What is the ideal thermostat temp for a 29 foot with twin
350`s ? Mine are 140.

Stock merc carbed engines use 140* Tstats. If thats what you have, then thats what you should run.

I have twin 350 MAG mpi. 160 degree t-stat.

Quicksilver #807252Q5

Yep, mercruiser(black) mpi engines use 160's.

all Mercruiser carb OEM stock black motors 1996 (serial # break for 1996 only will determine either a 140 or 160 degree t-stat ) and earlier models used a 140 degree T-stat.

All MPI motors use a 160 degree and some closed cooling MPI motors use 170 degree t-stat

When Mercruiser ( really GM) went to the 8 bolt intake manifold (vortec) for the carb motors is when the 160 degree t-stat was used

Thanks for the responce, I put 140 back in it. Leaving for Marathon this weekend and just doing some maintenance.

forgot to add in my post above all Carb motors went to 160 degree t-stat very late 1996 for model year 1997 to current. There is no more OEM stock carb motors from Mercruiser starting in 2012 just new carb repowers for 2011 and back motors. Volvo got rid of carb motors in 2009 or 2010 FYI.

If thats what you have, then thats what you should run.