detonation troubleshooting

Need some advice on detonation causes in my old two stroker. Just out of the blue one day, it started what sounds like detonation when under a heavy full throttle load. Internal engine rattling/pinging and loss of power.

CDI and coil is working fine (even swapped out with a spare to make sure), new carburetor has maybe 30 hours on it (I took apart and cleaned anyway and checked float setting), pulled the jug and piston and rings and bore all look fine. Thought maybe it was leaning out through a bad engine seal, so replaced both. New tank of gas, mixed 32:1 as always, fuel filter flows freely, air cleaner is clean, changed the plug. Still, detonation and loss of power when I open her up. This bike has been a very consistent performer, jetted just perfectly....but now something is amiss.

What the heck is going on here?! Could perhaps the ignition timing (stator) be giving me problems, or maybe I've got a hot piece of glowing carbon causing pre-ignition...maybe a long shot.

Any ideas would be much appreciated!!

Mark

possible ignition timing change, hot spot or too rich mixture. It happened on my WR300 when mixture was too rich

Thanks for your thoughts. I haven't been able to find any hot spots in the cylinder, head, or piston, but will probably pull it apart again and take a closer look for carbon that my be acting as a glow plug.

Timing changes "shouldn't" happen unless the stator mounting gets loose or the rotor rotates on it's shaft for some reason. In my mind (and I could be wrong) stators are there only to generate a timed input to the CDI/coil in order to fire the plug. The stator either produces the input, or produces a weak input, or it doesn't produce one at all. For the stator to be the cause of detonation would infer that the timing got moved significantly advanced. My stator is very secure and hasn't moved.

On mixture, I don't understand how a richer fuel-to-air ratio would cause detonation, unless it built up carbon and got the glowplug effect. But if you're referring to richer oil-to-gas premix, then yes it could cause a leaner air-to-fuel ratio and hotter running.

Still a mystery.

Not dud fuel?

Nope, changed fuel sources and still the same issue. Gonna take a closer look this weekend....swap out the carb, swap out the stator, check/adjust the timing, etc. Will report back the results for those that are interested.

Well...this "problem" bike isn't my Husky either. It's my old '73 MX 360 Yamaha.
Today I swapped out the carb, the CDI, and the stator with known good working condition units. Still the same issue presents itself. Last thing to do is to swap out the coil and look once again for possible air leaks (both crank seals are brand new, installed in my first attemt to rectify this).
Been doing these damn old 2 strokers for almost 40 years, and this one is beginning to piss me off.

Jim,
Hate to ask but the silencer isn't plugged is it?? Just a thought

I'd say the same.. If the silencer is oils oaked and ceramitized the packing(usually catch's on fire inside) it will choke off the motor.. Repack and I'd say you are back to the "RACE" mode again..


Chow, Carl

I can relate to the dog food thing...happended many moons ago to my old MGB where mice decided to take up residence in the heater box. Little bastards!
But, back on topic, I'll check the silencer but I kinda doubt it. The thing ran fine the first couple times out this year, then the problem appeared. It's worth looking at, though. No idea is a bad idea at this stage.
Thanks for the comments.

And the winner is....DIRTDAME

Let's put this thread to closure. Lots of great input from everyone, thanks! The problem was not timing related, ignition related, fuel octane related, fuel restriction related, carb jetting related, preignition hot-spots related, or compression related.

Took her out this weekend and positively tore up the mountainside with my bike running like it's never ran before. Night and day difference. Remember, she's 37 years old and I brought her back from a long 30 year sleep. Checked/rebuilt/fixed everything and was happy enough with how she ran until early this year when things started going downhill. The only thing I didn't check was....(drum roll).....the silencer. Yep, plugged tighter than thight with years worth of baked on deposits. Just took the Skyway apart, torched the screen to burn off all the goo, and MY GOD!!! does she run strong now.

Lesson learned, it's usually the simplest things that stump us.

Girls always got more attention than me for some reason anyway! Glad you got it fixed.

Damn! Sorry Scott, I didn't look back far enough to notice that indeed YOU are the winner after all.
A heartfelt thanks is in order. Sure wish all my dirt bike problems were this simple.

I know the gas has been replaced..but this is interesting article about gas

http://articles.superhunky.com/4/41

Good reading, thanks for sharing Mike.

It amazes me that my 37 year old two stroke runs as well as it does with non-leaded 87 octane, considering what it ran with back in the day. Of course, ignition advance isn't as great as some bikes (stock on this on is 2.5mm, and I run it at around 2.3). If it's detonating, I sure can't hear it anymore now that I cleaned the stinkin' silencer (duh!!!).

Now, on one of my other toys - an '87 930 Turbo - I pay particular attention to getting the highest octane gas I can find. When you're cramming 15 psi of air into the engine with very advanced ignition timing, one can't be too careful of melting holes in rather expensive pistons. But for the 'ol dirt bike, she does just fine on 87 octane.