3.0 High RPM Misfire

[alldodge]

The issue which is unknown is the cam lobe, are we getting a full stroke for the pump?

How to test:
Disconnect power to coil.
Remove fuel pump
Measure the distance the rod moves as the motor is rotated

The pump on this engine is the lever arm type: 8M0073435 so I'm not sure how to measure the stroke, I can say that the wear area from the cam is in the right place on the lever.

Ah don't know what to say

 

[sgtnoah]

Thank you for the advice so far, I sure appreciate it.

I made some changes and headed out to the river for testing today, still not fixed unfortunately. This one is really turning into a head scratcher!

- Replaced fuel lines
- Converted to an electric fuel pump, 30 gph @ 6 psi
- Flow tested from line going into carb into bucket -- very strong fuel jet, easily more than the 12-ish gph that this thing uses at full power

My manual is a 1999 edition it does not list the carb on this motor, a "864940 21 4406". I am running 1.60mm main jets and a 4 x .65 power valve. Would these be right for that carb?

My pride is fighting me, but I'm wondering if I need to break down and take it to a marine mechanic? As a diesel mechanic it would break my heart to do so, but dang is this troubleshooting process dragging on!

Any thoughts would be appreciated.

-- Pete

 

[alldodge]

- Converted to an electric fuel pump, 30 gph @ 6 psi

It now has an electric fuel pump but does it have the same issues?

 

[sgtnoah]

Exact same, misfire at 3800-4000. Plugs look way too white.

 

[Scott06]

Plunger above power valve that allows it to open stuck? Slide freely? Maybe try a higher float level or larger main jets. Double check jet size wasn't leaned out if boat was used at altitude.

My 91 3.0 generally (for 25 years) had two plugs that were black, two that were close to grey ash perfect, no matter what I did with carb etc. it would tend to build carbon inside the chambers I didn't get a high rpm ping but dieseling after a good hot ski pulling, had to let it idle. If I ran sea foam or water down the carb to remove carbon that would help. Would be a cloud of black smoke after the sea foam sat in there 20 min, make sure the neighbors aren't home.

 

[Scott06]

Missed you had jet sizes in there, this doc lists some 3.0s with 1.65 main jet and .75 power http://www.boatfix.com/merc/servmanl/13/13b5r2.pdf. I think up to a .90 power valve is available.