OK. On the bench I have a friends 020T. He's a tree surgeon and he gives me a lot of wood, so I'd like to fix it for him.
Symptoms (as described by him): erratic idle, if you set it so that it won't stall when climbing it runs too fast, if you set it to a sensible idle, then it stalls out.
Easy I thought. Turn both H and L to one turn out, and try to tune for 2800 rpm as per the book. Nope. Response to the L screw is erratic and inconsistent. If you turn the throttle screw all is good to about 3200 rpm, then it just dies. I checked the high end, and set it to a conservative 13K. No problems at the high end, it cuts like a champ.
Must be an air leak. Vac tested it, and it held 0.5 bar vac for several minutes while I spun and yanked on the crank. There was a tiny leak in the impulse which I fixed, but that could have happened as a stripped it down. Checked the piston as I sealed the exhaust - no scoring, all looks good.
Got it all back together. No difference. Damn.
Must be the carb. Stripped it and had a look. Some little bits of plastic in the metering chamber, and a bit hanging out of the valve jet. A-hah! So it all went in the ultrasonic bath, came out squeaky clean. Gaskets are all good, fuel pump diaphragm is good.
Got it all back together. No difference. Damn. Damn. Arrrgh!
So I looked at the Service manual again (Thanks Dibbs!
) Under "stalls at idle" it says "small plastic plate in valve jet does not close". Now I can't find any pictures for Zama carbs, but I'm guessing that the bits of plastic I cleaned out are the remains of the "small plastic plate".
Does anyone know what this "small plastic plate" does? Is it a plausible cause for the problems? Is the solution a new carb? Please help, because this one is driving me nuts!!
Symptoms (as described by him): erratic idle, if you set it so that it won't stall when climbing it runs too fast, if you set it to a sensible idle, then it stalls out.
Easy I thought. Turn both H and L to one turn out, and try to tune for 2800 rpm as per the book. Nope. Response to the L screw is erratic and inconsistent. If you turn the throttle screw all is good to about 3200 rpm, then it just dies. I checked the high end, and set it to a conservative 13K. No problems at the high end, it cuts like a champ.
Must be an air leak. Vac tested it, and it held 0.5 bar vac for several minutes while I spun and yanked on the crank. There was a tiny leak in the impulse which I fixed, but that could have happened as a stripped it down. Checked the piston as I sealed the exhaust - no scoring, all looks good.
Got it all back together. No difference. Damn.
Must be the carb. Stripped it and had a look. Some little bits of plastic in the metering chamber, and a bit hanging out of the valve jet. A-hah! So it all went in the ultrasonic bath, came out squeaky clean. Gaskets are all good, fuel pump diaphragm is good.
Got it all back together. No difference. Damn. Damn. Arrrgh!
So I looked at the Service manual again (Thanks Dibbs!
) Under "stalls at idle" it says "small plastic plate in valve jet does not close". Now I can't find any pictures for Zama carbs, but I'm guessing that the bits of plastic I cleaned out are the remains of the "small plastic plate". Does anyone know what this "small plastic plate" does? Is it a plausible cause for the problems? Is the solution a new carb? Please help, because this one is driving me nuts!!
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