I have an old Yamaha ET500 petrol-driven generator. It runs OK, but the output voltage is about 380VAC instead of 230VAC, even with max load on it. Could the problem be the AC regulator (if there is one), and is it cheap and easy to replace? I have traced the circuit, but I do not have a service manual.

The reason is due to something that occurs in the generator due to impedance and something called Middlebrook Criteria. The simple version is that the power supply in the fridge does not like it and wont accept it. The answer is a high quality power conditioner or a new generator. If you get a power conditioner make sure you tell the company you need a stable supply for a SMPS ( switch mode power supply) and check the return policy because it may not be enough to fix the issue and electrical parts are not returnable in some cases.

you can find the downloadable owners manual here https://www.manualslib.com/manual/835407/E-T-Systems-Et-500-12v.html there are also videos here that may help here https://video.search.yahoo.com/search/video;_ylt=AwrJ61dVLfxcTy8A5SVXNyoA;_ylu=X3oDMTE0Y2FtY3BmBGNvbG8DYmYxBHBvcwMxBHZ0aWQDQTA2MDdfMQRzZWMDcGl2cw--?p=ET500+Gate+motor+owners+manual&fr2=piv-web&fr=cb-max

The YAMAHA ET500 uses an NGK Spark Plug # B5ES 6410 or equivalent. I hope this is helpful.

Locate where the choke lever is for your generator (under the petrol tank), you ought to notice a star headed shaped screw connected to the governor shaft (if you bend and look closely further beyond the choke lever; still under the petrol tank) which is linked to the carburettor housing/assembly. You need to increase/turn clockwise (using a star screwdriver) on this screw.

Your generator engine sound will change/high a bit because the engine speed will increase.

Reconnect your 1800w chop saw (load) and see if your generator can "effortlessly" accommodate it without shutting down otherwise, increae the turns on this adjustment screw attached to the governor shaft till your generator operates/stabilizes with the load (1800w) connected

On the engines carb. is a governor rod should connect to the front case of the engine. This rod should have an adjustment to slow the engine speed. BUT,,, are you reading the generator unloaded or without any thing plunged into it? Once you have the generator loaded with a device, the voltage should drop to a constant 240vac. 150 rpms should not make that big a difference on the voltage output, and if running under load the RPM's should drop also..

Those voltage readings MAY be normal.

The AC waveform may cause your meter to read inaccurately. You would need to use a TRUE RMS meter to accurately read the voltage.

The siple meters actually read the AVERAGE voltage and then are calibrated to mulltiply it by the "form factor" for a sine wave. OFTEN these generators don't produce a clean sine wave. A square-ish wave will read higher on the garden variety meters.

Compare the brightness of a lamp comparing to when run on your utility.

The frequency depends on engine speed ... yours is running a bit fast...check it under at least half load and if off, adjust the govenor slightly.

go to there web sit and than go to dealer locate type in your zip code it will display the closes dealer to you. let me know if this helps i may have another solution

engine rpm is adjusted to high... I set engine rpm according to a loaded condition usually while running a power tool to puts max load or close to it on the generator. I try to get it at 125VAC loaded

Most generators use a flyball type governor within the engine to control rpm; also there is a voltage regulator that tries to maintain voltage at about 115v. The voltage regulator varies the voltage to the armature increasing /adjusting magnetic force. Use a meter with htz or freq setting to adjust engine governor to 60 cycles. When the generator is producing 60 cycles, the engine should be turning 3600 rpm, max power range. At 3600 rpm the voltage should be 115 to 120 volts. Loading the generator will cause governor to try to maintain 3600 rpm. Check engine manual for proper adjustment of gonvernor and linkage. The governor mech should last the life of the engine, however governor system does wear out. Expensive repair. enjoy