165 merc inline 6

[roymarine]

I m running a 1977 inline 6 165hp mercruiser engine in a 21 ft Starcraft islander. The outdrive is rebuild with 1.65 std gear ratio. running a 14.5 x 19 alum prop (new) engine run fine no problem to get in plane but will top of at 4000 rpm, yesterday i tried a 17 pitch prop with same result. The boat will do 39 mph GPS at 4000rpm with the 19 pitch prop and will do 37mph at 4100 with the 17 pitch prop I have point ignition. Im thiking that i could be running lean on fuel but not sure.

 

[ziggy]

Re: 165 merc inline 6

your within specs. with either prop.
i would have expected to see a 400 rpm diff. between the two props though. but dia. blade rake, and many other variables could perhaps account for the lack of 300 rpm. maybe. speed decrease don't surprise me. hopefully the hole shot was better though..
i went from a 21p to 19p and did see 400 rpm diff. the 21p was a fan blade type prop. the 19p has much more rake to it.

i assume your trimmng out for best rpm prior to porpoising or ventilation?

spec is 3900-4300...
myself and fwiw. i also find that in the hot, humid air of nebraska in the dead of summer that i loose a little rpm. it comes back as the air cools down and the humidity goes away at beginning and end of season.

i also have kept my points w/o intention of changing at this point. i've got the same engine set up you got but a 1975..

 

[Blk-n-Blu]

Re: 165 merc inline 6

Have you checked the choke operation?

 

[dbkerley]

Re: 165 merc inline 6

Same set-up running a 21p and topping out at around 4300. The speedo was reading right along with the tach running the 21p so I dropped the speedo duriing the refit.

I recently converted to electronic ignition and will be switching to an electric choke in the next few days. I don't have any new numbers yet. Although it does seem to be showing more timing advance than I had expected. I'm working my way back to spec.