Wow, been watching this thread and sort’a smiling.
First of all, on a small four cycle engine, setting the throttle lever to “full throttle” is not the same thing as “wide open throttle”. A “wide open throttle” would over rev the engine turning it into scrap steel within a very short time, possibly within seconds. Engine RPM is determined by the governor adjustment when the throttle lever is set in the “full throttle” position; the throttle butterfly plate will only go to “wide open” if engine RPM drops below the governed RPM setting. Most small four cycle engines have their governor set to 3600 RPM’s maximum (with lever at full throttle position), which is perfectly safe… and in fact “full throttle” is where they run the most efficiently, run the coolest and experience the least amount of wear-&-tear when under load. You do more damage to a small four cycle running it at something less than “full throttle” (under load).
Second, most log splitter hydraulic pumps are rated for 3600 RPM’s (to match the engines they’re coupled to), and develop maximum flow and pressure at something very close to that (usually somewhere between 3400 and 3600 RPM’s). Running your new engine at half or three-quarter throttle may only be turning the pump at 2200 RPM… and that ain’t gonna’ build squat for pressure and have very low flow (slow ram speed). Possibly the governor was adjusted wrong at the factory and three-quarter throttle is turning the engine at less than 2000 RPM’s… and that’s gonna’ build pressure measuring even less than said squat. Just because your last engine ran fast enough at three-quarter throttle doesn’t mean this engine is turning the same RPM (after all, you did say it sounded quieter, and seamed to be working less).
You need to find the specs for your pump and engine… and put a tachometer on your engine. Your probably gonna’ find out that your pump is rated for 3600 RPM and your engine is rated for 3600 RPM. If the tach shows your engine is only running, say 2900 RPM at full throttle, the pump is never gonna’ reach maximum rated pressure… and if you’re then running it at three-quarter throttle around only 2000 RPM the ram is gonna’ stop when you try splitting old chewing gum… engine horse power has nothing to do with it if the pump ain’t turning at something close to rated RPM.
Some have suggested that if the engine ain’t stalling the problem has to be in the hydraulics… that ain’t necessarily true because if the engine isn’t turning the pump fast enough it can’t build enough pressure to load the engine enough to stall it. But one thing is for sure… if the engine ain’t stalling, or loading down, then horse power, or lack of it, is not the problem… but RPM is another story. The pump works on RPM, not horse power… the higher the GPM, the more horse power required to maintain RPM under load.
addendum; adjusting the throttle/governor relationship on most small four cycle engines is a simple task.