Merc 140 performance

[trailshredder]

Just got my 76 Merc 140 back together, new head, gaskets, spark plugs, rebuilt carb and rebuilt 19 degree prop. This is my first ever boat so I am not really sure what to expect out of it. Just seems like it should have a little more ?umph? than what it does, although, how do you measure ?umph?! It will do about 35, downwind, full throttle at about 4100RPM. Occasionally will miss at full throttle and stall at idle which makes it a real pain trying to trailer it at the ramp! I have turned the idle up now (900-1000 RPM) to keep it from dying out and adjust the 2 idle/air mixture screws to the smoothest idle out of the water. Wondering if the valves need adjusted again after running a while?

 

[Don S]

Re: Merc 140 performance

You could have a hundred different things wrong. If you keep it idling at that rpm, you won't have a drive.
Have you checked the dwell and timing? Do a compression check, that should give you an indication of tight valves.
check your idle mixture, book specs are just preliminary settings.

 

[trailshredder]

Re: Merc 140 performance

I dont recall seeing in the manual if valves should be adjust with lifters "bled off" or pumped up. Does it matter? engine didn't run for at least 2 weeks prior to adjustment.

 

[Bondo]

Re: Merc 140 performance

Just seems like it should have a little more ?umph? than what it does, although, how do you measure ?umph?!

Ayuh,... It's the bottom of the line, entry level motor...

?umph? , ain't somethin' it's noted for...

 

[45Auto]

Re: Merc 140 performance

My '82 Mark Twain trihull (16') with a Mercruiser 120 would do 32 MPH going downhill.

As Bondo says, the little 4 cylinders aren't well reknowned in the boating world as real "powerhouses". However, mine would deliver about 7 MPG which is GREAT for boat!

 

[trailshredder]

Re: Merc 140 performance

I took it out again last night and noticed a few things other than just "umph". It has miss/cutout to it but not consistent. I added a clear glass inline filter to the fuel line from the tank. I noticed quit a bit of crap in that filter. Appears to be corrosion chunks so I am guessing there is some rust in the tank. If there is rust, I would have to assume there is moisture in there. I am leaning toward this being my issue. Any opinions? Can these old metal tanks be cleaned out or am I better off getting a new plastic one?

 

[45Auto]

Re: Merc 140 performance

You can hook the fuel line to a clean external tank (5 gallon outboard tank, etc), and a test run will make it obvious real quick whether it's your internal tank or a problem somewhere else.

 

[trailshredder]

Re: Merc 140 performance

I wondered that. Wasn't sure if there was anything special about the old metal tank or not. Is it simply just a baffled metal tank with a suction hose that goes in it? Wondering if I should tear the carb down again and give it a good cleaning.

 

[Don S]

Re: Merc 140 performance

You can hook up a remote tank, but it's not going to do a thing with the dirt already in the carb and fuel pump. Try the tank, but you might have to rebuild the carb, and replace the filter in the fuel pump to cure your problems.

PS, Props are in inches of pitch, not degree's.

 

[trailshredder]

Re: Merc 140 performance

man, what a learning experience! I didn't even know there was a filter in the pump itself! Sometimes I just feel like a real idiot! So, now I have 3 filters! Thanks again Don!

 

[trailshredder]

Re: Merc 140 performance

Compression looks good (115-120-110-105). Timing was a bit off, nuts on now. Going to look for a fuel filter, fuel supply hose and try to borrow an outboard fuel tank. How can you tell if the points should be replaced?

 

[Bamaman1]

Re: Merc 140 performance

You can tell if the points need replaced by visually inspecting them for wear. Mercruiser 140's seldom need the points replaced due to relatively low hours usage.

You're going down the right path. Your 19 pitch prop should be the right one for general use. Like it's been said, these engines are not known for out of the hole power, but will usually run fine after the boat planes off.

Too bad nobody's ever figured out how to install a nitrous kit on these motors to use until the boat planes off. You'd have power out of the hole--and cruise with great fuel usage.

 

[RIDEPATE]

Re: Merc 140 performance

Trail, that "clear glass inline filter" is not a good idea. Glass breaks, you go boom.............at that point you'll know what "umph" is all about.

 

[alilley]

Re: Merc 140 performance

that is funny at you mention that abotu the nitrous kis cuz me and dad have been talking about trying to figure out how to put a turbo on one.

i am suprised that some one has not mentioned this before but you compression numbers look a little low. now grant it mine is fresh rebuilt but my numbers 2 of them are at 135 and the other 2 at 140...... how many hours are on the motor? yoyu said you went throught the head, but did you do anything to the bottom end??......just a thought here

 

[dubs283]

Re: Merc 140 performance

trying to figure out how to put a turbo on one.

don't know how well a turbo would work with wet exhaust

but a supercharger would work

however, not too many people other than tinkerers would have any interest in a supercharged 3.0 liter

this is what you call yummy:

http://www.acms-usa.com/bulletin_files/images/superchargedMercruiser.jpg

 

[pwpns]

Re: Merc 140 performance

I used to have a 81 Bayliner Mutiny 1750 with a 120 Merc in it and the best I could ever do was 32.5 and I was running a 19 pitch prop also. Sounds like its about right to me.

 

[trailshredder]

Re: Merc 140 performance

Guess the speed is in the ball park then, just need to get this miss/cutout issue resolved. Not sure how to tell if points need replaced. My compression numbers were done on a cold engine, I always thought it wasn't really the number itself that was important but the variance in number of all the cylinders. Just did the top end as a result of a blown head gasket and craked head. Previous owner said he replaced the water pump and it is my guess that when the pump went he limped it back to shore overheating. Thanks for all the tips, I am leaning toward a contaminated fuel issue right now. Possibly points if anyone knows a way to check them for being worn, they appear smooth and alligned but I haven't actually checked the gap on them.

 

[mkast]

Re: Merc 140 performance

Not sure how to tell if points need replaced.

I always thought it wasn't really the number itself that was important but the variance in number of all the cylinders.

Look at the points, are they burned, pitted or wearing unevenly?

Even compression numbers between cylinders indicate the engine cylinders are wearing evenly.
Most troubleshooting guide lines mention a 10% variance between cylinders, indication of a problem.
Obviously, if the numbers all read 140 PSI engine is good shape, if all the numbers read 95 PSI, engine is wearing evenly, but, worn out.

 

[ziggy]

Re: Merc 140 performance

just replace the points. they're not expensive. i replace mine yearly and consider them to be a part of summerization. also do a tune up and set dwell (point gap), then timing...

agree with others. the compression numbers are getting close to being to low. anything below 100psi and something is up. http://www.boatfix.com/merc/Bullet/97/97_25.pdf

 

[Don S]

Re: Merc 140 performance

I to would strongly suggest at this point that at this point you need to do a full tuneup, including the compression test ziggy linked to. Just to make sure it was done right.
That tuneup includes points, condenser, cap, rotor, sparkplugs (NOT Champion) and even the plug wires if they are looking old and tired, or the ends show any corrosion at all.
Then you need to set the dwell, not just point gap. (YES, you need a dwell meter) you then need to set the timing to spec. and confirm the advance timing is working.
May also want to rebuild the carb. Complete disassembly, soaked in carb cleaner and all passages blown out with compressed air and verifiied. Replace all fuel filters. Reinstall and after engine warm up, adjust idle mixture and idle speed.
If you think the valve adjustment could be off, readjust them. But do it properly, not how you think it should be done.
Everything has to work together, at the same time for your engine to run it's best.

After all that, see how it runs.