with 8oz. of castor oil per gallon. Vigorously mixed of course. Learned later that it was a bit rich...6oz to gallon was recommended next time.
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Evan York

www.evan-york.com

@EvanYork

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"That which you manifest is before you" - Enzo the dog

Ace Rossi · Posted: Tue Dec 04, 2012 9:35 pm Post subject:

I may be wrong but my understanding is that although more oil is in fact a rich mixture it can cause a lean condition!
To simplify, imagine a funnel is your main jet and you pour honey thru it. It goes thru slowly
Now pour a thinner mixture such as water and it goes thru faster!

I know, not a good example and pretty elementary but you get the idea! Sirt of like saying a thicker consistency flows thru the system slower or something to that nature!

Evan York · Posted: Tue Dec 04, 2012 9:39 pm Post subject:

Never thought of it quite like that. Thanks for the advice! The sparkplug was a touch on the gray side, so I know for a fact that I was running pretty lean. Just didn't think it would lead to loss of compression.
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Regards,

Evan York

www.evan-york.com

@EvanYork

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"That which you manifest is before you" - Enzo the dog

Ace Rossi · Posted: Tue Dec 04, 2012 9:42 pm Post subject:

Since ive been bashed recently over other opinions I had you may want to wait for the pros to chime in!

Randy Mckee · Posted: Tue Dec 04, 2012 10:01 pm Post subject:

That sucks. I hate sticking a motor.

Might have been too lean, or it could have been a mechanical failure for various reasons.

Can you pull the cylinder and take some pics of the head, piston and inside of cylinder? It might help us help you diagnose the issue so you can prevent it in the future.

Some things that could have caused failure (in no particular order)

Too lean is most common reason. What jets do you have in there? (pilot and main)
Clip, ring or spark plug failure -- unusual but it happens
Debris in the motor. Are you running an airbox with no filter?
Cold stick -- not likely unless you were getting on it hard when motor was cold
Incorrect piston, ring, gaskets, or some other out of spec piece that caused tolerances to be too tight or loose.
Loose cylinder or head studs.
Bad gas, though what you ran seems fine.
Timing is off by a bunch. This can also occur if the CDI goes gad.

I'm sure there's a dozen other odd-ball reasons.
_________________
Randy
#123 TonyKart - S4 Stock Honda

Matt Dixon · Posted: Tue Dec 04, 2012 10:03 pm Post subject:

What jets were you running and do you know what the adjusted altitude or air density was at the time. That info may help clarify the situation.
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Matt Dixon
94y Energy Corse/Swedetech

Benn Herr · Joined: 18 Jul 2001 Posts: 1584

Ace,

Not to bash here, but I doubt that's the issue here. What you are talking about is important when you are on the ragged edge of correct jetting in heavy competition, not short 5 lap break in runs. Sounds more like the main jet was way to small or something in the engine is letting go.

Evan,

At a minimum you will need a piston and ring. I'd look closely at what the bearings and reeds look like. What size main jet are you running? What kind of fuel system? 8 ounces of oil is fine, we run that all the time. The most it will do is smoke a little more while you're warming it up.
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Evan York · Posted: Tue Dec 04, 2012 10:17 pm Post subject:

Randy - I'm planning on pulling the motor apart sometime in the next week. Perhaps tomorrow if I get the new piston and ring, and I'll be sure to post photos.

Matt - I'll have to go back and check. I ran whatever jets were already in the motor, and they were used in the summer time in Florida (a bit different in the winter in North Carolina - probably way too lean for cold, dense air). Definitely my mistake and I'm willing to bet that was the problem....

Benn - I have ordered a complete top-end kit. I have a new piston and ring coming. I'll definitely check the reed cage. What bearings are you talking about? The main bearings or the bearing on the end of the rod?
_________________
Regards,

Evan York

www.evan-york.com

@EvanYork

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"That which you manifest is before you" - Enzo the dog

Benn Herr · Joined: 18 Jul 2001 Posts: 1584

Evan,

While it is hard to see the main bearings, try to look between the sides of the crank and the crankcase. Look for marks on the case of things coming out (bearing cages). You probably won't see anything there. The rod bearing should be looked at too. You have to look through the slots in the rod. The rollers should be shiny and the bars in-between them even and light white/yellow in color. Make sure the wrist pin bearing isn't missing any rollers or is discolored. Also make sure you can find both wrist pin clips and don't use them over - new every time!
_________________
Benn Herr
Come see our Superkart Build Off @
http://www.lostenduros.com/?page_id=1534
beherr3@cox.net
http://www.southwestkarters.com/
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Kart Design
Chandler, Az.

Greg Wright · Posted: Wed Dec 05, 2012 6:49 am Post subject:

What was the ambient air temperature? Your castor could have seperated in the fuel.
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Greg Wright
Rapid Racing Inc.
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Jim McMahon · Posted: Wed Dec 05, 2012 7:47 am Post subject:

Well that sucks...
FWIW, with the 95 cylinder it's not considered a stock moto. Even though the motor is a stock (kinda), moto.

So did it just loose compression with no sign of the skirt sticking? Reason I ask is that the skirt will tell you something about how it stuck. Since its a new build it sounds like you have a lot of variables to isolate (incorrect jetting, airleak, mechanical breakdown, oil separation, overadvanced ignition, injestion, list goes on).
Can you post some pics of the piston crown, skirt and cylinder bore?
Was the ring gap checked as part of the build?

8oz/gal is "OK", but as pointed out it will cause the jetting to be leaner, so if you are already close to the edge it may be enough to push it over. Its more to do with the amount of gas that gets displaced by the additional oil rather than the change in viscosity or gravity. Each time it pulls a charge past the reeds, theres less fuel available for combustion with 8oz/gal as opposed to 6oz. Most people run 6oz, so for the sake of simplicity I would say run that.
To add to the list the need to do a vacuum/pressure test after a rebuild. All the seals in the motor must be working correctly in order for it to run safely. It should hold 5psi for about 5mins.

With all of what you've said though... between oil ratio and change in air density I think it might simply be jetting. Might as well use this opportunity to check everything that has been pointed out in this thread to ensure smooth running from here. If funds allow, an air density gauge would be nice to have. Failing that, you could at least use the reported readings from the closest weather station using a mobile device app.
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Todd Kageals · Joined: 15 Jun 2012 Posts: 271

Hey....were you running at MMX when that happended? If so, they are going to kick us Honda guys out of there if this keep happening (just kidding). I blew my '94 CR125 there. Turns out my lower crank bearing went out and took everything else....including the cylinder...with it. Good luck with your engine. I have two I am building right now and will be back at MMX soon.

Todd

Evan York · Posted: Wed Dec 05, 2012 12:14 pm Post subject:

Jim - Much thanks for the insight. I will definitely post pictures probably next week when I do the rebuild. Currently in the middle of exams at UNCC.

Todd - I was actually at CMP breaking the motor in when it happened. I was at MMX trying to diagnose the issue last Sunday. I'm sharing a garage with one of my clients there so I'll be there a lot. Swing by garage 9 if you see me!
_________________
Regards,

Evan York

www.evan-york.com

@EvanYork

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"That which you manifest is before you" - Enzo the dog