20 hp twin struggles to turn over even with new battery

I would first remove and check cables for corrosion, they sometimes corrode from the inside out. That's easy and cheap.
You could then remove the starter and get it tested. AutoZone might do it, but I would find a local place that specializes in starters/alternators.
Mike

The problem could also be the starter solenoid. try bypassing the solenoid to get 'full' power to the starter.

For 16 horse power and higher I recommend a 360 CCA battery or higher.

The first test is to check voltage at the starter, min 9.5v cranking, if it's less there's a problem in connections or starter. Remove spark plugs and retest, if it's anywhere close to 9.5v with plugs out the starter is worn out (if all conn's are clean and tight). Whats the code #? First 2 digits are the year the engine was made, I sell a lot more opposed twin starters than anything else, same size armature for twice the CI.

B & S recommends a min 230CCA battery.

The starters should free spin about 9500 RPM's, it you have or can get a vibro-tach that's another way to see how they perform.

DO NOT GET THIS STARTER REBUILT!!!!!!!!!!!!!
The armatures wear and most rebuilders don't replace the armature, the cheaper, less than $100, price point starters are cheap Chinese pieces of crap, go with OEM B&S, or with Outdoor Power Equipment, OPE brand, the only Chinese built one I've seen that's half way decent.

My daughter's lawn tractor does the same thing....new battery, new starter, and new solenoid. Sounds like a compression release problem where valve adjustment may be needed.......haven't got around to that yet. In the meantime I told her to NOT apply the choke until the motor has started spinning. Once the starter gets past the first one or two compression strokes THEN pull the choke handle.....starts every time. Not a fix or solution but it gets the engine going.

SABL said:

My daughter's lawn tractor does the same thing....new battery, new starter, and new solenoid. Sounds like a compression release problem where valve adjustment may be needed.......haven't got around to that yet. In the meantime I told her to NOT apply the choke until the motor has started spinning. Once the starter gets past the first one or two compression strokes THEN pull the choke handle.....starts every time. Not a fix or solution but it gets the engine going.

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If you have an OHV engine, yes adjust the valves. rpmp3 has a flat head twin. It is possible for the comp release on the cam to go bad, and excessive clearance will effect it's operation also, but flat head's usually loose valve lash while OHV gain valve lash as they wear (key word- usually).

I should have ckecked the specs for the OPs engine......oops!! Thought it was OHV......

Nice to see more posts by k2skier!!