88 M70, very strange fuel issue

[curtis73]

Motor in question is a 1988 Mercury M70 3-cylinder. We bought it new in 88 and ran it all summer, every summer up until 2004 it lost a fuel pump and a cylinder. We had the powerhead rebuilt (Dad handled it so I can't tell you exact specs, but it was bored, new Wiseco pistons, Boyeson reeds). It worked perfectly up until three years ago when it started dumping fuel out of one carb. It could be any one of the three, but one would apparently push fuel past the needle and it just dumped out the throat. Sometimes the top, sometimes the bottom, sometimes the middle carb. Just depended on which needle gave up first.

The seats in these carbs are not replaceable; they are cast into the aluminum. New floats, needles, and about 10 rebuilds and sonic baths later, it was still happening.

-The seats don't appear to be dirty or pitted, but we also resorted to re-machining the seat. I put some valve lapping compound on a drill bit and slowly spun the bit in reverse to polish up the seat. That improved things slightly, meaning now it just dumped slightly less volume of fuel out the throats.

-My next thought was that the fuel pump was over-pressurizing for some reason. I replaced the fuel pump. Same issue.

-Bought used carbs from a running motor that I tested on earmuffs. Ran great on the parts motor, cleaned and installed on my M70, ran fine for 10 days, then same issue.

This is now the third summer that I've been fighting with this. Three marinas and a total of 7 techs are clueless as well, as is the rebuilder. Its pretty obvious that it is not a carb issue. After all that work to the old carbs, and then a different set of carbs shows the same symptom, it pretty much has to be something about the motor, right?

Thoughts?

 

[curtis73]

And, FYI... new coils last year. New stator two years ago. New plugs every year. New wires last year. New spark box with rebuild in 2004. One thing I have yet to do is a compression test since I don't have a tester here at the camp, but it appears to be satisfactory. Thumb over the plug hole isn't a good indicator, but my ear is. When its not dumping fuel, it runs flawlessly. Before these carbs started acting up, I trolled for 2 hours at 600 rpm, and it was able to make 5500 rpm with two of us in the boat just like always. It certainly seems as though compression is good, but I will test it as soon as I can.

 

[gm280]

:welcome: curtis73. Great to have you aboard...

Not being there and doing any testing myself, I have to say it sounds like too much fuel pressure. I realize you stated you replaced the fuel pump, but that is the only reason fuel could possibly over flow the needle and seats. So if you don't want to buy yet another fuel pump (and I can understand that), I would actually test the fuel pump output pressure. But test it at the higher RPMs and not merely at idle. And if you really want to fix the issue, buy and install a fuel pump regulator inline from the fuel pump to the carbs... JMHO!

 

[Ant-Knee]

I have to say,
Only things that come to my mind is the valve needle seats or fuel pump, I agree with gm280
Only other thing that I think would cause something like that is maybe a plugged tank vent, But I don't think that would do what you describe...
The fact that it is any of the 3 carbs that leaks leads me to the fuel pump Or maybe the vaccum to the pump..

Just a thought

Ant...

 

[curtis73]

My thought was fuel pressure as well.

Possibly the Boyeson reed valves making greater crankcase pressure/vacuum and therefore making the fuel pump move more fuel?

 

[Faztbullet]

Possibly the Boyeson reed valves making greater crankcase pressure/vacuum and therefore making the fuel pump move more fuel?

Nope,,,valves are like a door ..open/close to allow fuel/air mix in and trap it in crankcase. What you problems is the fuel lines are degrading internally and the minute particles are sticking under needles holding them open.You will see little black specs that look like pepper in fuel bowl....

 

[Ant-Knee]

Nope,,,valves are like a door ..open/close to allow fuel/air mix in and trap it in crankcase. What you problems is the fuel lines are degrading internally and the minute particles are sticking under needles holding them open.You will see little black specs that look like pepper in fuel bowl....

Would make perfect sense...
I just had to replace all my fuel lines cause All the black was coming off them...
Yours could be worse then mine were so its clogging the needle seats........ Think its do to the ethenal in the fuel...

Ant....

 

[curtis73]

Nope,,,valves are like a door ..open/close to allow fuel/air mix in and trap it in crankcase. What you problems is the fuel lines are degrading internally and the minute particles are sticking under needles holding them open.You will see little black specs that look like pepper in fuel bowl....

Would love to agree, but not possible. First of all, in the dozen or so times I've been in these carbs, zero debris was found. Second, the fuel lines have all been replaced with the nitrile vinyl stuff. They crack, but don't degrade and dissolve like rubber.

I would love for it to be that simple, but we have gone way beyond the simple stuff... a dozen times.

My point about the reed valves is that aftermarket reeds are designed to trap more air/fuel in the crankcase for better performance. Thus the difference between peak pressure and least pressure is increased. More differential = more push/pull on the fuel pump diaphragm = more fuel pressure.

 

[Faztbullet]

My point about the reed valves is that aftermarket reeds are designed to trap more air/fuel in the crankcase for better performance

Its not the reeds as they cannot "trap " extra air/fuel . If they are 2 stage, the small set are weaker thus takes less effort for atmospheric pressure to enter carb/crankcase. A engine does not suck, it forms a vacuum in crankcase which outside air pressure trys to rush in and fill. The only way you could trap more is to enlarge crankcase or super/turbo charge it...as for adding hp reeds at wide open are wide open no matter who makes them and that won't affect/increase the top end performance.

 

[curtis73]

Its not the reeds as they cannot "trap " extra air/fuel . If they are 2 stage, the small set are weaker thus takes less effort for atmospheric pressure to enter carb/crankcase. A engine does not suck, it forms a vacuum in crankcase which outside air pressure trys to rush in and fill. The only way you could trap more is to enlarge crankcase or super/turbo charge it...as for adding hp reeds at wide open are wide open no matter who makes them and that won't affect/increase the top end performance.

Ok, maybe I'm not explaining myself properly.

Stock reeds are cheap. They provide a little bit of restriction to the air/fuel rushing in to fill the lower-pressure crankcase. Then as the piston falls, it increases the pressure and the reeds "close." Since it takes a tiny bit of outflow to close the reeds, a very small amount of pressure is lost. Aftermarket reeds offer less restriction both ways, so more air/fuel can flow in during low pressure, and it takes less outflow to close them. Therefore, more air/fuel getting trapped in the crankcase. That is 2-stroke 101. How could that not affect delta P?

(and I'm speaking of less than WOT for right now)

 

[Faztbullet]

They provide a little bit of restriction to the air/fuel rushing in to fill the lower-pressure crankcase. Then as the piston falls, it increases the pressure and the reeds "close."

No....you need to restudy 2 stroke101, the reeds close due to vacuum in crankcase is gone, the downward piston travel pressurizes the crankcase so when intake ports are exposed by piston the fuel/air charge enter cylinder and help push out exhaust . They will and cannot increase capacity as crankcase only has X amount of capacity and the atmospheric pressure which flows thru carb/intake is constant at around 14 psi, and only can be increased by turbo /supercharger. It just like when you breath, you exhale and lungs are in vacuum state,diaphram contracts and air rushes in.To increase the air in lung you must contract diaphragm tighter which increases chest area so lungs expand more thus you get more air....

 

[curtis73]

I understand that. Completely. But the properties of when and how the valves close and how efficiently they allow atmospheric pressure to enter the crankcase alters the amount of fuel/air trapped in the crankcase. Especially during operation, VE < 100. Increasing the amount of air/fuel that goes past the reeds means there is greater mass in the crankcase. As the piston pressurizes the crankcase, there is greater mass and therefore greater pressure.

All I'm saying is that HP reeds change the amount of air/fuel that make it into the crankcase during low crankcase pressure. Your assumption that it always equalizes to atmospheric is incorrect. At most loads, it is significantly below atmospheric. Therefore, altering reed properties could alter the delta P; the difference between highest and lowest pressures seen in the crankcase.

Am I making that clear? I'm not the best with words. Assuming that absolute pressure in the crankcase with stock reeds varies between 12psi and 7psi, I'm simply saying that more efficient reeds might change that to 13 psi and 7 psi, thereby increasing the Delta P seen by the pump diaphragm.