Thanks for info. I will try to send pictures . Maybe you can see the problem

I ran 70+ with an inline without a jackplate. Jackplates are for V-6's, you can't put a V-6 setup in an inline powered boat, the needs are totally different.

Might need a prop with more bowlift, excessive positive trim will induce chine walk. I like a 26" small ear Chopper for that combo.

I am having trouble downloading pictures. Says file is too big. Steering is dual Ride Guide. Little to no play. Mounts seem pretty tight, not much play. Factory mounts.is it possible a jack plate is needed or just going up in mounting holes?

Can you post a picture of the gearcase? I'm thinking the fairing on it has a problem.

How loose are the motor mounts? Does the engine move around much at speed?

Do you know what brand of steering it has?

The boat should run near 70 as it is.

:D I have a 1979 Ventura II with a 1985 Merc 115 hp. with a 23p Turbo prop,dual steering and low water pickup which I probably don't need since prop shaft is about 2 inches below bottom. No jack plate but bottom is recessed in about 6inches from transom. Runs and handles great until about 50mph then starts chine walking if trimmed above level. Tops out at 58mph but have to fight to keep under control. I can raise motor without drilling new holes about 2inches. Does capitate if trim is not down the whole way when getting on plane. I am more concerned with handling than getting ultimate speed. Any advice would be greatly appreciated.