Broken engine ... comments on failure mode?

[BrianPetersen]

I hurt a motorcycle engine yesterday :-/

The engine is a fuel-injected Honda 125cc SOHC liquid cooled single that has a rolling-element bottom end. After about 10 minutes of highway running at approx 10,000 rpm (redline is 11,000 rpm) it lost a bit of power momentarily. I think "that's odd" but carried on about 5 km to my exit; on arriving I hear the dreaded knocking ... I topped it up with oil (it was low - 2 weeks ago at a regular check, it was okay) limped it home and took the engine apart last night. The engine still started easily and ran okay - it just knocked. The knocking was consistent with both RPM and load on the engine.

Camshaft, rocker arms, look fine. Valve clearances are within spec. Nothing scary came out in the engine oil that I drained out.

It's quite apparent that the piston and cylinder wall have issues ... The piston and cylinder wall were scored on the thrust side. The opposite side looks okay. Wrist pin was partly seized into the bore in the pistons, but the fit at the small end of the con-rod seems fine and the wrist pin itself looks okay (but I'm going to replace it anyway). Con-rod moves easily on the crank with no apparent sloppiness and the crank spins freely. (remember, rolling-element bearings in the bottom end of this one).

Piston was coated with black deposits on top. Don't know if that came from the engine burning its lube oil when the piston tried to seize, or whether it was like that all along.

Sparkplug is white. To me it looks lean as heck but someone's telling me that it's normal on fuel injected bike engines. (I'm not convinced.) If this was carbed I'd have gone up in main jet size long ago based on sparkplug alone.

2600 km since previous oil change (scheduled interval is 4000 km), oil last used was Motul 300V ester based synthetic.

Comments? Questions?

 

[MikeHalloran]

Check that the cooling system is okay.





Mike Halloran
Pembroke Pines, FL, USA

 

[BrianPetersen]

Cooling system is fine. Temperature gauge stayed at normal reading the whole time.

 

[patprimmer]

It leaned out and nipped a piston.

The rod will be fine, the rollers in the big end might be contaminated. I would very carefully clean the big end or replace it.

You will need a piston and maybe a rebore.

It may have been a bit hot or the oil might have been a bit hot. On that engine, would the low level effect cooling of the oil or supply of oil to the bore and piston dome underside?

Regards

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[RossABQ]

I know nothing about this engine; do you believe it is intended to run continuously for extended periods at 10k RPM? An 11K redline likely indicates some impending limit on one or more parts, that doesn't necessarily mean continuous operation at any RPM below that is acceptable. I'd be curious what the piston velocity is at 10k RPM.

 

[Artisi]

check the temp gauge.

 

[thundair]

Brian
Could it be a gear tooth missing. I have had this happen on another vehicle and it sounded just like a bearing failure.

Cheers

I don't know anything but the people that do.

 

[BrianPetersen]

I'm pretty sure that patprimmer had the correct answer ... I suspected as much, just wanted confirmation from an independent source because I've had different (respected) local bike mechanics tell me different things; I had one shop tell me that a wrong air/fuel ratio won't cause a problem like this. (Bah-loney!)

The fault is most certainly NOT with the temperature gauge or the cooling system! And there are certainly NOT any missing gear teeth or anything of the sort; the bottom end checked out perfect. As for whether this engine is designed for continuous operation at high speed, on a 12 horsepower bike, it had better be. Piston speed at actual redline (10,400 rpm, not the 11,000 rpm that the gauge shows) is 16.36 m/s.

The engine is back together and has run about 1500 km since the rebuild. No problems - BUT - I've been careful to not use full throttle at highway speed until some way gets figured out to get a little more fuel in there. I've also put a resistor in line with the intake air temperature sensor. Sparkplug still looks too light for my liking, but at least it has *some* colour on it now.

I really need to borrow my friend's wideband data-logger once he's done with it ...