Replaced Fuel Pump Diaphram - Will this affect mixture?

[Smig]

Motor is: 1976 850 (85 HP) 4 cyl, 2 carbs.

I replaced the fuel pump diaphram and since then, can not get the motor to run properly (sooty, oil-fouled plugs, backfire.) I checked the carbs and everything is clean, but I am having a heck of a time getting the mixture adjusted correctly.

I had everything perfect last fall and the fuel pump is the only change that I made - could new diaphrams cause this type of problem? (the old one was seemed fairly stiff, but no holes)

Thanks
Smig

 

[emckelvy]

Re: Replaced Fuel Pump Diaphram - Will this affect mixture?

Improper installation of the fuel pump or ruptured diaphragm can allow fuel to be drawn into the crankcase and cause the symptoms you're experiencing.

I'd recommend you inspect your installation and make sure you have the correct gaskets and proper order of assembly, to ensure the fuel pumps aren't causing the problem. There are some very subtle differences in fuel pump styles and if you got the wrong gaskets, they won't seal properly. Just something to look for.

BTW, a good indication of this problem is that fuel keeps pumping in the motor and won't stop, with the primer bulb obviously flowing fuel and never getting hard. And the fact that you don't have fuel pouring out the carbs, in which case it's pretty obvious where the fuel's going.

Gas is only going to get in the motor at 2 places; carbs (where it's supposed to), or fuel pumps (where it's not!).

I just worked on an old Johnrude 40 Lark that was running poorly, when I went to run it the bulb wouldn't pump up; I found fuel pouring out the diaphragm side of the fuel pump due to broken diaphragm. It was dumping all that raw fuel right into the crankcase.

Ran a lot better after I fixed that problem!

HTH & G'luck........ed

 

[Smig]

Re: Replaced Fuel Pump Diaphram - Will this affect mixture?

Thanks ed,

I'll check the diaphrams - anything tricky that I should look for?

I haven't had trouble with the motor not stopping, in fact the opposite - I use to be able to idle it down to around 500-600 rpm in gear. Now it's so rough I can barely get down to around 800 in gear

 

[Smig]

Re: Replaced Fuel Pump Diaphram - Will this affect mixture?

I pulled the fuel pump apart and don't see fuel on the wrong side. However, there were two strange things -

First, the diaphram curled and wrinkled up as soon as I took the pump off. There is also an indication that the diaphram touches the check valve retainer when flexing away from the crank case. Are both of these normal? The old one didn't do this when I took it off, but it was pretty stiff and much thicker.

Second: the crank side chamber and diaphram is covered with soot. I know that the engine was running rich but I thought the fuel pump connected to the crank, so how would soot get in there (btw: when I saw the soot I had a heart attack so I checked compression and all 4 are good and even)

Finally, when I replaced the fuel pump, I only did the diaphram and gaskets. Should I also replace the check valves?

Thanks
Smig

 

[Smig]

Re: Replaced Fuel Pump Diaphram - Will this affect mixture?

...I'd recommend you inspect your installation and make sure you have the correct gaskets and proper order of assembly, ...

I checked the part number and they are from the correct OEM gasket and diaphram kit and in the right order. I can get a new kit (that includes the valves) and try that, but I really would like to figure what the problem is.

Thanks
Smig

 

[andy6374]

Re: Replaced Fuel Pump Diaphram - Will this affect mixture?

Smig-
All all your plugs getting soot on them or just a couple?

I am having a very similar problem on my engine (88' V6 Black max). But I am only fouling the top two plugs (cylinder 1 &2) with soot. I am going to check my fuel pump too.

I cleaned the carbs and rebuilt the fuel pump...then the problem occured. And I am having serious doubts that it's the top carb.

 

[Smig]

Re: Replaced Fuel Pump Diaphram - Will this affect mixture?

#1 is sooty (dry) and #2-4 are wet fouled. I was expecting the wet because I couldn't get the mixture correct and set it rich, but what has me worried is the soot in the fuel pump - how did it get there?

Do you know what causes wet vs dry fouling in a 2-stroke? I will be curious to see what caused your problem since it didn't happen until the rebuild. Did you replace the check valves in the fuel pump? I didn't and am wondering if that could be part of the problem.

Smig

 

[andy6374]

Re: Replaced Fuel Pump Diaphram - Will this affect mixture?

I just took my fuel pump apart and it was clean and free of soot. So that isn't the problem. I the rebuild kit I purchase included check valves.

My top two pistons are covered in carbon. Like I've said in my other post, I've run 2/3 of can of powertune thru that carb to no avail. I can't figure this out and its driving me nuts.

What are the odds that two coils are producing weak spark? Less than small.

 

[Smig]

Re: Replaced Fuel Pump Diaphram - Will this affect mixture?

I have decarbed several times and can't get rid of that coating on the pistons either, but my idle was (before this new problem) smoother and I could idle it down lower than before the decarb.

Have you run the motor since you rebuilt the fuel pump? I saw another guy in a post say that his idle was much smoother after changing the check valves - maybe the plugs are fouling at low speeds?

Smig

 

[emckelvy]

Re: Replaced Fuel Pump Diaphram - Will this affect mixture?

Just for a lark, try pulling the distributor cap down and see if it's very dirty inside. I've seen cases of rough idling/misfiring due to "trash in the cap" which causes arcing to the wrong set of contacts inside the cap. You can use some carb cleaner to flush out the cap if it's full of carbon dust etc.

Also check the spring-loaded carbon contact in the middle of the cap; if it's not free to move, or is pushed back in the cap, it can't make full contact with the rotor. Seen quite a few that were gummed up pretty bad and had to be pulled out/cleaned.

Now, if you don't have a distributor (I forgot in your original post you said it's a '76 which should have ADI ign), you might be having issues with the stator.

If you have (4) coils on the port side of the motor and the CDI box mounted on the stbd side, you have ADI (Alternator Driven Ign).

Big Batch of Problems with ADI stators, I just got in a '76 850 with running problems; when I checked the stator it had the correct resistances, and plenty of spark.

After starting and idling for 5 minutes, it lost spark and quit. Checked the stator and the resistances were way different. Needless to say it's getting a new one!

CDI/Rapair replacements are around $105 at www.boatfix.com, don't think I've seen them cheaper anywhere else. P/N is 174-5454 K1, Merc P/N is 398- 5454A26.

You can check the stator out with an ohmmeter; the low-speed windings (the blue and blue with white-stripe wires) should be 5700-8000 ohms. The hi-speed windings (red and red with white-stripe wires) should be 56-76 ohms. If the resistance on either winding is out-of-spec the stator is bad. Disconnected the (4) wires from the CDI box before testing. The stator should also be replaced if any low-speed or high-speed windings' wire is grounded.

The back side of the fuel pump sees crankcase vacuum/pressure so the soot is representative of the combustion quality in the motor. Since you noted the area was dry, it's not likely fuel is being sucked into the crankcase past the pumps. Maybe your motor's problem is just coincidental to the fuel pump refurb; ign probls might be masking the real problem.

Otherwise I'd look to issues with the carbs (i.e., leaking fuel inlet needle or float adjustment) if you can't sort out the rich-running/soot problem.

HTH........ed

 

[Smig]

Re: Replaced Fuel Pump Diaphram - Will this affect mixture?

Thanks Ed,
I hope it isn't the stator!! I'll check it out tonight...both cold and warmed up. I did check spark already and can jump over 1/2" bright blue in the daylight.

I don't know if this helps pinpiont the trouble, after a ton of messing with the needles, I was able to get a holeshot, but no top end over 4500 rpm. Every once in a while it would surge up and then come right back down or bounce up and down (the engine speed that is).

Smig

 

[andy6374]

Re: Replaced Fuel Pump Diaphram - Will this affect mixture?

I just spoke with a mercury outboard assistance rep... although Merc seemed hesistant to try and diagnose the problem over the internet (understandably they didn't want to shoot themselves in the foot) but they were susipicous of the bleed lines. If the check vavles in the line get stuck then it would cause this probelm.

 

[Smig]

Re: Replaced Fuel Pump Diaphram - Will this affect mixture?

...they were susipicous of the bleed lines. If the check vavles in the line get stuck then it would cause this probelm.

The cylinder bleed lines??!! I thought all they did was bring crankcase ooze from the bottom up to the top to lubricate the top bearing? If stuck open it seems the opposite, they would put to much down in the bottom cylinders? Hopefully someone can explain this for me...

Now that you mention it - I did make one other change this spring that I had forgotten about: I had to replace a bleed hose that split!


Smig

 

[Scaaty]

Re: Replaced Fuel Pump Diaphram - Will this affect mixture?

Make sure the diaphragms you replaced are EXACTLY the same..

 

[Smig]

Re: Replaced Fuel Pump Diaphram - Will this affect mixture?

...You can check the stator out with an ohmmeter; the low-speed windings (the blue and blue with white-stripe wires) should be 5700-8000 ohms. The hi-speed windings (red and red with white-stripe wires) should be 56-76 ohms. If the resistance on either winding is out-of-spec the stator is bad. Disconnected the (4) wires from the CDI box before testing. The stator should also be replaced if any low-speed or high-speed windings' wire is grounded...

I checked the stator cold - everything checks out ok. I think it's time to put it back together with new fuel pump check valves, start it up and see what happens. I will check the stator again when warm.

 

[Smig]

Re: Replaced Fuel Pump Diaphram - Will this affect mixture?

Make sure the diaphragms you replaced are EXACTLY the same..

I just triple checked and the diaphrams are the correct ones according to the Mercury books. My original seemed thicker and stiffer and this new one seems really thin. I did find that I had the little tab that sticks out coming out the bottom of the pump instead of the top, but it doesn't look like that affects any of the cutouts in the gaskets.

Now here is my next question - I went ahead and bought a new kit that includes the check valves and the new kit came with a filler block, spring and cap. It looks like these are to keep the diaphram from hitting the valves. I do know that the diaphram that I just took out has a mark from hitting the valve - could that cause a problem with the fuel flow?

Thanks
Smig

 

[Scaaty]

Re: Replaced Fuel Pump Diaphram - Will this affect mixture?

I just triple checked and the diaphrams are the correct ones according to the Mercury books. My original seemed thicker and stiffer and this new one seems really thin. I did find that I had the little tab that sticks out coming out the bottom of the pump instead of the top, but it doesn't look like that affects any of the cutouts in the gaskets.

Now here is my next question - I went ahead and bought a new kit that includes the check valves and the new kit came with a filler block, spring and cap. It looks like these are to keep the diaphram from hitting the valves. I do know that the diaphram that I just took out has a mark from hitting the valve - could that cause a problem with the fuel flow?

Thanks
Smig

I should clairify...screw P'N's...lay the diaphragm OVER the old one...Been, done...that..right P/N, had Oval cutouts where ya least would suspect..and this was a prob for my Merc Guru rebuilding my 135 TOP, a good friend of Ed's, and those two know there stuff..

 

[Smig]

Re: Replaced Fuel Pump Diaphram - Will this affect mixture?

I should claifry...screw P'N's...lay the diaphragm OVER the old one...Been, done...that..right P/N, had Oval cutouts where ya least would suspect..and this was a prob for my Merc Guru rebuilding my 135 TOP, a good friend of Ed's, and those two know there stuff..

Ahh, now I know what you mean - I did compare them when I first changed the diaphram. The diaphrams are identical in shape, but the new one seems really thin and floppy - I just attributed that to the old one stiffening up. There were slight differences in the gaskets though, the holes were all in the same places, just slightly different shapes - where the old one had blobby corner holes, the new one has circles. Otherwise they are the same.

 

[Scaaty]

Re: Replaced Fuel Pump Diaphram - Will this affect mixture?

OK, so we can dismiss that possible problem. So, hmmm. ya got me..out of ideas my end

 

[Smig]

Re: Replaced Fuel Pump Diaphram - Will this affect mixture?

There are two more things to check - Andy's suggestion for the bleeder valves and Ed's for the hot stator.

But first I am going to try replacing the check valves, putting the whole thing back together and start from the very, very beginning with a fresh link & sync.

I'll try using the new block, spring and cap that came with the check valve kit - who knows maybe it will help with that flimsy diaphram. If it works, great - it will bother me that I don't know why, but at least it will work. If it doesn't, I'll check the stator and bleeders.

If that doesn't solve it, well I have a 500 that needs some wiring done - at least it will get us on the water!