Motor Speed

[amateurfishing]

Finally able to test my 75 HP '84 mercury motor to its full potential....had it out today but only mustering about 10 mph (on HB gps) at full throttle, should it not be faster than this? it did do a lot of sputtering at full throttle like it was starving for fuel, fuel pump on seemed to help a bit but was still not completely smooth but would think i could get more speed than this

[Brad B]

I would take it in and get it looked at. a 75HP motor regardless of age should easily do 30 plus MPH. I have a 2011 40HP Yamaha 4 Stroke and I get 26MPH out of that at wide open.

[boatfixer]

No doubt something is wrong. Do you have a tachometer?

Have you done a compression test? That would be first.

Check for spark on all cyls, use a timing light to look for steady spark at higher rpms

[amateurfishing]

i am mechanically illiterate but did assume something is amiss after yesterday, did not know for sure if it could be year old gas but doubt that would hold it back that much. i know i just had a lower unit test done to check for possible leakage there and mechanic said all was in great shape with a compression test on that. dont know if it would have anything to do with a fuel pump or not, i know that the fuel pump is not necessary but previous owner said he used it to keep fuel flowing smoother/better at higher speeds for tubing/skiing. i did use it a few times yesterday during the sputtering, seemed to help a little but not 100% better, more like 40-50% better with fuel pump on at high speeds.

[boatfixer]

Ummmm, fuel pump? Are you using an external fuel pump? You shouldn't have to use anything other than the one on the engine. If that one fails, you should get a diaphragm kit and repair it. Not to mention, the use of an automotive type electric pump is highly discouraged.

[fishingstar]

year old gas would give a lot problems.

Start with fresh gas

[amateurfishing]

yes there is an external fuel pump....previous owner told me used it to keep gas flowing better at high speeds for tubers/skiiers, is there no such thing as boat/motor external fuel pump set up? boat and motor are all orginal setup together (to best of my knowledge), everything is 1984...boat, motor, trailer. if there are no boat/motor designs for external fuel pump, i can look into that. i told mechanic, he looked at it & shrugged his shoulders

if its a gas issue, should i put stabil or something in it to help run (even though i did that to winterize last fall), or just better off dumping all 18 gallons? would prefer burning it off if possible

[boatfixer]

There is no need for 2 fuel pumps. For around 8 dollars, you can buy a diaphram kit for the original pump. Much cheaper than buying an electric pump. It was not part of the original set up.

As far as the old gas in there, adding Stabil, Seafoam, etc will do nothing. Once gas goes bad, its bad.

If you can remove it and dispose of it properly, that would be best.

[wastewaterguru]

Getting rid of 18 gallons properly is no easy task. First take a sample from the bottom of the tank. Make sure it isn't separated or water contaminated. If the boat is new to you......then we are starting from scratch and can't assume it ran well before you got it. Any information on the history of the issue would be helpful here. A compression test of the motor should be done next. (Should always be done before anyone buys a boat since the tester is only 50 to a hundred bucks). The electric external pump is a red flag. Poor crankcase pressure would deem the engine diaphragm pump worthless.....and could be caused by crankshaft seals, reed valves, or a bad cylinder, etc......all of which are spendy.

Does fuel spray out of the carbs when running??

Still a lot of unknowns to pin it on a fuel problem.

[Fishjet]

Water in your gas

[amateurfishing]

well the motor runs but obviously at maximum efficiency

hopefully i will have time to burn most of gas out this year and go from there...will also talk to mechanic about scenarios

no tachometer on boat

i know a compression test was done earlier this year for possible lower unit leak issues and was told all was great, dont know if that is the same compression test that was mentioned above or or diff't one.

thx for all the info, help, ideas, & things to look for, will also have to dig into the "need" for external fuel pump issue.

[dukhntr]

i know a compression test was done earlier this year for possible lower unit leak issues

Huh? A compression test is for the engine. A compression tester is screwed into the spark plug hole in the head and the engine is turned over. The reading is in pounds of compression. All cylinders should be within 10% of each other. If the engine had a compression test,the shop should give you the compression reading for each cylinder.

A leak test is done on the lower unit to test for leaks and the integrity of the seals.

dukhntr

Also, your auxillary fuel pump is an accident waiting to happen. If there is an engine problem, that pump will continue to pump fuel to the engine, and possibly into the boat. Is the engine hard to start? If so, my guess is the crank seals need to be replaced. If the engine starts properly, rebuild the stock fuel pump as others have said.

Good luck!

[TruthWalleyes]

External fuel pump

Sounds like you've got a 1 of a kind there man. Could do some major damage continuing to run like that. Bring it in, remove the external fuel pump, which is most likely just compensating for internal fuel pump not working.

Imagine a leak in that fuel line...

[amateurfishing]

k, so external fuel pump is a problem no issues with motor starting

had some kind of pressure test done on lower unit to make sure all seals were good, maybe NOT a compression test

just wondering why motor would be sluggish at full throttle, looks like some tests need to be done including internal fuel pump, any idea on cost for a 75 HP '84 mercury?

thx for all the suggestions, ideas, info, & help......continue to love this site

[20lbSloughShark]

fuel pump kit for my '84 Johnson 40 hp was $20 shipping and all. Change the spark plugs too. Siphon the old gas and run it in your car or truck, who cares if that seems sluggish. Get new 91 octane non-oxygen gas. I think you should just get rid of the external fuel pump. If the fuel pump kit, new plugs, and new gas doesn't work, clean the carbs.

[amateurfishing]

can i dump boat gas into truck with no issues?

motor is 1984, is it ok to use non oxy gas when motor was made b4 that was around?

[wastewaterguru]

Non-oxy gas just means no oxygenation additive. Ethanol is the main oxygen additive. Non-oxy has been around longer than you have.

[TruthWalleyes]

Did you mix OIL into your gas? If you did, you do not want to run that through your vehicle.

A quick compression test would be good too. Remove spark plugs, either screw in compression gage or press into spark plug hole. Should be 80psi or higher when cranking. Anything below 80, or 1 cylinder being drastically higher or lower than another is a sign of some major problems.

I agree with new spark plugs. Pump kit for sure. If you can, get rid of the gas and try new, or just hook your motor up to a smaller tank with good gas if you can.

Really never a good idea to leave that much gas in the tank over the winter, try to run it out at the end of this year if possible.

[amateurfishing]

yes, it is premix gas

sounds like my first priority will be getting rid of gas, how long will it take to burn off 18 gallons on 75 hp motor?

spark plugs were brand new last year, only used handful of times. i think mechanic did compression test last year but will double check that. external fuel pump has been on since original owner (has its own switch) previous owner said he never used it, i only used it to siphon out gas last summer so would like to keep it for that reason but will not use it for boat operation. im guessing old gas that i need to burn off with a compression test close 2nd.

again, thx for all info and help. looks like i have some boat rides in my near future to get old gas burned off asap

[SkunkedAgain]

sounds like my first priority will be getting rid of gas, how long will it take to burn off 18 gallons on 75 hp motor? .... im guessing old gas that i need to burn off with a compression test close 2nd .... looks like i have some boat rides in my near future to get old gas burned off asap

[amateurfishing]

k, thx, will get it checked out asap

appreciate all the info, advice