3.0 mercruiser low rpm's

[trabich]

I had recently bought a 1984 vip 19 foot ski/fish boat. Overall this boat is very well kept. The first time we took the boat out we were hitting about 45 mph and at about 2800 rpms. The last couple of trips out to the lake we were only hitting 30 mph and about 2200 - 2500 rpm. I just cant figure out why the top speed has lowered 10 mph and my rpms are so low. There is a 21p prop on the boat which I am looking for a 19p. I changed plugs, fluids, and am getting ready to run some sea foam through the carbs. Any Ideas would be greatly appreciated.

 

[67cuda340]

Re: 3.0 mercruiser low rpm's

Hi,

I do have a 14 x 19 pitch prop I took off my 84 seaswirl 3.0 in/out board
mercruiser. I changed to a 17 pitch. If interested let me know.

As far as the low rpm's make sure the throttle is opening fully.

 

[45Auto]

Re: 3.0 mercruiser low rpm's

The first time we took the boat out we were hitting about 45 mph and at about 2800 rpms.

You need to verify your tach and use a GPS for speed. Something is wrong somewhere, no way you were doing 45 MPH at 2800 RPM on a 3.0L. At least one of them is wrong or you mis-read it.

That would be like me claiming that my 5.7L Suburban cruises down the freeway at 150 MPH turning 1000 RPM.

The last couple of trips out to the lake we were only hitting 30 mph and about 2200 - 2500 rpm.

Those speed vs RPM numbers sound about right. If so, first thing is to verify that your engine is in decent shape. It should be turning up into the low to mid 4000 RPM range, probably somewhere around 40 MPH. When was the last time it was tuned up and had a compression check performed?

 

[fat fanny]

Re: 3.0 mercruiser low rpm's

I have to agree w/ 45 ther is no way you were turning 2800 @ 40 mph my bet is the speedo is incorrect. GPS will correct that issue immediately.

 

[Bondo]

Re: 3.0 mercruiser low rpm's

I changed plugs, fluids, and am getting ready to run some sea foam through the carbs. Any Ideas would be greatly appreciated.

Ayuh,.... Do a Full,+ Complete Tune Up, 'n Service... Plugs,+ snakeoil Ain't a tune up...

I agree, your #s are Way Off... Probably the tach...
I'd throw in a Compression Test, while you're doin' the Tune Up....

 

[trabich]

Re: 3.0 mercruiser low rpm's

OK, I guess that did not come out right. As of now I am only turning 2200 rpms at 25 mph. My top speed is only hitting 25 mph. I read before that at top speed I should be turning 4 - 4200 rpms. I am getting ready to check the timing but am wondering where my rpms are. Compression test came out good.

 

[45Auto]

Re: 3.0 mercruiser low rpm's

You have engine problems somewhere. All the screwing with props in the world won't get you the 2000 RPM you're short. You're just wasting your time even thinking about props until you get the engine working correctly.

An engine needs 3 things to run right:

1) compression
2) fuel
3) spark at the correct time.

You're missing at least one of these. If your compression is good as you claim, then the spark or fuel (or both) must be incorrect.

Go here and check everything on the list. Check your fuel pressure and timing. Have someone who knows what they are doing set the points if your boat still has them:

http://forums.iboats.com/showthread.php?t=295070