My friends 35 Fountain twin 502 Magnums hit a big wake the other day and now one engine Cuts in and out while using the trim buttons. The trim button on the throttle and the trim button on the dashboard do the same thing if he doesn't touch the trim the engines run great, but as soon as he touches the trim up or down it starts popping and bucking Port engine
any help would be appreciated

Very well could be a Plate inside battery for the port engine is loose from that impact...trim is using more power than the hurt battery can provide now. Been there done that. Maybe it won't take you as long as it took me to figure out because I had no one to tell me to check it.

Hopefully easy enough to check by swapping batteries side to side. vs buying a new battery and it doesn't fix it.

Battery is broken internally .

Thanks fore your replies. I told him your answers and he wanted to test the batteries and charging system before he went and bought a battery or 2. He said he did a long load test with a tester and it tested ok. Alternators ok
Is it possible to have a broken plate inside a battery that shows nothing wrong on a load test. After his testing he went for another ride test and everything ran great until he touched the trip buttons.. This time the starboard engine popped also...
Is this a power shorting issue or a grounding issue?

To test out the battery theory, swap batteries and see if it goes to the other motor.

I'm thinking a bad connection, and would check from bat to starter post, including the 90 amp fuse. Also check grounds on trim pump and bat, both ends and look under the insulation jacket

Loose end connector on the wire from battery to trim pump.
Check all wires and connections.

If he has the two engine's batteries connected to each other thru the switches - then the good battery won't be able to support the hurt battery. Make sure they are seperated when using and testing.

And unfortunately load testers don't always tell us off issues. Trim uses a bunch of amps, thus why it's solenoid is close to a battery connection ie: starter lug in most app's.

both batteries were change today and we checked all wires under the dash engine compartment grounds Etc and it still does it pops backfires onlu when trim swiches pressed... but only does it while pressing the trim switches above $3,500 RPMs anything below 3500 RPMs everything works fine trim up and down no issues
repeat it's only happening above 3500 RPMs
I just found this out today

The higher the rpm the more current it takes, so still think you have a bad connection

I don't think it's the batteries.