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I agree it is likely a fuel problem, the 1GM is pretty straight forward. Start by checking the cheap stuff first.Have you tried changing the fuel filter(s)? When was the last time you changed them? I get knocking when the filters start to get clogged up.
- start by changing the Fuel Filters (note, there may be an extra Racor Filter in the line). [ about $5.25 ea. / 104500-55710 ]
- make sure the Tank Vent line is clear.
- look for any cracks in the hose or leaky fitting that may be letting air into the system when it is running.
- the Fuel Feed / Lift Pump has a diaphragm that eventually wears out. Yanmar no longer sells rebuild kits, so it's pretty easy to replace the pump as a unit. [ $81.96 / 105582-52010 ]
- fortunately the 1GM has only one cyl. so it only needs one Fuel Injection Valve - [ $117.35 / 728170-53100 ]
- at this point all you have left is the Fuel Injection Pump, however at $344.60 it might be prudent to get a qualified mechanic to take a look. [728171-51100]