I think most guys will tell you to use a scraper of some sort to clean the head. But scrapers leave scratches, which do nothing good for your heads. I suggest you consider using a pressure washer on them?? If you try this, you will have to tie the the heads down (My pressure washer will blow them away, risking other damage).
Lacquer thinner, BRASS brushes, and a variable speed 3/8" drill at a lower RPM. The attachment is a cheap set at Harbor Freight for $5 that works really well for me. I keep the carbon soaked with lacquer thinner sorta like I'm "wet sanding" to avoid scratches. I use lacquer thinner because I can buy a liquid Quart for about same price as a 16oz aerosol can of spray carb cleaner. Like BigT said.... The reason you want to use brass brushes, especially on the pistons, is that every little scratch collects carbon again and becomes a hot spot. Be aware that if your engine has never had the heads off, Onan made some of their original OEM head gaskets out of asbestos, so be careful about using those brushes dry on a drill and getting dust particles airborne. Keep them really soaked with lacquer thinner to avoid any dust. Any replacement gaskets you buy will be Graphoil. Brass Rotary Wheel and Brush Set 5 Pc.
Love the B43& B48 Onan engines. Only real difference was the B43 had a 66.5mm stroke and the B48 stroke was 73mm. While you're doing the decarbon, you can run your pistons down to the bottom and measure your bore dimensions(B43&B48 are the same). 3.2500/83.55 is the max for standard pistons. Head bolt torque is 16ft-lbs for Graphoil gaskets. Factory Service Manual download is available in the Resources Section of this Forum. I always thought these were almost perfect engines, except for some genius Design Engineer pointed the exhaust outlets up rather than down so you have to de-carbon the heads every 200 hours. I guess nobody ever explained to him the way gravity works to eject exhaust carbon from a combustion chamber. Little secret to cut down on the de-carbon jobs. If you don't run your tractor in the winter (Snowplowing/blowing), run straight 40wt oil. Onan even said straight 50wt if the ambient temp is above 80 degrees in the Service Manual (That's 1:00AM here in MS, in July.), but it's getting hard to find any more. Multi-viscosity oils tend to carbon up more than the straight weight oils.
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