mercruiser I/O top end problem

[slshoe]

Please bear with my lack of knowledge on the subject. I just bought a 78 Sea Ray with a Mercruiser 140hp. I took it out for the first time today and the top end speed was very weak, maybe 20mph I figure I will replace spark plugs, wires, and maybe fuel filter, anything else I might want to look at. Oh and I had a hard time starting it at first, and after some investigating I think it was partially becaause I didnt run the motor blower p[rior to starting it. (didnt even know what it was---first I/O). Thanks in advance for any and all help.

 

[180shabah]

Re: mercruiser I/O top end problem

The blower will not help you get the engine started, it will help you get rid of fuel vapors so you don't EXPLODE when you try to start it.

As for the low speed - don't worry about speed, it will be what it will be. What was you WOT RPM?

 

[QC]

Re: mercruiser I/O top end problem

Sounds like a good plan, but I can assure you the blower did not contribute . . . What RPM would she make at wide open throttle (WOT)? Welcome to iboats!!

 

[slshoe]

Re: mercruiser I/O top end problem

top end rpm's were 5500 and pardon my stupidity but whats WOT???

 

[Dante04SRT]

Re: mercruiser I/O top end problem

top end rpm's were 5500 and pardon my stupidity but whats WOT???

Wide Open Throttle

 

[Gary H NC]

Re: mercruiser I/O top end problem

If it was turning 5500 rpms you should see better than 20mph.
It really depends on the weight,size of the boat and hull design.
I would do the complete tune up,Including setting the timing and dwell,put some Seafoam in the fuel in case its water contaminated.Change the fuel water seperator filter.Then see how it does.

 

[Dante04SRT]

Re: mercruiser I/O top end problem

If it was turning 5500 rpms you should see better than 20mph.
It really depends on the weight,size of the boat and hull design.
I would do the complete tune up,Including setting the timing and dwell,put some Seafoam in the fuel in case its water contaminated.Change the fuel water seperator filter.Then see how it does.

I thought the 3.0 maxed out around 4400-4800rpm? Maybe its just too high and way outside its powerband.

 

[180shabah]

Re: mercruiser I/O top end problem

Depends - 3.0L is 4200-4600, 3.0LX is 4400-4800. His is a decade older than yours and I believe they were 4200-4600 then, which puts him 1000RPM over WOT. If his tach is correct he is WAY underpropped.

 

[180shabah]

Re: mercruiser I/O top end problem

slshoe click here and read

Should help answer alot of ?'s

 

[Don S]

Re: mercruiser I/O top end problem

180shabah
That link goes to a manual for the 90 to 97 engines, not his 77 or 78. Lots of changes in 20 years.

 

[slshoe]

Re: mercruiser I/O top end problem

okay, thanks. Im going to change the stuff I originslly planned, then go from there. Make dure myu prop is the right size, RPM reading is correct etc.. I'll let you know how it goes.

 

[firehog6305]

Re: mercruiser I/O top end problem

that old of a searay is a very heavy boat, I had a 19ft alum starcraft with a merc 140, it was a 1995, and fot I could only get about 26mph, so that might be right, I know that searay has to be at least twice as heavy then that starcraft

 

[ParallaxBill]

Re: mercruiser I/O top end problem

With rpms that high along with the slow speed could the prop be actually slipping?

4200-4600 should be correct WOT rpms.

BTW, (for comparisons sake) my 18' 72 Glassmaster will do 42 @ 4250rpms with a normal load with a 165 Mercruiser and I'm told here that it is a heavy boat. I think his boat has something more than a weight problem.

 

[180shabah]

Re: mercruiser I/O top end problem

With rpms that high along with the slow speed could the prop be actually slipping?

If the prop hub were spun he would have high RPMs, but he would lose all propulsion, not just some of it.

 

[ParallaxBill]

Re: mercruiser I/O top end problem

Yep, that's true.
I still think something is wrong unless his boat is way heavier or way bigger than is normal for a 140 MC.

 

[Don S]

Re: mercruiser I/O top end problem

Did you have the drive trimmed to far up or down? Did you try different settings of trim?
What size prop do you have.
I know some people that have under propped there boats intentionally because they can only be used in low speed cruising or trolling while fishing, but still keeping the engine rpm us so the engine lasts longer.

 

[slshoe]

Re: mercruiser I/O top end problem

I changed plugs today, no place open for plug wires and dist. cap like I wanted, the old plugs were in bad shape. Boat started much easier but again no top end speed. I have a 18.5 ft 78 sea ray fiberglass V-hull. I think I could expect at least 30mph. Its not even tubing speed. How can I tell what size prop I have?? Tomorrow plug wires and dist. cap are being replaced. I also added some seafoam to the gas. Also I trimmed it up, down and all around, the problem is, it doesnt quite go fast enough for it to plane out all the way so im not real sure whats optimum yet. Thanks for the input guys keep it coming.

 

[slshoe]

Re: mercruiser I/O top end problem

I also thought I would add, the props I have (came with 2) the one not attached has a serial #48-78120-19 nad eith 0 or D at the end, on the side it has a #S891 can you tell from these #'s what size it is or do I have to measure it somehow?

 

[180shabah]

Re: mercruiser I/O top end problem

....serial #48-78120-19.....

19" pitch

 

[Don S]

Re: mercruiser I/O top end problem

What about the prop you are actually running? Do you have a number for it?
You might even put the 48-78120-19 on and see what it does