What caused this, too lean or lack of oil? Pics attached
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Yup most likely bad gas.
If you are lacking fuel, then you are lacking oil.. That is one of the main downfalls with 2-strokes. Getting the proper fuel is crucial for these engine...
Strizzo
Well-Known Member
while i agree that the cause of the pictured damage is textbook lean running, i feel like the no fuel = no oil idea is a bit of a myth for the most part on these skis. The case where this is definitely true is if the ski were converted to premix. I don't think that is the case here, as the OP didn't mention it, and we've all seen the aftermath of failed oil systems, they don't look nearly as clean as this (with the lean condition the damage/erosion to the piston was blown out through the exhaust, no oil engines don't get a chance to blow the damage out - metal pieces on top of the piston/embedded in the head).
if it was required to have fuel moving through in order to "carry" the oil as i've seen posted before (not necessarily you, can't remember who), then the DI and RFI skis oil systems would not work.
The DI and RFI skis are different. In MOST cases, less fuel means less oil. And 2-stokes will NOT survive without oil. A prime example is, Grey Tempo fuel lnes. As the begin to plug up, less fuel and less oil gets to the engine. The air to fuel ration is way off and he piston gets destoyed. Both by detonation and less oil .
Strizzo
Well-Known Member
how are the DI and RFI oil systems different from the carb skis? they still inject oil into the intake tract to be atomized by incoming air and lubricate the RV in the RFI and bottom end in both. The RFI injects the fuel in the transfer port between the crankcase and top end, so the fuel cannot "carry" the oil to the bottom end.
I may be wrong, but I thought the carbed skis pulled the oil along with the fuel. The DI and RFI skis had the fuel injected into the cylinders without the oil. No?
Strizzo
Well-Known Member
the oiling system on the carbed skis is separate from the fuel system. it doesn't mix the oil and fuel together before going into the carb. the oil injectors are just below the carb in the intake manifold ahead of the RV on 787 and down skis, not sure exactly where the injectors are on the 951 but i'd bet a similar location ahead of the reed boxes. the amount of oil flowed is dependent on throttle position. It is section 7 of the 1999 shop manual i've got.
Very good. I'll do some reading. Thanks.
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Sportster-2001-951C-Stock
Well-Known Member
In the case of the hole burned in the piston, we need more info, IMO
I'm betting on some sort of fuel issue, b/c to see a holed piston is consistent with a fuel issue of some sort.
How fresh was the fuel?
Was there premix in the fuel tank?
Was the oil injection system setup and operating, or deleted?
Was the engine overheating?
Were the carburetors rebuilt, was the high speed mix adjustment at spec?
Was the engine experiencing any performance problems such as lean hesitation?
Were the spark plug insulators white (too hot)?
Were the spark plugs the correct heat range?
Was the engine being run hard (lots of heat), or babied?
What was the cylinder compression before failure?
Describe the oil puddle in the bottom of the crankcase, large enough to wet the crank wheels?
Was the squish band out of spec?
Is there carbon on the underside of the piston crown? (I bet answer is yes, I can see piston was overheated!)
In the case of lack of fuel, I want to point out that lack of fuel can cause detonation and excess heat. Excess heat will often lead to oil film failure. Thus as the logic goes, lack of fuel leads to lack of lubrication.
If the engine was running exceptionally strong and bouncing off the rev limiter, I'd have to say the high speed fuel mixture was too lean. If the fuel quality was old, stale or questionable then the octane was too low. Either of these can cause detonation (premature ending of combustion, maximum cylinder pressure occurring too soon during combustion stroke).
I'm betting on some sort of fuel issue, b/c to see a holed piston is consistent with a fuel issue of some sort.
How fresh was the fuel?
Was there premix in the fuel tank?
Was the oil injection system setup and operating, or deleted?
Was the engine overheating?
Were the carburetors rebuilt, was the high speed mix adjustment at spec?
Was the engine experiencing any performance problems such as lean hesitation?
Were the spark plug insulators white (too hot)?
Were the spark plugs the correct heat range?
Was the engine being run hard (lots of heat), or babied?
What was the cylinder compression before failure?
Describe the oil puddle in the bottom of the crankcase, large enough to wet the crank wheels?
Was the squish band out of spec?
Is there carbon on the underside of the piston crown? (I bet answer is yes, I can see piston was overheated!)
In the case of lack of fuel, I want to point out that lack of fuel can cause detonation and excess heat. Excess heat will often lead to oil film failure. Thus as the logic goes, lack of fuel leads to lack of lubrication.
If the engine was running exceptionally strong and bouncing off the rev limiter, I'd have to say the high speed fuel mixture was too lean. If the fuel quality was old, stale or questionable then the octane was too low. Either of these can cause detonation (premature ending of combustion, maximum cylinder pressure occurring too soon during combustion stroke).
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Sportster-2001-951C-Stock
Well-Known Member
The 951 oil injectors are in the intake, upstream of the reed valves. The carburetor isn't calibrated for premix, if setup as per spec.
Sportster-2001-951C-Stock
Well-Known Member
You're sort of right, but in the case of oil injection the fuel and oil aren't mixed before passing through the carburetor metering, they mix in the intake and crankcase but even that is unnecessary b/c a puddle of oil is maintained in the bottom of the case and is slung upward by the crankshaft, so it's really a sort of splash oiler.
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Just got finished putting a top end on a 787. Pistons looked almost identicle to these ones. After getting the truth out of the guy it had been sitting for 3 years. Bad fuel. When he got it he had me rebuild carbs and replace tempo lines. Never said it was sitting for 3 years. I just figured he had been reading about the tempo lines and wanted them replaced and carbs rebuilt as a PM. Had about 30 min of run time on it when it died.
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