mercruiser 165hp 6 cyl 1977...performance issues

[Captainwally]

Ive got a 1977 Searay with twin 165s in the boat. 2 years ago I had then freshened up and new out drives put on. My question is is their a way to increase HP for getting up to plane..

Ive changed props one lower and one higher but no real improvement with out loosing either rpm or top end. its a 25' and tops out at about 29 @ 3400 rpm... any ideas?

 

[fishrdan]

3400RPM? That's almost 1000RPM shy of the engine's WOT RPM, causing the engines to lug and not perform as they should. I assume both engines run the same RPM and you have ruled out tach issues. (like a wrong setting, 4-6-8)?

What props are you using and which ones have you tried, supply both pitch and diameter, aluminum or SS, 3 blade or 4 blade. This sounds to me like a prop issue, so it may need to be moved into the prop forum..

 

[thumpar]

When the new drives where put on were they the same ratio as the old? If you are not reaching full WOT you will not get it to perform right. 1000 RPM is going to be hard to fix with just a prop change.

 

[Fishermark]

DonS wrote a good post regarding low RPMs - it is in the stickies at the top of the page. ( Or you can click here.). In addition to the list and suggestions above, is there a chance your hull may be water logged?

 

[Alumarine]

Why did you change both drives?
What was top speed and WOT with the old drives?
What is the ratio of the new drives?

 

[alldodge]

Ive got a 1977 Searay with twin 165s in the boat. 2 years ago I had then freshened up and new out drives put on. My question is is their a way to increase HP for getting up to plane..

Ive changed props one lower and one higher but no real improvement with out loosing either rpm or top end. its a 25' and tops out at about 29 @ 3400 rpm... any ideas?

:welcome: to iboats

Specs on your engines are:
Timing 6 BTDC @ 650-700 rpm
WOT 3900-4300 rpm
Point dwell 31-34

What did the boat do before the new drives in WOT rpm?

 

[FreeBeeTony]

I don't think the max WOT of an IL6 is as high as 4300RPM...........I think its more on the order of 3800-3900RPM.

 

[alldodge]

I don't think the max WOT of an IL6 is as high as 4300RPM...........I think its more on the order of 3800-3900RPM.

I pulled it out of the manual no 3 at boat info, have a look for your self to verify, near the end of the manual
http://boatinfo.no/lib/mercruiser/manuals/mercruiser3.html

 

[Alumarine]

I don't think the max WOT of an IL6 is as high as 4300RPM...........I think its more on the order of 3800-3900RPM.

No, WOT is 3900-4300 for the l6-165.

 

[FreeBeeTony]

I stand corrected........I think I was recalling what the WOT of my IL6 was: ~3800.

 

[fishrdan]

When the new drives where put on were they the same ratio as the old? If you are not reaching full WOT you will not get it to perform right. 1000 RPM is going to be hard to fix with just a prop change.

I missed the part about the new drives... I agree with thumpar, check the drive ratio's, most likely way too low of gears. As I recall it should be something like 1:81 to 1.

 

[fishrdan]

What elevation is the boat operating, close to sea level or high elevation 5000'+

 

[NHGuy]

Well, if you are trying to gain 600 rpm and the motors are running right you need to go down by 4 to 6" of pitch. Are you running the props from your old drives? If so you are likely to have the wrong drive ratio.
As was said, it's tough to prop your way out of a wrong ratio. But if you let us know the prop pitch and drive ratio we can be more help. If you can't find the drive ratio mark that's OK, prop pitch will still be a help.

 

[Alumarine]

I missed the part about the new drives... I agree with thumpar, check the drive ratio's, most likely way too low of gears. As I recall it should be something like 1:81 to 1.

My manual says 1.65 for that year range.

 

[Scott Danforth]

Hit the stickies - something amiss if you cant hit WOT.

Start with a compression test, then check/adjust timing and verify the throttle plates are opening fully

The manual states 3900-4300, however nowhere near the top end of the RPM range where you can damage components. Its a balanced inline 6 with 7 bearing journals. you run out of air-flow into the cylinders with the poor intake runner design before you can damage the motor in any way shape or form

 

[fishrdan]

My manual says 1.65 for that year range.

Probably correct