1978 140 hp mercury power problems

[aps71285]

Im almost to the point of sinking this boat. New spark plugs, clean fuel screens, good spark, compression is 116-118 on all cylinders. I know its a huge no no but this engine will not start without starting fluid. Once running it does good....half of the time... Sometimes though it just wont build rpms no matter what i do. If i rev it in neutral it does fine. If i have it on muffs its always perfect. Any help would be appreciated

 

[Chris1956]

OK, first let's make sure you are starting that motor correctly. Pump up the primer bulb, motor tilted all down. Lift cold start lever all the way. Hold the choke button down and crank her until she fires. Now feather choke until she is warm. If that procedure doesn't work, there is something wrong with the motor. Check to make sure the two choke plates close properly. If they do, chances are she is not cranking fast enough. Clean the electrical connections. Run some new battery cables from the battery to the solenoid (+12VDC) and lower cowling (-12VDC). Make sure the starter ground cable is clean and sound.

Battery cables can corrode under the insulation. Look for bumps in the wire to find corroded areas.

Now if she won't rev up, either the carbs are dirty or misadjusted or some of both. Also, I would inspect the spark plug wire for arcing, and the entire harness for failing insulation.

 

[aps71285]

Ok. When i run cables from battery to motor it cranks significantly faster but still not starting. Ive cheched all spark plug wires and dont see any leak marks. As far as the carbs go should i just rebuild them? It will rev up in neutral and when its on muffs just fine, but wont build spee/rpm under load

 

[Chris1956]

OK, do the choke plates close? Choke the daylights out of her to start.

Muff operation means nothing, due to no load on the motor. If you have not changed the idle mixture adjustment, it is time to rebuild the carbs. The idle passages can easily get clogged. Clean the carbs, install new gaskets, floats and inlet needles and readjust the idle mixtures. The idle mixture needs to be adjusted to give good acceleration, in the water, motor warm, under normal operating conditions.

 

[aps71285]

Yes the choke plates are closing. Do you know the procedure to setting the needles

 

[mr 88]

When you prime the bulb does it get hard ? Those are known with todays gas to go to hell quickly,I ended up puttin a 3/8 ID one way check valve between the bulb and the engine to eliminate that issue.Then I over prime it when cold,basically forcing gas into the system,starts right up when doing it that way. I believe you lightly seat the needles and then turn them out 2 1/2 turns. Then check acceleration in the water. If it wont get up and go ,start with the top card and turn it out about a 1/8 of a turn,try accelerating, if it doesnt get up and go move doen to the next carb and repeat procedure with the bottom. Then start at the top and repeat untill it gets up and goes..

 

[mr 88]

BTW I trailer that boat and always fire it up on muffs before heading to the ramp, that way I am good to go,fuel is in the system and it turns over a lot quicker on muffs than in the water due to no back pressure.Hit the ramp,click the key and I am gone.No need to prime it the rest of the day,it fires right up after the initial cold start for the rest of the day

 

[aps71285]

Ok ill give that a try. Idk what/if any work has been done to this motor so anything to check is a go with me. And yes the bulb gets hard but only if the motor is tilted all the way down.if its up gas just runs out the carbs

 

[Faztbullet]

Does motor have distributer or switchboxes???

 

[aps71285]

It has a distributor.

 

[aps71285]

Ive ordered the carb kits, fuel pump kit, and am going to re wire everthing by the end of the weekend as long as all my parts come in. Any thing else i could try while waitin tho id surely love to try. Also someone ive talked to mentioned reed valves. Is that an issue with these motors?

 

[demarko210]

I had a problem starting my 115 when I first got it, new boat owner here. It has switch boxes and over time i realized that i do not need to choke it, or push the throttle to start it. I have a 3/8 fuel line and all i do is pump the bulb until hard or a little beyond hard to when you see some gas push through carbs. Motor trim all the way down and it fires. I do not need to spill fuel out of carbs any more now i just pump bulb until hard. It doesnt start the first time, around the third try and she fires up.

 

[Chris1956]

Reed valves are pretty robust, and hardly ever go bad. It is very-very unlikely all of them went bad.

Look under the brass elbows and tees where they screw into the carb covers. There is usually a small stainless steel screen. These can get clogged. Make sure the carbs are clean, including the three small idle passages in the carb throat and the idle restrictor tube. Make sure the carb floats are adjusted properly. Set the idle mixture 1.5 turns out from seated, and adjust on the water, motor warm, in gear. Normally the idle mixtures need to be opened a/2 turn or so on the top carb and less on the other carbs, but need to be set by trial and error. You are looking for smooth acceleration and decent idle. The carbs need to be a bit rich to achieve any acceleration.

 

[aps71285]

Ok chris i will check those screens. And i will do the carb adjustment after rebuild. All my parts should be here Saturday

 

[aps71285]

Ok guys, im in the process of rebuilding the carbs (watching a guy that knows carbs better than me) and after second carb it looks as if these were freshly rebuilt. Ill let ya know how they perform when im done but im not convinced this was the problem

 

[aps71285]

That did all the tricks but pretty sure i blew the engine up. After the best day on the water ever with this motor it went kapooy. Cyl 1 is down to 90 psi and 3 is at 30. Lots of metal deposits welded to the end of those spark plugs also. Does this mean rebuild?

 

[cuenta619]

That did all the tricks but pretty sure i blew the engine up. After the best day on the water ever with this motor it went kapooy. Cyl 1 is down to 90 psi and 3 is at 30. Lots of metal deposits welded to the end of those spark plugs also. Does this mean rebuild?

Sorry man.
The engine give you any signs before blew
or what you think you make wrong when u rebuild the carbs.

Because I'm clean the mine and planning test the motor tomorrow

 

[aps71285]

No warnings at all just running perfect then bam done. Id say all the times of starting fluid. Unless i over revved it but idk the limit either. I was at 5k rpm when it went

 

[cuenta619]

No warnings at all just running perfect then bam done. Id say all the times of starting fluid. Unless i over revved it but idk the limit either. I was at 5k rpm when it went

Is my same history many time used starter fluid
I scared blew my engine

 

[Chris1956]

Those motors will get damaged if they overheat, run lean or have over advanced timing. Mechanical failure can also damage them, ie. throw a rod. A compression test will tell if the motor has been damaged.

The starter fluid if used sparingly, is probably not a problem, as residual oil is in the motor. However, it is far better to get her to start on gasoline instead.