WOT RPM Low. MAG350 MPI B3

[gcarmich]

My Mag 350 MPI only hit 3870 RPM with throttle lever all the way down. Clean hull. New valves, heads, injectors.

Could it be a linkage issue since I had to reassemble the linkage?

 

[Scott06]

My Mag 350 MPI only hit 3870 RPM with throttle lever all the way down. Clean hull. New valves, heads, injectors.

Could it be a linkage issue since I had to reassemble the linkage?

Couple of things I would look at -

first yes of course it could be the linkage make sure it is opening all the way

Verify firing order and the the indexing of the distributor. There is a procedure that Mercuiser has for this if you google it or find it in the manual.

Compression test could be something simple like rocker arm being too tight

@alldodge would probably have some things to verify as well.

 

[Scott Danforth]

it could be a throttle linkage issue
it could be a fuel system issue
it could be a wrong prop issue
it could be an ignition issue

what troubleshooting has been done?

compression test?
fuel pressure?
does your motor have the crab cap?
are the injectors from Mercruiser or ebay or amazon?

 

[gcarmich]

it could be a throttle linkage issue
it could be a fuel system issue
it could be a wrong prop issue
it could be an ignition issue

what troubleshooting has been done?

compression test?
fuel pressure?
does your motor have the crab cap?
are the injectors from Mercruiser or ebay or amazon?

Haven't compression tested since new heads/valves. Can do soon.

Props are original.

Is fuel pressure available through VesselView and/or RINDA?

Has crab cap.

Injectors are OEM. Just recently serviced and tested good.

I'll reconfirm firing order.

And check linkage and ensure full open all the way forward.

Thank you.

 

[Scott06]

this thread has the indexing of dist

Mercruiser 5.0 MPI distributor issue

Short version: Installed new distributor (Merc 5.0 MPI with crab style cap -- shaft separated from pressed fitting on top). Boat ran great, but chugged at idle. Friend came and looked at it and believed it was off by a tooth, so moved it by a tooth. Boat now idles great, but misfires above 2000...

forums.iboats.com

 

[Scott Danforth]

you test fuel pressure with a mechanical pressure gauge at the schroeder valve

verify the crab cap is in good order. get a new one if the old one is more than a year old

test compression with a known good gauge. should be between 140 and 150 psi on all cylinders

 

[gcarmich]

you test fuel pressure with a mechanical pressure gauge at the schroeder valve

verify the crab cap is in good order. get a new one if the old one is more than a year old

test compression with a known good gauge. should be between 140 and 150 psi on all cylinders

Does the fuel pressure change with RPM?

My crab cap is 2yrs old.

 

[Scott Danforth]

Does the fuel pressure change with RPM?

Not enough to notice. it should be about 45-50 psi

My crab cap is 2yrs old.

I would change it

 

[alldodge]

Not enough to notice. it should be about 45-50 psi

Scott, it's a Merc so most likely 43 psi, Volvo Penta is 50-60

Does the fuel pressure change with RPM?

FP can change a few psi, like 40 at idle and 43 or so at WOT

Props are original.

What are originals and what rpm was being able to turn prior to motor work?

Is fuel pressure available through VesselView and/or RINDA?

Depends on your motor serial number, most likely NO
Rinda should be able to provide injector pulse width

What is motor serial number?

 

[gcarmich]

Serial Number : 0W067905
Model Number : 4442077TS
Sterndrive S/N OW260110

Finally got a fuel pressure gauge with correct Schrader valve fitting. Fuel rail pressure is 34-36 psi. Seems to hold around 34 psi.

Still need to test compression
Still need to index distributor per manual instructions
Crab cap has less than 10 hrs on it but I can replace if needed
Linkage looks good. Wide open with throttle down as expected

I can check pulse width tomorrow with Rinda - what values am I looking for?

 

[Scott Danforth]

34psi is low

 

[alldodge]

Finally got a fuel pressure gauge with correct Schrader valve fitting. Fuel rail pressure is 34-36 psi. Seems to hold around 34 psi.

You have an issue with the fuel pump or regulator, should be 40 to 43 psi
FUEL PUMP Assy

The Reg is item 10

Fuel pump test procedure

 

[gcarmich]

You have an issue with the fuel pump or regulator, should be 40 to 43 psi
FUEL PUMP Assy

The Reg is item 10

Fuel pump test procedure
View attachment 410997

Thx. I don't see Fuel Shut Off Tool (91-805918A1) image or availability on-line. How do I shutoff fuel flow? Hose Pinch Pliers?

 

[alldodge]

No pliers pinching

Search the part number and one shows up for $12 on ebay

 

[gcarmich]

No pliers pinching

Search the part number and one shows up for $12 on ebay

Says sold out but there are similar looking valves advertised for "Fords". Do you think they are interchangeable?

 

[alldodge]

Don't know for sure but I'm thinking yes.
Reason is because Mercury no longer has it available either. Even superseded part is not available and it looks the same as what is pictured in the cool fuel manual 40

Edit: no it doesn't look the same

 

[alldodge]

Scratch that previous

Your connecting to the low pressure side with a Tee fitting

 

[gcarmich]

Thx. I found that advertisememt on a site but when I click through, it says item not found. If i can figure out the size and thread on existing connection, can i just insert a marine grade fuel cutoff valve?

 

[alldodge]

Marine or Auto is no issue. Many of the fuel lines are same as auto and brake lines