Evinrude 85hp low fuel pressure

[Chevyls6]

I've got a fuel pressure issue with my 77 rude.

Rebuilt the fuel pump using Sierra parts to correct a full throttle surging issue.

I fired it up after that with a gauge installed to check the pressure. Pumping the primer bulb gets me about 12psi on the gauge.

With the engine at idle it reads nothing on my gauge which starts at 3psi. Revving up to around 1500 rpm just barely bounces the needle.

Does this sound right ?

 

[racerone]

No it does not sound right.

 

[F_R]

Sounds ok to me. Grabbing a service manual at random. it says 1 PSI at 600 RPM, 1.5 PSI at 2500-3000 RPM and 25. PSI at 4500 RPM. None of these readings will cause much excitement on your 3 PSI gauge. OK, so it wasn't the correct manual.

 

[Chevyls6]

Gives me a baseline though since all these pumps are kind of the same.

I have another gauge that may read lower I just need to find it.

 

[F_R]

Correction: That's 2.5 PSI. not 25 PSI. What a difference a dot makes!

BTW, I just looked at an 85hp manual, but wrong year because I don't have the correct year. Nevertheless, it says the same PSI's

 

[Chevyls6]

Thanks F_R i figured you meant 2.5 haha. I've never heard of a carburetor running on 25psi.

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[Chevyls6]

How likely is it that the check valves in the fuel pump are bad?

My pump had the old style metal reeds instead of rubber valves.

They looked ok when I had it apart but they are old.

 

[F_R]

Actually, i prefer the metal valves. Maybe that's just me.

 

[racerone]

There is very little that can go wrong with these pumps !------What is your motor doing / not doing that leads you to investigate the fuel pump.----What other trouble shooting has been done ?

 

[Chevyls6]

Ok so the fault is whenever I throttle up it starts to hesitate and cough as soon as it hits 3500-4000rpm.

After doing that for a few seconds the rpms start to drop quickly even if I don't pull back the throttle.

Once I throttle down and throttle up again it will repeat the same process.

It feels just like hitting the rev limiter in a fuel injected car but with bogging right before.

In my mind that translates to running on the fuel in the bowls till it's gone and then the fuel pump can't keep up so the engine slows down.

So far I've checked the fuel pickup for debris, tried running with the gas cap off, rebuilt the fuel pump, and replaced all hoses including the vacuum line from the intake manifold to the pump. All coils are putting out strong spark. 3 are aftermarket with 1 OEM.

One odd thing that I noticed while spark checking is that the non oem coils don't seem to fire on every compression stroke while cranking. Sometimes there is a pause like it missed a spark. Could this be my issue ? It starts up and runs ok at low rpm and very well at mid rpm.

My next move is probably gonna be an on water timing check at wot and fuel pressure check at speed.

 

[Chevyls6]

Oh one more thing I pulled the plugs out and the 3 connected to the aftermarket coils were dirty and oily while the 1 OEM coil plug had a clean spot on it like maybe the spark was burning the carbon off?

 

[racerone]

Or no spark at all on that clean plug.----Have you done a compression test on this motor ?---If so post your numbers.-----And you can run with a timing light one cylinder at a time to see how spark behaves when motor falters.

 

[Chevyls6]

Clean plug definitely has spark it jumps a 7/16 gap well.

I did a compression test months ago. As I recall all cylinders were around 115-125psi.

I found my other fuel pressure gauge that starts at 1 psi. I'll give that a try too.
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[Chevyls6]

Ok I got out to check it again today.

Fuel pressure at idle is around 2.5psi

Fuel pressure at 3000rpm is 1.7
​​​​​​I didn't rev any higher than that since I was on muffs.

I also tried spraying some water on the coils and wires to check for voltage leaks. They all seemed fine.

 

[Chevyls6]

Just to be thorough I poked around in the ignition a bit. I think I may have found the problem.

All 4 coils read 0 ohms on the primary side.

On the secondary side 2 read 270 ohms 1 read infinite ohms and 1 read 1250 ohms.

I already cleaned all the coil grounds and the one from the coil bracket to block and power pack to bracket.

This seems like a smoking gun to me. My new theory is that I've got spark breakdown happening under heavy load since the fuel pressure has been tested and is fine.