B
Ben
Guest
Hey I’m glad to hear that you’ve gotten things sorted out as far as the rebuild. But I’m still concerned about the meltdown. You said he rode it until the engine stopped and then put it away, and it was coughing right before it gave up its life. But I respectfully disagree with your diagnosis. Under-revving a two-stroke engine simply won’t produce these results, no matter what gear you’re in or how poorly it is operated (in this case, by an inexperienced 9-year-old). Under-revving will only make you go slower and not realize the full potential of a tuned exhaust ramming the unburned mixture back into the cylinder at the power curve peak before the up-shift. By not spinning very fast, more load is put on the engine at lower speeds in higher gears, as every stroke of the piston is fighting to hold angular momentum to keep the crankshaft turning. In the end, you just won’t go very fast, but no harm whatsoever will be done to the engine, and it will never, never score the piston and cylinder like you’re seeing here.
Missing a gear will sometimes cause over-rev, and I have seen several instances where part of the piston skirt has broken off and knocked a sizeable hole in the crankcase as a result of this. At other times, rod bearings have spun out and knocked themselves around the crankcase too, but I have never seen pistons as badly treated as this in any of those instances.
You noted that part of the skirt is missing in this case, but if the engine was over-revved, it would be a fairly clean break, and I would not expect the rings to be seized to the piston, but they are very clearly seized in your pics.
Now this all logically leads me to see the cause of failure as overheating. However, we still don’t have a cause of overheating. I’m afraid that whatever caused this failure will still be present when you finish the rebuild, and your friend will find himself with another blown engine too soon. I initially thought that it was a lack of lubrication, but you said your friend was certain he mixed the oil and fuel. Does he remember what the mixing ratio was?
Was there a recent rebuild on this engine with the correct sized piston for the bore? A piston without enough clearance will seize as it wears quickly, can’t seal, and overheats because heat transfer to the cylinder is reduced.
If it was rebuilt, were the port windows properly beveled? I’ve seen some Blaster and other barrels that were bored and bored and bored and the ports were never beveled. Bevels in a two-stroke engine allows the rings to expand into the ports (all rings do this into the ports, some more than others) by gently guiding the rings in and out. Without the bevel, the ports act like scrapers that effectively remove oil from the piston and rings. This wears out the rings very quickly and consequently overheats things. Also without a bevel, the rings can become snagged in the ports and broken up, possibly causing the results you’re seeing. The top of the exhaust port deserves special attention as the exhaust window is around 65% of bore width (if I remember right) on a stock Blaster.
When he converted to pre-mix, did he change the jetting? Larger than normal jets must be used when changing to pre-mix to allow for oil, as well as fuel to be passed through the carburetor to the engine. Without these, a lean fuel mixture is introduced, and overheating is a possibility.
Something else I thought of was the float level in the carburetor bowl. I don’t really remember these being a problem on the stock Blaster, but if the carburetor has been replaced, I wouldn’t hesitate to check the fuel level.
Anyway, I encourage you check into these things to help avoid another costly meltdown. So I guess in the end, it may not have been the kid brother’s fault (unless he tried to fuel it up and didn't put oil in the gas, which is another possibility). He just happened to be riding when it died. Although he did lie about not riding it, and he knew he shouldn’t have been riding it in the first place…
Missing a gear will sometimes cause over-rev, and I have seen several instances where part of the piston skirt has broken off and knocked a sizeable hole in the crankcase as a result of this. At other times, rod bearings have spun out and knocked themselves around the crankcase too, but I have never seen pistons as badly treated as this in any of those instances.
You noted that part of the skirt is missing in this case, but if the engine was over-revved, it would be a fairly clean break, and I would not expect the rings to be seized to the piston, but they are very clearly seized in your pics.
Now this all logically leads me to see the cause of failure as overheating. However, we still don’t have a cause of overheating. I’m afraid that whatever caused this failure will still be present when you finish the rebuild, and your friend will find himself with another blown engine too soon. I initially thought that it was a lack of lubrication, but you said your friend was certain he mixed the oil and fuel. Does he remember what the mixing ratio was?
Was there a recent rebuild on this engine with the correct sized piston for the bore? A piston without enough clearance will seize as it wears quickly, can’t seal, and overheats because heat transfer to the cylinder is reduced.
If it was rebuilt, were the port windows properly beveled? I’ve seen some Blaster and other barrels that were bored and bored and bored and the ports were never beveled. Bevels in a two-stroke engine allows the rings to expand into the ports (all rings do this into the ports, some more than others) by gently guiding the rings in and out. Without the bevel, the ports act like scrapers that effectively remove oil from the piston and rings. This wears out the rings very quickly and consequently overheats things. Also without a bevel, the rings can become snagged in the ports and broken up, possibly causing the results you’re seeing. The top of the exhaust port deserves special attention as the exhaust window is around 65% of bore width (if I remember right) on a stock Blaster.
When he converted to pre-mix, did he change the jetting? Larger than normal jets must be used when changing to pre-mix to allow for oil, as well as fuel to be passed through the carburetor to the engine. Without these, a lean fuel mixture is introduced, and overheating is a possibility.
Something else I thought of was the float level in the carburetor bowl. I don’t really remember these being a problem on the stock Blaster, but if the carburetor has been replaced, I wouldn’t hesitate to check the fuel level.
Anyway, I encourage you check into these things to help avoid another costly meltdown. So I guess in the end, it may not have been the kid brother’s fault (unless he tried to fuel it up and didn't put oil in the gas, which is another possibility). He just happened to be riding when it died. Although he did lie about not riding it, and he knew he shouldn’t have been riding it in the first place…
