Mercruiser 165 tachometer

"Fellas, I got an interesting

"Fellas, I got an interesting one
Recently, I changed the upper and lower units on my boat. The engine in it is an inline 6 Mercruiser 165. The upper is labled as an HP 165, serial number 3392390, which by the literature I have been able to find on line is a 70-73 model, with a ratio of 1.65:1. Here is my issue, when I am running wot, I my tach is only reading about 2500rpm. I repitched with a new prop, and I still get 2500 rpm. the current prop is a 14 wide, 18 pitch 4 blade composit prop. I got spark on all cylinders, , but I am nowhere near the 3900 minimum rpm I should be running. Can this be a bad tach? or, is the inline 6 just to weak to push a 20 foot 4500 pound boat?"

"I've had similar issues,

"I've had similar issues, it could be the tach, can you get a timing light with a tach built in to verify? 18p Should be Ok. You may have a fuel/carb issue or a low compression issue.

Is the engine a mercruiser ? is there a mercruiser serial number tag at the bell housing on the starboard side? just above the starter?
If not it could be an auto/truck engine with an under performing camshaft.

How did the engine do before the outdrive change or do you not know.. can you ask the PO ? (previous owner) Could be a mechanical advance issue (under the points plate in the distributor)
Could also be a slipping, aged engine coupler.

But yea.. could be a tach. If you are thinking about changing the gauges, check out greatlakesskipper.com they buy out stock and have a great selection, usually complete with harnesses.."

"Brian,
The engine has

"Brian,
The engine has a mercruiser tag on it. The outdrive I had before was actually the wrong ratio, I used it off a smaller boat I scrapped. It was from a 3.0 4 cylinder (mercruiser 140), I don't know what the ratio was but I was getting 3500rpm with a 22p 4 blade prop on it, which was still to low, but I figured that was because I was overpitched. I had to replace first the lower unit because the drive shaft broke, then the upper because the gears shredded, so I looked around to get the correct ratio thinking maybe that's why I trashed them before.
I haven't checked compression since last year, but I rebuilt the engine then, so unless I've gouged the cylinder walls or something drastic that should not be an issue. i have an electrical meter with a tach built into it that I can try, but the problem is getting someone to go out with me that can drive the boat while i check the meter."

"I did the same thing, intenti

"I did the same thing, intentionally while my original drive was rebuilt. I used a 1.98ratio which is what is used on 2.5L 120hp & 3.0L 140hp mercruiser engines, I used a 3 blade 23p and still maintained proper RPMS of 3800-4200. Nope you wouldn't have trashed them from being the wrong ratio, you would just need more pitch to make up the difference, it's like on a car going from 3.73 to 4.11, you'd get larger tires. You gain mechanical advantage.

Did you have the cylinder head rebuilt? that could be the cause of compression loss. hows the adjustment of the rockers? I know mine doesn't like the 1 full turn, it likes a 3/4 turn. I also have 160psi across the engine except for 2 cyliners 150 & 155. Another issue that I had was with bad fuel from the steel tanks that were rusting from the water the methanol was pulling in.

I'm going to find a xls I did on gear ratio and prop selection just for reference."