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Detonation is caused by being too lean. That doesn’t mean the jetting was off, what it means is the engine didn’t get enough fuel. Maybe the fuel line got pinched and starved the engine, maybe the floats stuck and starved the engine, maybe dirt in the fuel blocked a hole in the carb or plugged the filter and starved the engine. No matter the cause, the end result was the engine got too lean and hand grenaded.
As far as reading plugs, did you give it a hot chop or did you just idle in? If you just idled in or let it idle at all, your plug would load up with fuel and you wouldn’t be able to accurately read the plug. It would suggest that it was fat when in fact it could be lean.
If you are going to read the plug, you have to do a hot chop. What that means is you rev it out to max and then shut it down with out idleing. Just coast back in. That way you can get an accurate reading. Better method is to remove the pipe an look in the exhaust port or use a bore scoop to read the top of the piston. Just looking a your piston photo, it is screaiming, I need more fuel.
What engine, what oil, what mix?
You might look into the method you are using to get a clean plug reading, or ditch reading your plug and read the piston more. The lean condition can be in most of your running rpms and your rich plug reading is just while performing your “clean cut off.”
A cool day is great for making power unless your jetting is still set for a warm race day and 100 octane may work fine untill you run into a cool day with better air.Andy,
You need to give everybody a baseline of info. Like Doug said, engine, fuel, oil, ratio, fuel filters, air box filter, water temp,etc, etc. Could be anything.
Sorry and hope it didn’t ruin your jug!
Are you sure this is the right piston for the engine? It looks strange to have detonation on the pin side of the piston as usually it is concentrated on the exhaust side if lean. The squish clearance could have been altered when the piston was changed and the base gaskets were not the same thickness as the original piston was set up for from Barry. The only time I have seen this type of detonation is when the clearance between the piston and the head varies from each side. You should have .030 minimum clearance between the piston and the head all the way around with a minimum cc of 10.5 if the head is not o-ringed.
We once had a Leary CR125 that would detonate much like this. It was not do to the normal causes like jetting, compression or timing. It was a bad detonation ring.
If this is a billet YZ125 you should call up Barry and see if he can help you.
KR
isn’t 120 degrees too cold on the water temp?
how could you be lean, melt your piston and the temp be only 120?
not chopping your motor will give you false plug readings.
sorry for your problem. Barry will take care of you.
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