Only 3000 RPMs?! Help!! 67 Mercruiser 6cyl

[keith204]

67 Mercruiser. Original engine. Inline 6cyl. I bought this a couple of weeks ago.
This thing runs GREAT but doesn't seem to have the getup it should. The local boat shop told me that this thing should be running at about 5,000 RMPs instead of 3,000.

I replaced the terribly beat up prop with a brand new one, and it seems to be a bit quicker, and very smooth. However, RPMs are stil staying at 3,000. Any ideas?

 

[ziggy]

Re: Only 3000 RPMs?! Help!! 67 Mercruiser 6cyl

Any ideas?

get an oem mercruiser service manual for it. do a compression test. see if ya got anything to work with. do a minor tune up to it. prop was a good start if the old one was chewed. don't know about a 67. but a 72 L6 wot specs is 3900-4300. 5k seems a bit fast..what prop pitch ya got. it should be stamped on the prop...

 

[keith204]

Re: Only 3000 RPMs?! Help!! 67 Mercruiser 6cyl

What kind of prop pitch numbers am I looking for? I see a 19 on the end of the number, and i think the number begins with a 13. Like this: 13-xxxx-19. If 13 is way off, then it may be an 18... unsure what these numbers mean.

 

[ParallaxBill]

Re: Only 3000 RPMs?! Help!! 67 Mercruiser 6cyl

Keith what size boat is it running in?
I run a 14.5"x19p aluminum prop on my 18' tri-hull 72 and WOT is 4,300 rpm or a little lower with more people or load on the boat. Just as it should be for a 72 L6 Mercruiser. But, like ziggy I cannot be sure about the 67 but I will look it up for ya tonight. 5K is definately too high for a GM L6. Even in a car or truck they just were not designed to be high revving motors.

 

[ParallaxBill]

Re: Only 3000 RPMs?! Help!! 67 Mercruiser 6cyl

From the numbers on your prop I would bet it is an original Mercruiser prop. If you can decipher the missing numbers a google search just may bring up the prop info. It worked for me on my SS Mercruiser prop. Also run the numbers on Ebay for information.

 

[keith204]

Re: Only 3000 RPMs?! Help!! 67 Mercruiser 6cyl

Keith what size boat is it running in?
I run a 14.5"x19p aluminum prop on my 18' tri-hull 72 and WOT is 4,300 rpm or a little lower with more people or load on the boat. Just as it should be for a 72 L6 Mercruiser. But, like ziggy I cannot be sure about the 67 but I will look it up for ya tonight. 5K is definately too high for a GM L6. Even in a car or truck they just were not designed to be high revving motors.

It's a 18ft. Suppose it's a V-bottom or something.. not a tri-hull. I did find that the RPMS should be around 4600. I'm thinking this is something fuel related. Just that, my old beat up prop.. so terribly beat up.. gave me 3000 RPMs and my brand new prop gave me 3000 RPMs. It's almost as if... the gas is being restricted in a way that it can't give enough gas to push it over 3000. I could easily be wrong, as I am new to the boat thing... The boat place said that this new prop will easily give me a few hundred more RPMs than my old one, but it gave me none. Any ideas?

 

[Coors]

Re: Only 3000 RPMs?! Help!! 67 Mercruiser 6cyl

search, and read the threads in here
,
you need to understand how engines operate.

 

[keith204]

Re: Only 3000 RPMs?! Help!! 67 Mercruiser 6cyl

Coors, do you have any particular posts in mind?

 

[ziggy]

Re: Only 3000 RPMs?! Help!! 67 Mercruiser 6cyl

from a post by don s...............

 

[keith204]

Re: Only 3000 RPMs?! Help!! 67 Mercruiser 6cyl

dad gum... I saw that list earlier, but it's so long, and I sure dont wanna go through it!

Ok fine, you win. This will probably be bulletproof. Slowly, and surely... I will work my way through this list, and afterwords, have a good understanding of how all this stuff works.

Thanks for the list.

 

[ParallaxBill]

Re: Only 3000 RPMs?! Help!! 67 Mercruiser 6cyl

Keith, also check to see if your tach is working correctly. When I bought my boat it wouldn't pull more than 3,400 rpm but it turned out it was getting much higher revs. The lightbulb in my tach had been installed by the previous owner too deep and the bulb itself was stopping the needle assembly at 3,400. Fixed the problem and now I know how much it is actually turning. Your problem may be different but my example is one example of "sometimes it could be anything". Eliminate the easy to fix problems first if you can.

Keep us informed on your progress.