I have a late 80’s model Mercury 25 2-stroke that won’t stay running…
I took it out last month to Morris from the Folly landing and on the way out and back I had to keep pumping the bulb or it would shut off at full throttle. It would run at idle and putt/troll speed fine though… The motor did this about 6 years ago and at that time I replaced the gas, spark plugs, and did a carb kit. One or all of these thing seemed to fix it and it ran like a champ…
Now 6 years later it does the same thing so I did the same thing; new sparks, new gas, and new carb kit and it still won’t stay running. Not even if I pump the bulb and not even at idle speed… Tonight I even replaced the gas a second time as well as re-cleaned the carb and insured I put it back together correctly and no change…
It cranks, fires and runs for maybe 5 seconds and then shuts off. I was thinking maybe fuel pump, but I’m not even sure if this old motor has one or where it is if it has one,haha. I’m at the end on my limited combustion engine knowledge (fuel + air + spark = run), so any help is much appreciated.
Thanks!!!
Joel
I can’t speak for the Merc, but I’m pretty sure it has a fuel pump. I know on my mid-80’s Johnson, and the 25hp Evinrude we used to have (an 86 model) both have fuel pumps.
That would be the first thing to check. Usually you can get a rebuild kit fairly cheap for them, and they should’t be all that hard to rebuild. Normally it’s just plastic parts and washers, and a few screws. It probably just runs off of vacuum generated by the engine.
First just check for a clogged screen on the fuel pump.
Second, pull the carb and run a fine copper wire down through the main jet. You are obviously getting enough fuel to idle, but your high speed jet could be clogged up. Just soaking it, or spraying it, won’t work if the jet is plugged. In fact, if you just rebuilt this carb, go ahead and pull it off again now, and check this.
If those don’t work, rebuild the fuel pump (or, rebuild it anyhow if you’ve never rebuilt it, it’s cheap insurance).
Third, if these don’t check out, check for fishing line wrapped around the prop, it can snake it’s way far up in there.
… The Cross of Christ is the anvil upon which the hammer of evil wore itself out.
Thanks for the response redfish!!!
Fuel pump does make sense, I’ll see if I can go that route. A buddy of mine has a similar year/model Johnson and I think he did his fuel pump last summer. I’ll see if I can do a little beer bartering for his help.
I’ll also check the prop for line. Good tip, regardless of whether or not it helps the “fuel” issue.
Thanks again!!!
“Pura Vida”
Smitty
India '07
15’ Alum. Fisher w/ Mercury 25HD
12’ Tarpon 120 w/ Two Arms and a Paddle - SOLD…
Yeah, I would pull the fuel pump and see if it has a screen that’s clogged, and then pull the carb and run a fine copper wire through it. Both of those places are very well known in Johnsons for causing your symptoms, and I can’t see where Merc would be much different. The good news is, both of those are nearly “free” to fix if that’s all it is, shouldn’t need any parts unless you damage a gasket on your carb.
You can get copper wire from any random electric cord, esp. the cord from low voltage “wall wart” type DC converters like cell phones and such.
… The Cross of Christ is the anvil upon which the hammer of evil wore itself out.
I don’t know if you have a manual, but this is a good site for them. http://boatinfo.no/lib/library.html You can’t download the whole manual, but can print as many pages as you need. It really helped when I was working on my trim pump for my '87 Mercury 45hp. 50 classic.
Bob Van Gundy
Marine Designs,Inc.
Custom Aluminum Fabrication
803-727-4069
I went crazy trying to figure out the same type problem years ago,ended up just needed a new fuel line and connector from gas tank to engine,My mechanic loves me:frowning_face:
Have you checked the fuel filter to make sure it isn’t clogged? Also check for a pin hole in the bulb or fuel line that might let the engine suck in air instead of gas. If you have the old style Mercury connector check to be sure you haven’t lost the o-ring that makes them seal. Finally, once you have the problem corrected, stay away from ethanol gas if you aren’t already.
I pulled up the owners manual and It is a side mounted fuel pump on the carb that is made up of the fuel pump body, cover, and two gaskets. The problem is I don’t have the body…
The body is a small metal plate with holes in it that is sandwiched between the cover, the two gaskets, and the side of the carb. When I rebuilt my carb this weekend with new gaskets it was never there… When I rebuilt it six years ago it was never there either… How has my motor been running all this time without a pump and the cover plate smashed to the side of the carb?
I know y’all are gonna ask did you loose it this weekend, but I promise I didn’t. When I pulled the carb to rebuild it, the two gaskets were sandwiched together with no fuel pump body between them… I assume this was the case six years ago two when I did it, put the motor ran fine until now…
I guess I’ll go through all my connection hoses and gaskets to see if there are leaks, but what should I do about this magic pump, or lack there of? Does anyone actually have this motor they could reference from?
“Pura Vida”
Smitty
India '07
15’ Alum. Fisher w/ Mercury 25HD
12’ Tarpon 120 w/ Two Arms and a Paddle - SOLD…
EDIT…
Just found another manual with a diagram of the carb mounted fuel pump and it does NOT have the fuel pump body. Apparently there are two different carbs that came on these motors. Mine is correct and not missing anything …
I’ll start working through all my lines, hoses, etc. for leaks and “snake” the carb with a wire…
“Pura Vida”
Smitty
India '07
15’ Alum. Fisher w/ Mercury 25HD
12’ Tarpon 120 w/ Two Arms and a Paddle - SOLD…
sucking air someware! ck bulb and pick up in tank.
okay, lets start at the beginning so we aren’t chasing our tails
compression test (just to make sure something bad isn’t happening)
spark check (no need to clean a carb or rebuild a fuel pump if you don’t have spark)
if the fuel pump was the only problem, then pumping the primer bulb would make it run
when you took the carb apart to clean it, did it have fuel in it?
if not, you need to find the fuel obstruction
pull the fuel line off the carb and pump the primer bulb to see if fuel comes out to check for an obstruction
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Chris,
To run through your list…
Comp test - would need to bring the boat in to test as I don’t have the means
Spark - cranks every time, just won’t stay running
Fuel pump - gaskets have been replaced, and pumping bulb does not keep it running
Carb - had fuel in it when removing it, also has fuel coming out the over flow hole in the side of the fuel pump when pumping bulb
Fuel lines - pulled line at carb, pumped ball, fuel comes out. Pulled line at filter, pumped ball, fuel comes out. Pulled line at engine, pumped bulb, fuel comes out.
I did however notice a small leak around the gasket where the fuel filter screws on, going to replace that this week and see if it helps… Also, I’m going to pull the carb again and snake it with copper wire.
“Pura Vida”
Smitty
India '07
15’ Alum. Fisher w/ Mercury 25HD
12’ Tarpon 120 w/ Two Arms and a Paddle - SOLD…
Compression test-- you can buy reasonably priced compression testers at Advance or similar places. Take your spark plug in with you so that you can make sure it has the right adapter. A blown head gasket can definitely cause problems at anything but idle. This happened to my Johnson-- it blew the gasket out between the pistons. At wide-open, it sounded “a little funny” and would only give me about half speed. RPM’s was about right, spark was good, but I popped the head off and could see that there was nearly no head gasket left between the cylinders. Swapping a head gasket on these isn’t all that hard, fortunately, as long as the bolts don’t freeze in place.
Spark test— a $5 spark plug gap tester. I got mine at Park’s on 17-A in Summerville.
… The Cross of Christ is the anvil upon which the hammer of evil wore itself out.
if you don’t follow the steps, I cant help
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Chris,
I don’t follow your comment? Did I offend you? If so, I apologize…
A compression tester is $45 at advanced and the other tests you listed are simply my time and cost nothing. As stated, I did find a leak in my fuel filter gasket, which is $5 to replace. I figured I would do that first. If it works, great, if not, then the last thing to do from a cost stand point would be to do the compression test. I understand that if the compression test comes back bad then all the other test would likely have been unnecessary. However, I had done all of those thing prior to testing the compression even came up as a suggestion.
As always, I appreciate everyone’s help and input.
Thanks,
Joel
“Pura Vida”
Smitty
India '07
15’ Alum. Fisher w/ Mercury 25HD
12’ Tarpon 120 w/ Two Arms and a Paddle - SOLD…
no, not offended in any way
just unable to assist any further without accurate info
cant build the walls of a house until you have a foundation
cant build the roof until you have walls to hold it up
harbor freight tools sells a compression gauge, for I think less than $20
just because it cranks doesn’t mean you are keeping spark on both cylinders
got to verify spark under load
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www.joinrfa.org
10-4 Chris, I follow you.
Replaced the filter gasket today… Still very small leak there, need a thicker gasket…
Gonna go buy a comp test kit and spark gap tester today and start from the foundation 
“Pura Vida”
Smitty
India '07
15’ Alum. Fisher w/ Mercury 25HD
12’ Tarpon 120 w/ Two Arms and a Paddle - SOLD…
Did a comp test today; ran three test on each cylinder by pulling the cord five times for each test, as the gauge automatically stays on the highest reading. Results were as follows:
Top - 145, 145, 140 - Ave. - 143
Bottom - 120, 135, 130 - Ave. - 128
This is a 10.5% difference, which seems to be a nat’s ass past tolerance…
Also, checked the spark while the pugs where out and they seemed fine. Spark was bright blue and easily jumped the gap…
Thoughts? Keep looking for air holes? Pull the carb back off and triple check for obstructions? Replace all fuel lines including bulb to be safe?
“Pura Vida”
Smitty
India '07
15’ Alum. Fisher w/ Mercury 25HD
12’ Tarpon 120 w/ Two Arms and a Paddle - SOLD…
quote:
Originally posted by elcidsurfer
Did a comp test today; ran three test on each cylinder by pulling the cord five times for each test, as the gauge automatically stays on the highest reading. Results were as follows:
Top - 145, 145, 140 - Ave. - 143
Bottom - 120, 135, 130 - Ave. - 128
This is a 10.5% difference, which seems to be a nat’s ass past tolerance…
Also, checked the spark while the pugs where out and they seemed fine. Spark was bright blue and easily jumped the gap…
Thoughts? Keep looking for air holes? Pull the carb back off and triple check for obstructions? Replace all fuel lines including bulb to be safe?
“Pura Vida”
Smitty
India '07
15’ Alum. Fisher w/ Mercury 25HD
12’ Tarpon 120 w/ Two Arms and a Paddle - SOLD…
Did you dip the carb in carb cleaning solution? You can get the a gallon dipping container form any auto supply store… there could be shellac somewhere you cant see…Just make sure you full break it down to the metal components before dipping. You don’t want to put any of the plastic parts or gaskets in the solution…
“Endeavor to Persevere.
Give,Give… Never Take.”
EC
compression is fine
spark sounds good at start up
still don’t know if its losing spark and shutting off (power pack) issue
try squirting some fuel in the carb to keep it running
try covering the carb throat with a rag and see if choking it helps
if you cant keep it running like that, I would be leaning toward losing spark
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