Mercury 78 140hp inline smoking and stalls

H

Holster100

Cadet
Joined
Feb 5, 2011
Messages
11
Hi,
Have a problem where when it is idling sometimes it will stall and then fuel drips from the bottom throttle shaft. It is also smoking a terrible amount sometimes chokes you when trying to adjust carbys.
Has fresh fuel and correct ratio of oil. I have also adjusted carby mixture etc.
I have to use the warm up lever and choke to get it fired up again.
I have checked the timing, rebuilt carbs with new needle seats, new plugs, and fuel pump.
I have compreesion of 120-125psi and all cylinders. Powerhead had been rebuilt in 2002.
Any ideas?
Thanks
 
C

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
29,036
Re: Mercury 78 140hp inline smoking and stalls

The floats go bad on those motors. The originals were hard foam, and they deterioate. The replacements were hard plastic and they crack. I recomend you check them out, as it would seem your carb is overflowing.

Another common problem is the fuel pump diaghram leaking. You can remove the pulse hose from the fuel pump, and hit the primer bulb. If fuel pours out of the pump pluse line, it is bad.
 
H

Holster100

Cadet
Joined
Feb 5, 2011
Messages
11
Re: Mercury 78 140hp inline smoking and stalls

Thanks Chris for the reply. I have now put in new floats, new carby flange gaskets and bowl gaskets.
Still no better. I have also re-done the float levels. Still stalling after a idling for about 30 seconds to a minute and starts dripping fuel from the throttle shaft.
I even tired lowering the float level slightly but still drips fuel. I have taken out the spark plugs and checked them and they do have a bit of a oily deposit on them.
So far i have rebuilt all the carbs with new needles and seats, new floats and gaskets and reset all the floats level.
Im running out of ideas.
Thanks :)
 
Faztbullet

Faztbullet

Supreme Mariner
Joined
Mar 2, 2008
Messages
15,938
Re: Mercury 78 140hp inline smoking and stalls

Does the carbs flood or drip fuel if you pump bulb up tight? Also check fuel pumps for split diaphram or sticking check valve/s.
 
H

Holster100

Cadet
Joined
Feb 5, 2011
Messages
11
Re: Mercury 78 140hp inline smoking and stalls

Hi Fatzbullet,
No it dosent flood or drip if you prime the bulb hard. I have put new diaphram gaskets etc in the fuel pump.
Only thing i havent replaced is the check values.
 
H

Holster100

Cadet
Joined
Feb 5, 2011
Messages
11
Re: Mercury 78 140hp inline smoking and stalls

Anyone have any ideas?
Still no better. I have now fitted new primer bulb, fuel lines and even a new tank. I wondering if the dripping is just unburnt fuel. I remove the carby's again and noticed of lot of oily mixture sitting in front of the reeds.
Could it be faulty reeds? Maybe the spark is not strong enough as even the plugs are still oily. Adusting the mixture dosent make any difference to the amount of smoke.
I also did another compression check and all cylinders 130psi.
 
daveswaves

daveswaves

Ensign
Joined
Mar 22, 2002
Messages
901
Re: Mercury 78 140hp inline smoking and stalls

Holster100 said:
Anyone have any ideas?
Still no better. I have now fitted new primer bulb, fuel lines and even a new tank. I wondering if the dripping is just unburnt fuel. I remove the carby's again and noticed of lot of oily mixture sitting in front of the reeds.
Could it be faulty reeds? Maybe the spark is not strong enough as even the plugs are still oily. Adusting the mixture dosent make any difference to the amount of smoke.
I also did another compression check and all cylinders 130psi.
Click to expand...

Clearly you have excess fuel coming from somewhere. How does the engine run at speed? Are all the plugs oily? When it is running does it spit fuel out the front of the carb? Did this happen suddenly? Did it start after you re did the carbs or did you redo the carbs to try and fix it? What ratio of oil to gas did you use? Was it marine oil?
 
H

Holster100

Cadet
Joined
Feb 5, 2011
Messages
11
Re: Mercury 78 140hp inline smoking and stalls

daveswaves said:
Clearly you have excess fuel coming from somewhere. How does the engine run at speed? Are all the plugs oily? When it is running does it spit fuel out the front of the carb? Did this happen suddenly? Did it start after you re did the carbs or did you redo the carbs to try and fix it? What ratio of oil to gas did you use? Was it marine oil?
Click to expand...

It runs perfect at speed. The bottom 3 plugs are oily. No fuel comes out of the front of the carby when running. Its always dripped the fuel since i got it. Thats why i rebuilt the carbs. Im using 50:1 quicksilver oil.
Thanks
 
daveswaves

daveswaves

Ensign
Joined
Mar 22, 2002
Messages
901
Re: Mercury 78 140hp inline smoking and stalls

The bottom 3 plugs are a puzzle. The other thing to consider is it was rebuilt in 02, that is 9 years ago, depending on the level of rebuild that was done you may be seeing worn reed blocks.
 
H

Holster100

Cadet
Joined
Feb 5, 2011
Messages
11
Re: Mercury 78 140hp inline smoking and stalls

daveswaves said:
The bottom 3 plugs are a puzzle. The other thing to consider is it was rebuilt in 02, that is 9 years ago, depending on the level of rebuild that was done you may be seeing worn reed blocks.
Click to expand...

Thats what i was thinking. Dont know what was done during the rebuild as i didnt own it then.
 
H

Holster100

Cadet
Joined
Feb 5, 2011
Messages
11
Re: Mercury 78 140hp inline smoking and stalls

Still have a few issues with this motor.
I have got the idle pretty good now. Not stalling at idle anymore. Still very smokey. Only problem is when i turn it off it will not fire up again within a few minutes without choking and using the throttle lever. When i do this it fires up instantly and then will idle fine.
Have i still got the idle mixture wrong? I am starting with the screws out 1 1/2 turns. I then turn counter clockwise until it starts to run rough then turn the screw clockwise until it runs rough again. I then turn counter clockwise again until get a good idle between the two rough idle points i had.
Is this correct? Should i be doing this from top to bottom carb etc?

PS: I took the spark plugs out and had a lot of smoke comming out of number 2 and 3 plug holes?

Thanks
 
daveswaves

daveswaves

Ensign
Joined
Mar 22, 2002
Messages
901
Re: Mercury 78 140hp inline smoking and stalls

Holster100 said:
Still have a few issues with this motor.
I have got the idle pretty good now. Not stalling at idle anymore. Still very smokey. Only problem is when i turn it off it will not fire up again within a few minutes without choking and using the throttle lever. When i do this it fires up instantly and then will idle fine.
Have i still got the idle mixture wrong? I am starting with the screws out 1 1/2 turns. I then turn counter clockwise until it starts to run rough then turn the screw clockwise until it runs rough again. I then turn counter clockwise again until get a good idle between the two rough idle points i had.
Is this correct? Should i be doing this from top to bottom carb etc?

PS: I took the spark plugs out and had a lot of smoke comming out of number 2 and 3 plug holes?

Thanks
Click to expand...

It sounds like you have the idle mixture ok as long as it does not fall flat on its face when you accelerate and come up on plane. Put the plugs back in and the smoke wont come out, 2 and 3 just were at the right position for vapors to come back, ie exhaust port uncovered.
All three carbs must be adjusted correctly if that answers your question. It is not unusual for the merc inline to resist re starting if it has heat soaked after a hard run. The fuel vaporizes and you have to use the fast idle and sometimes the choke to get it going.
 
C

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
29,036
Re: Mercury 78 140hp inline smoking and stalls

Did you check the fuel pump diaghram for leaks?
 
H

Holster100

Cadet
Joined
Feb 5, 2011
Messages
11
Re: Mercury 78 140hp inline smoking and stalls

Update:
Finally got it to the lake today. Started up ok. Went to throttle off from ramp would die as soon as you went past 1500rpm. Start it up again put it into gear and push the choke in and shoots out of the hole no problems. Has heaps of power in all throttle ranges and runs at WOT fantastic. Sometimes if you stop and idle for a bit and then throttle off goes ok without stalling.
Turned the idle screws out 1/8 a turn and then to 1/4 turn made no difference.
Has all new fuel lines from tank to carbs and primer bulb. Also new tank with fresh fuel.
Any ideas?

Thanks
 
daveswaves

daveswaves

Ensign
Joined
Mar 22, 2002
Messages
901
Re: Mercury 78 140hp inline smoking and stalls

Holster100 said:
Update:
Finally got it to the lake today. Started up ok. Went to throttle off from ramp would die as soon as you went past 1500rpm. Start it up again put it into gear and push the choke in and shoots out of the hole no problems. Has heaps of power in all throttle ranges and runs at WOT fantastic. Sometimes if you stop and idle for a bit and then throttle off goes ok without stalling.
Turned the idle screws out 1/8 a turn and then to 1/4 turn made no difference.
Has all new fuel lines from tank to carbs and primer bulb. Also new tank with fresh fuel.
Any ideas?

Thanks
Click to expand...

Well, first of all Im Jealous since our lake is still frozen.:( When you overhauled the carbs you would have had to do a link and sync procedure. If you did not then that is likely part of your problem. The timing must be all in before the throttle butterflys crack open. There is a straight forward procedure on the FAQ section of this forum. Double check that all the "events" described in the procedure are happening at the right time. After you do the link and sync you will have to start over adjusting the idle screws. you may have compensated for an out of sync situation by adjusting the carbs. You will have to make sure that all the butterflys are the same degree of "open" at idle as well. They can be adjusted via the linkage between the carbs. Since your lake is open you could fly a frozen Canuck down there to fix it for you.:facepalm::facepalm:
 
C

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
29,036
Re: Mercury 78 140hp inline smoking and stalls

The "dying of the motor as you advance the throttle" and the "choke helping it accelerate" are symptoms of the idle mixture being too lean. Richen the top carb 1/8 turn and try to accelerate. Repeat both steps with the mid carb and then the bottom carb. Repeat sequence from top - mid - bottom until you get reasonable idle and smooth acceleration.
 
You must log in or register to reply here.
Top