low WOT on mercruiser 5.7 with 19p prop

josh84 · **Joined:** Sun May 19, 2013 1:48 pm **Posts:** 10

hi guys new to the site i have a 86 liberator with a 5.7 with 56hour i just got the boat a month ago an have ran it a few times an noticed that the wot is a little low i have a 5.7 with alpha 1 an 1.47 gear an 14.5 19 pitch prop 3 blade ss an im getting like 3,900-4,000 rpm at wot i had changed both the carb filter an the water sep an nothing changed with rpm so i had the carb rebuilt just to know it was done an still nothing changed i checked the timing today an its spot on so i pulled the cap cuz i seem to have a little skip an it was moist an corroded. do u guys think a tune up with fix this problem an with a 19 pitch prop what should i see for rpm ? thanks for any in put

rpengr · **Posted:** Tue May 21, 2013 12:01 pm

I assume this boat is new to you, that you don't know what RPM it was getting in the past?

A moist and corroded dist cap (and rotor) will certainly cause problems, and you might find you were running on 7 instead of 8 cylinders. (pulling the spark plugs could help you tell if that was the case). You could also have a bad spark plug wire, or bad spark plug.

As far as the prop question... it would help to know what speed you are obtaining (by gps), and what size libby you have. If the RPM is low and the speed is low (less than 37-40mph?), then it is most likely an engine issue. Assuming you have a 21' or so, then I would guess you should get around 50mph at 4700 rpm. I would also guess that the prop should be a 19 or 21 pitch.

I have never had a Liberator, so someone else should have more accurate information.

afw200 · **Posted:** Tue May 21, 2013 2:24 pm

Quote:

As far as the prop question... it would help to know what speed you are obtaining (by gps), and what size libby you have.

josh84 wrote:

i named my 86 211 liberator ''just fore play'' some people get a kick out of it when they figure it out :lol:

rpengr · **Posted:** Tue May 21, 2013 2:53 pm

Very good. My previous post assumed a 21' for my Prop/Speed comments.

josh84 · **Joined:** Sun May 19, 2013 1:48 pm **Posts:** 10

thanks for the in puts guys. an yes the boat is new to me i just picked it up bout a month ago. with that said the boat is a 1986 21' liberator with a 260hp merc with a alpha 1.47 gear an a 14.5 19 pitch prop i put a new cap an rotor an now im getting like 4100 rpm so i think im gonna do the plugs an wires too. but as it sits with me an my gf in the boat an 30gals of fuel at wot 4100 its doin 48mph on the gps an the last time i took it out with half tank of fuel i seen it at 52 mph at 4000 rpm.idk if that helps u guys but i dnt know much bout the boat cuz i just got it but the motor is newer with 60hours on it an the out drive is rebuilt all fluids are topped off theres not bottom paint an the transom is dry i drilled a hole thru it for my transducer an the wood is dry if theres any other questions let me know an thanks for all the help guys

ric · **Posted:** Mon May 27, 2013 9:54 pm

Are you trimming up?

rpengr · **Posted:** Tue May 28, 2013 3:31 pm

I cannot be sure, but my guess is that 260 HP on a 21' Libby would give you around 50mph, or low 50's.

If you are getting near 50 at only 4100 rpm, it sound like:
..1.. Your engine must be running well, and is producing good horsepower.
..2.. 19 pitch may be too much pitch for your gear ratio...therefore you are only getting 4100 rpm
..3.. You would get more horsepower out of your engine if you could get the prop and/or gear ratio adjusted so that you can obtain an RPM near the engines maximum allowable RPM (perhaps 4600?)

With your current gear ratio, it sounds like you should run a 17p prop if you value better hole shot and pulling power, or stick with the 19p for better gas mileage while cruising.

josh84 · **Joined:** Sun May 19, 2013 1:48 pm **Posts:** 10

Yeah ur probably rite that mite be all the motor will turn with a 19 pitch prop an 1.47 gear in the out drive. But I was reading that most ppl run 21pitch props on the 21' libs ? I really dnt care bout hole shot to much I like the top speed Lol so how much mph will I lose with a 17 pitch ? My last boat was a 21' maxum with a 4.3 an had a 14.5 19 pitch prop an I use to run that boat up to 4800 rpm but I think the gear in the out drive is different like 1.82 but I'm not sure but thanks for all ur help

rpengr · **Posted:** Wed May 29, 2013 2:03 pm

I would not expect that you would lose any MPH with the 17p prop. Hopefully you will gain speed. (but you will use a little more gas also). The reason is that at 4100 rpm, your engine is not able to put out 100% full horsepower. Your probably only getting 85-90%

josh84 · **Joined:** Sun May 19, 2013 1:48 pm **Posts:** 10

ok great im goin to see if i can get my hands on a 17 pitch to try am see what it does thank u so much for all ur help

jvthundercat · **Posted:** Thu May 30, 2013 3:47 am

If you are that worried about a the extra couple mph, go to BBlades and have them hook you up with a labbed prop. Cheaper than you think and tuned for your boat and driving style. If it was me, I would throw a cheap 17 on it and enjoy the boat........not really, I would have that engine out and getting it up to at least 400 ponies so I could put a 24 pitch prop on it.. :mrgreen:

josh84 · **Joined:** Sun May 19, 2013 1:48 pm **Posts:** 10

well i have a 450hp sbc that i had in a pick up i was thinking bout detuning it a little like pull some timing an a smaller marine carb. its a 355 with vortec heads an 536 lift cam all roller top end an it made 450hp 440 ft lbs with 38 deg of timing an a 750 cfm double pumper holley carb so if i pull timing an put a 650 cfm edlebrock marine carb an get it down to like 400hp u think the outdrive will hold it ? thats the only thing stopping me from doin it

rpengr · **Posted:** Fri May 31, 2013 7:10 am

josh84 wrote:

well i have a 450hp sbc that i had in a pick up i was thinking bout detuning it a little like pull some timing an a smaller marine carb. its a 355 with vortec heads an 536 lift cam all roller top end an it made 450hp 440 ft lbs with 38 deg of timing an a 750 cfm double pumper holley carb so if i pull timing an put a 650 cfm edlebrock marine carb an get it down to like 400hp u think the outdrive will hold it ? thats the only thing stopping me from doin it

I have heard that the Volvo SX drive would hold 400hp.

I have no idea about the Merc Alpha I. If you don't get an answer here, then try the same question on iboats.com

jvthundercat · **Posted:** Sat Jun 01, 2013 6:38 am

josh84 wrote:

well i have a 450hp sbc that i had in a pick up i was thinking bout detuning it a little like pull some timing an a smaller marine carb. its a 355 with vortec heads an 536 lift cam all roller top end an it made 450hp 440 ft lbs with 38 deg of timing an a 750 cfm double pumper holley carb so if i pull timing an put a 650 cfm edlebrock marine carb an get it down to like 400hp u think the outdrive will hold it ? thats the only thing stopping me from doin it

You will blow out that drive in one pass. A Bravo can't even handle my 415 hp 502's.

josh84 · **Joined:** Sun May 19, 2013 1:48 pm **Posts:** 10

thanks for the info guys

so my new problem is i bought a cap an rotor from west marine its a sierra brand an the second time out it failed on me the boat was running on 4 cylinders anyone have problems with the sierra parts before ? i was talking to a friend that works at a marina an he told me the sierra stuff is junk to go with oem quick silver so i think its time for a complete tune up new plugs,cap,rotor,wires,an ill probably change out the sierra filters i just put on too. but as for more power i think my plan is a set of vortec heads an new cam an carb an use the stock bottom end cuz the motor only has 60 hrs on it but what size cam should i get anyone know ? thanks for all the help guys

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low WOT on mercruiser 5.7 with 19p prop

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