So I have been shacking out the gremlins one at a time and think I finally found the smoking gun I have been looking for. The video attached shows the fuel system tests, vacuum gauge tapped intothe pick up line between the fuel/water filter and the inline filter. Then a pressure gauge plugged into the fuel rail. I saw no issues on the pick up side as vacuum was under 1" Hg, however the fuel side has me troubled. When I crack the key, the pumps pressurize the system to ~48 PSI, but as soon as they shut off the gauge drops to 40 PSI. I start the motor and it idles around 40PSI, but as you can see when I tach the motor to about 4,000 RPM the pressure falls off to about 37 PSI No doubt this is a regulator problem, but do you think the pumps are suspect as well as the low 40 PSI is not being held or is there a normal range?

https://www.youtube.com/watch?v=0dn7j48WXsM

If those regulators are vacuum referenced the pressure should go down when you start it. Do both motors do the same?

yeah, I just ran the same test on the other motor and got identical results. When I pull the vacuum line to the regulator the pressure is a pretty solid at 45 PSI throughout the RPM range. Before putting new Merc regulators back in, anyone know who makes adjustable ones? My googling is coming back negative.

http://www.offshoreonly.com/forums/g...regulator.html

Have you contacted Raylar to get their recommendation on a regulator? He knows these motors better than anyone on the face of this planet. I don't really see the need for an adjustable regulator with the motors you are running. But I'll leave that up to the experts like Raylar.

You need to do the fuel pressure test under load. Your vaccum free revving to 4000 is not anywhere close to under load at 4000, nor are your fuel delivery requirements. Take her for a ride with somebody noting the fuel pressure and vacuum readings, then switch instrumentation to the other motor and compare.

First call was to Raylar, but they are out of the office this week. Matt, I just was doing some bench marking from engine to engine before finishing the break in, and was expecting a difference from one motor to the other (hoping for a smoking gun), even if not a real work environment...but sadly they are both the same. So I'm suspecting the the lower than expected PSI numbers are a result of this cheap gauge?. Many years ago I did have these ECUs flashed by Mercury as part of their tech brief dealing with transom soot....thinking maybe now I should have them flashed back?

Fuel PSI is correct. Its vacuum referenced and without load, it will not increase, it will decrease as the vacuum does.

Rewording since some made typos and to make this all clear -

As vacuum increases fuel pressure at the rail decreases.

As vacuum decreases, fuel pressure at the rail increases.

For fuel psi tests we like to:

Ck fuel psi with key on, engine off. (KOEO) Note fuel psi. There should be a Merc spec on this to compare.

Ck fuel psi with key on, engine running. (KOER) Make sure engine fully warmed up. Note fuel psi. There should be a Merc spec on this to compare.

Wide open throttle, full rpm. This is max fuel use as you are using max horsepower. Note fuel psi. This test is to see if fuel volume is adequate at highest use. If it isn't, the fuel psi will be falling off (going down).

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As others said above, and shown in your video (good vid by the way) steadily increasing rpm under a no load situation (on the trailer) doesn't require much throttle, thus the intake vacuum will rise causing your fuel psi to go down. This is normal.

People wonder why we use vacuum referenced regulators. It's because manifold installed injectors have their nozzles (spraying fuel) in the intake runners, which means they are subjected to manifold pressure/vacuum. This would change the flow rate of the injector (as manifold vacuum increases or decreases) as it's an opposing pressure.

So, the regulator being referenced to manifold pressure, keeps the acutal - at the nozzle - fuel psi constant.

Imagine trying to program a computer to constantly varying fuel psi ? Not gonna happen.

Again, not the fuel psi you see on the guage, as that will change - the actual psi at the tip of the injector.

Hopefully I explained that half way clear enough to understand and make the whole subject easier to understand.

Thank you all for your comments, they appear to be spot on related to vacuum regulated fuel pressure. This last weekend I did a series of benchmark readings with a tech in the back seat reading the gauge. From idle (40 PSI) to WOT the pressure would climb to 42 PSI then stabilize at 40 PSI until decelerating when the pressure would drop to 38 PSI. Based on what I see on these numbers, the fuel delivery system should be discounted as a potential source of my problems? Maybe I'm looking for a problem that isn't there (now), but this was the second motor in 12 years while the Port motor (450 hours) was still original. My next step is to remove the siphon tube, open the fuel tank to see if I have water or anything else that might be creating an intermittent problem. Suggestions always appreciated.