Help Please I am out of Ideas

[Fed up]

OK after quite a few threads I have encountered a new problem with my motor. Here is what has been done:
1978 115 Merc 6cyl
Carbs broke down and cleaned, newer model floats installed.
New fuel pump kit
new fuel lines, primer bulb, and motor fitting
fresh fuel with 50:1 Quick Silver oil
Link and Sync
New plugs and wires


Here is the issues that I am encountering, when the motor is cold and in the water idle is right around 1000 rpm (not quite sure it the tach is dead on) It is an easy start. Come off of the dock and idle through the no wake zone, jam the throttle wide open and she screams 5200rpm do not know the speed. Come back down to idle and it does good, hit the throttle again and we are back up and going with no symptoms.

Now stop and fish with the motor now fully warmed up, fish for 20-30 mins and go to restart and it will barely run with the warm up lever wide open, it will never go above 3000rpm. If I am lucky i can get it to run just above idle and get back to the dock, but most of the time I am back to the trolling motor to get back.

A compression test has been done and it was 120+-2psi on all cylinders.

I am completely puzzled on this one. This only happens when it is warmed up, runs great when it is cold.

When trying to restart it will almost pour fuel from the carbs it seems. It almost acts as it is completely flooded but I tried to disconnect the fuel line to see if when it was about to empty the bowls if it would idle up, it was a no go.

I am getting really aggregative with this motor.
I do have a spare motor with all the electronics that are known to be good, but before I start to replace everything I want to have a direction to go.

Thank you for all of your help.

 

[Laddies]

Re: Help Please I am out of Ideas

Sounds like the engine pulled some crude from the tank or line.

 

[F-16 Falcon]

Re: Help Please I am out of Ideas

Since it only does this after a good warm up I would suspect a timing issue. Get it to fail and then check the spark to each plug.

 

[Randy J]

Re: Help Please I am out of Ideas

fuel tank vent open ?? sticking anti siphon valve ?? just a cpl ideas ? does it have vaccume lines??

 

[Fed up]

Re: Help Please I am out of Ideas

I have two seperate tanks, both of which have two filters on them. A screen filter at the tank and a paper element type at the motor end. I tried swapping the primer bulbs to no avail. Also tried loosning the fuel cap on both tanks.

For some reason I keep leaning toward the thought of an electrical issue due to it only happening when the motor is heat cycled, but before I start swapping parts I would like to know what has failed.


I will try to see if I can get a spark test when it is acting up. I am thinking a timing light should be a quick way of seeing if I have spark.

 

[merc850]

Re: Help Please I am out of Ideas

I would suspect that the non-original paper element filter could cause a fuel flow problem. There are filters in the carb tops and maybe one near the fuel pump and these are wire mesh that don't swell with fuel saturation.

 

[Laddies]

Re: Help Please I am out of Ideas

Fuel does not run out of the carbs from a lean condition or ign. failure.

 

[vupilot]

Re: Help Please I am out of Ideas

I will be interested in what you find. I have a 1975 402, 40hp that runs perfect for the first 10 minutes when cold. Then I can barely nurse it back to the dock around idle, anything above idle and it stumbles and acts like it will die. The first couple times this happened the idle was fine to get back to the ramp but the last time it would barely even idle so I think the problem is getting worse. Youre problem sounds very similar to mine, I suspect electrical on mine as well, Thunderbolt IV.

 

[Faztbullet]

Re: Help Please I am out of Ideas

Is this the 1978 1/2 model with 2 switchboxes?

 

[Fed up]

Re: Help Please I am out of Ideas

Yes it has the electronic ignition.

 

[Faztbullet]

Re: Help Please I am out of Ideas

If the dual switchboxes, sounds like stator is dropping a bank(1 box). Check spark when this happens..

 

[Fed up]

Re: Help Please I am out of Ideas

That is kinda what I was thinking too, I have a known good stator along with switch boxes.

It will be at the earliest next week before I can get the boat back on the water.

 

[Chris1956]

Re: Help Please I am out of Ideas

Wasn't 1979-1/2 the changeover from Distributor to ADI for the inline six?

That model should have the backdrag carbs. They will have stainless steel strainers behind the brass fitting in the carb covers. You might check them.

The new floats are hard white plastic? Any cracks in them?

 

[Fed up]

Re: Help Please I am out of Ideas

No the floats were good to go, I pulled them from a spare motor that I have. If i remeber correctly the carbs did not have a strainer in them. But on Sat I will pull them back down to double check and see if there is anything in them.

 

[merc850]

Re: Help Please I am out of Ideas

Here's the strainer type fuel bowl covers

I had a lower carb filter full of "fur" and so I removed the screens, runs perfectly now.

 

[Fed up]

Re: Help Please I am out of Ideas

Ok so I pulled the carbs back off, and the screens were not there. I was looking for that inevitable "ah ha" moment, hoping to find something out of the ordinary, but it never happened. There was no trash in any of the jets, nor was there anything restricting fuel flow that I could see.

So after letting it idle to warm up I thought about what someone had suggested about losing spark, so after it was warm i used a timing light and went to each of the plug wires to see if I had spark. Everything was good except #4, so I swapped plug wires to see if the dead fault followed, and it did not.

So I changed the lower bank coils with a spare set I have, and still no spark. Next was on to swapping the switch box, and then I got spark back.
Sounds all well and good right, nope now I seem to be having other issues

now it will idle really rough and when you shut it down it back fires, same thing when you are trying to start it back up. There was one wire that was hooked up on the old switch box that was an orange wire, this does not correspond with any of the labeling on the newer model box, the orange wire seemed to make no difference whether it is hooked up or not.

Anyone got any ideas from this point?

 

[Fed up]

Re: Help Please I am out of Ideas

Ok I have now went back and put back on the old coil pack, still no change. But I do now have spark on #4...

So to sum it up, put on a new switch box and now the darn thing will backfire through the exhaust when trying to crank.

Does anyone have any idea of what might be going on here?

I am about to the point that I am ready to take it to a mechanic, because it is obvious that I can not fix this problem.

 

[Fed up]

Re: Help Please I am out of Ideas

Anyone with ideas?

 

[daveswaves]

Re: Help Please I am out of Ideas

Have you looked at the stator physically, wires ok? Cracks in insulation? Ground wire connected (if the type with a separate ground). Ground wires on switch box mounting screws?

 

[PHE]

Re: Help Please I am out of Ideas

I believe that the orange wire to the switchbox is the kill wire that turns it off when you turn the key off. It provides ground to the wire in the off position.

So, one switchbox does not provide spark, and the other gives you spark but will not run. I've heard that these boxes can go bad by drastically changing the timing. It could be that the new used one is also bad.