Logo
FULL SITE MENU

Possible carb flooding -79 Mercury 70hp 3cyl

johanwahlberg

johanwahlberg

Member
Hi!
All last season I noticed how it started smelling a lot of gas after a ride, a while after the engine was turned off. Also a lot of oil shimmer spreading on the water surface around the outboard. When tilting the engine up, fuel/oil drips down on the transom. Some weeks ago when I started doing all sorts of servicing with the engine in a barrel, i noticed what seems to be the cause; There are lots of fuel/oil collecting in the bottom of the air intake / sound attenuator area, below the two carburetors. It looks as if the lower carb is flooding. I wiped it dry, and ran it on idle to 1500 rpm for about 30-45 seconds, and then the intake area of the lower carb were wet again. I wiped it, and waited a couple of minutes with the engine off, and then it was wet again..!?

I took help from my mechanic neighbor. We took the carbs out and they looked fine, needle in good condition and good seal when blowing air through. My neighbor said the floats were an older model that might absorb fuel and get heavier with time so I'll look into replacing them, but the old ones should still work when adjusted, and the top carb doesn't seem to flood. We lowered the float about 0.1 inch but it made no difference.

The engine starts and runs fine. See attached pic. Any ideas? Should I continue adjust the float? Seems odd though - shouldn't both carbs exhibit the same problem if the floats were getting heavy?

I also think i got a drop of fuel on my finger when touching the bottom of the carb linkage/trottle shaft, which could possibly indicate a leak somewhere else. I can't imagine fuel pouring from the intake, around the carb back and upwards to the bottom of throttle shaft...

And yeah, the bottom spark plug was quite wet after winter storage.

IMG_7521.jpg
 

Attachments

  • IMG_7521_.jpg
    IMG_7521_.jpg
    45 KB · Views: 77
Last edited:
Sounds like both carbs need a new needle and seat, cleaning and adjustment. Those are good running carbs, by the way.


Jeff
 
Thanks fastjeff, I guess I'll be taking it apart again, after getting new floats. I got a tight seal already though. Why do you think I need to do both carbs?

Sorry, what do you mean by good running?
 
Why not?

I "vise test" carbs I work on. To wit: I clamp them in my vise, connect my test tank, and pump the bulb hard as a rock. If they leak I drop the float level a 1/16 inch at a time until they behave.

Jeff
 
Hi again. I'm revisiting this issue this season since I never got it solved. I have now "rebuilt" the carbs with new floats, seats, needles and gaskets. "Vice tested" them as fastjeff suggested with the bulb rock hard. No leakage - perfect. But after starting her up in a rain water barrel the issue remains. Fuel flooding out of (mostly) the lower carb (as pictured in my original post) and also making its way down and dripping from the bottom of the throttle shaft (See attached photo. The drops is not visible in the photo). I could see some fuel on the bottom of the upper carb's throttle shaft as well so both carbs seem to be affected. Remembering something about a bad fuel pump diaphragm could case fuel in the crankcase, but checked and the diaphragm is undamaged. I can't tell if the fuel is coming from the carbs fuel bowls or if it is somehow flowing back from the cylinders or the crank case. I'm clueless and so is my mechanic neighbour...
IMG_0043.jpg
 

Attachments

  • IMG_0043.jpg
    IMG_0043.jpg
    62.4 KB · Views: 99
  • IMG_0043.jpg
    IMG_0043.jpg
    62.1 KB · Views: 90
Last edited:
I'll bet the fuel hoses coming to the carbs AFTER the filter have deteriorated (ethanol gas) and allowed small rubber particles to mess up you rebuilt carbs. (Now, wouldn't that p*ss you off!)

Suggest replacing the hoses and cleaning the carbs AGAIN. At least, you know the drill by now (and those carbs are the easiest ever made to remove).

Jeff
 
fastjeff said:
I'll bet the fuel hoses coming to the carbs AFTER the filter have deteriorated (ethanol gas) and allowed small rubber particles to mess up you rebuilt carbs. (Now, wouldn't that p*ss you off!)

Suggest replacing the hoses and cleaning the carbs AGAIN. At least, you know the drill by now (and those carbs are the easiest ever made to remove).

Jeff
Click to expand...

That's an interesting thought. But pumping the bulb hard doesn't make them leak (after install). Seems it happens just after I kill the engine.
 
Been into the carbs once more, increasing the floats height to 0.75 in. (the new floats recommended 0.65-0.69). The top carb stays dry, but the lower seems almost to flood more. Weird thing is that it does really not seem like the fuels is coming from any of the jets or ventilations from the bowl. It just "magically appears". The only thing I can think of is that it's flowing out of the crankcase. It starts flooding about a minute AFTER i kill the engine. One full, maybe almost two teaspoons. Each time, I've run it for about 3 minutes on idle, revving up to 2000 rpm for a few seconds, a couple of times.

Another thing I noticed is that the round cover plate by the choke valve gets wet on the outside. There shouldn't be any liquid fuel up there? Both top and bottom carb. Don't know if this is related or not. Doesn't drip...

IMG_0059.jpg
 
Is the fuel pump diaphragm supposed to be wet on the INSIDE? These photos are taken just after replacing the fuel pump gaskets and diaphragm, and then starting up for a couple of minutes.
The rear, large chamber (to the left in the left photo) is dry, but the two smaller chambers are wet as indicated by the diaphragm surface. Wet areas are marked with red. I think fuel is supposed to pass on the OUTER side of the diaphragm, right? There are 4 tubes going into the outer part of the fuelpump (right photo) - fuel from the tank+to the carbs, and two other tubes whose purpose I don't know.
wet_areas.jpg
 
The check valves, the fuel goes through those.---------And crankcase compression may come from 2 cylinders.----------------Fuel / oil / air mixture is what is compressed in the crankcase and of course you would see that on the other side of the diaphragm------Post picture of the 3 plugs after the motor has been run.-------------Post the results of a compression test on this motor.-------------Post the results of the following spark test.----Does spark jump a gap of 3/8" or better on all 3 leads , yes or no.---------------checked the reed valves ?
 
racerone said:
The check valves, the fuel goes through those.---------And crankcase compression may come from 2 cylinders.----------------Fuel / oil / air mixture is what is compressed in the crankcase and of course you would see that on the other side of the diaphragm------Post picture of the 3 plugs after the motor has been run.-------------Post the results of a compression test on this motor.-------------Post the results of the following spark test.----Does spark jump a gap of 3/8" or better on all 3 leads , yes or no.---------------checked the reed valves ?
Click to expand...



Lower and middle spark plug is wet. Top is dry. Can't access the photos I took right now unfortunately. But I've been running it in a rain water bin for several weeks on low rpm so that might not be sign of anything really. Would probably have been wet anyway..?



Compression test was done last year and was ok. I’ll see if I can perform another one soon.


Spark test was also done last year with no signs of problems.


Reed valves seem ok (no spitting or puffing, and can’t see any bad visually).
Generally this engine starts and runs good with little problems.
The leaking has been going on for at least 2 years, as long as I owned it.
I'm thinking it's running rich somehow, rather than a needle/seat/float problem.
Top carb still dry and nice after carb rebuild.
 
A pin hole in the fuel pump diaphragm will cause the lower cylinders to run wet like that. it MIGHT even cause the gas accumulation on the lower carb.

Jeff
 
fastjeff said:
A pin hole in the fuel pump diaphragm will cause the lower cylinders to run wet like that. it MIGHT even cause the gas accumulation on the lower carb.

Jeff
Click to expand...

I read about this, so I checked the diaphragm but it seemed undamaged. Replaced diaphragm and gaskets just to be sure - no change in behavior. Checked the pump again, diaphragm still looking good but wet in areas I'm still unsure if they're supposed to be.
fastjeff - did you see the photos of wet areas in the pump a few posts up?

racerone, johnnygjr - I'll try to do the spark gap and compression tests next weekend. (Boat's at lakehouse, I'm working weekdays in city center...)
 
Still think there's some fine particles in that lower carb that's causing this. You COULD switch the carbs and see if the problem follows them. The only difference is the removable throttle linkage (and jets in some cases).

Jeff
 
Hi again,
summer came and we just had to put the boat in the water, leaky carbs or not. But after a couple of runs the carbs are actually quite dry. Not completely, but better. I'm not sure but I guess repeated idle starts in a rain water barrel might cause weird symptoms that's not normally there when running mixed high/low rpms.
I also put an extra o-ring in the lower throttle shaft opening to force excess fuel to flow out in front of the carb instead of down the shaft and into the middle of the engine.

So I'm fine for now, but I might continue to check compression, spark and switch the carbs later this summer.

Thank you everyone for your support!

Johan
 
johanwahlberg said:
Hi again,
summer came and we just had to put the boat in the water, leaky carbs or not. But after a couple of runs the carbs are actually quite dry. Not completely, but better. I'm not sure but I guess repeated idle starts in a rain water barrel might cause weird symptoms that's not normally there when running mixed high/low rpms.
I also put an extra o-ring in the lower throttle shaft opening to force excess fuel to flow out in front of the carb instead of down the shaft and into the middle of the engine.

So I'm fine for now, but I might continue to check compression, spark and switch the carbs later this summer.

Thank you everyone for your support!

Johan
Click to expand...


Was this resolved? Did you end up replacing the carbs?
I have an identical issue with all the same trial and errors.
 
Darryls said:
Sorry if i put this twice...Was this issue resolved?
I have the identical issue.
Click to expand...

It got better after I cleaned the carbs and replaced gaskets, needles, floats etc. I also added a (presumably missing) o-ring at the bottom of the throttle shaft (or was it the choke shaft?, the shaft that pokes down beneath the bottom carb. Always used to hang a drop of gas there…)
I never got it to stop puking back gas from the top carb. There was some from the lower as well but less.
The engine failed in the drive shaft last season so I don’t have it anymore.
 
I managed to rig up a hose to slip around the throttle shaft to catch the run off and direct it into a weed whacker gas tank I mounted in the boat.
I've swapped the top carb with bottom and still leaks after running making me believe this is not a carb issue.

Thanks for your reply!
 
Darryls said:
I managed to rig up a hose to slip around the throttle shaft to catch the run off and direct it into a weed whacker gas tank I mounted in the boat.
I've swapped the top carb with bottom and still leaks after running making me believe this is not a carb issue.

Thanks for your reply!
Click to expand...

When I think of it, it might have been the other way around, that the lower carb was puking the most. Anyway, i remember looking into the bleed system as well but i couldnt make sense of all the small hoses. I think the reeds was ok though. If bad i think they could cause gas backflow.

I ended up just accepting and living with it, until it broke down in the drive shaft splines on the engine side. Not worth fixing...

Good luck!
J
 
I was having the exact issue you are having. Does your boat have a fuel solenoid. The solenoid that allows fuel to run directly into the intake manifold when you press down on your key when cold starting? If so, yours may be stuck open. That’s what happened with mine. I cleaned the carbs thinking that’s what it was, but it still didn’t fix the problem of the lower carbs flooding. Thought it was a weak spark, but sparks seemed good. Then I realized that I couldn’t hear the solenoid click anymore when I would press down kn the key. Took it off and sure enough it wasn’t working. It was stuck on open. This allowed the fuel to bypass the carbs when they got full and flow directly into the intake manifold. That is why the lower carb is only flooding. It floods from the engine and then comes out the carb. You can test yours by plugging up both lines that go into the solenoid with bolts and zip ties. Hope this helps. I was going crazy trying to figure this out.
 
Back
Top