3.9 Hi Po motor RPM question

[michael silverman]

I have a '74 3.0 Coupe with a Metric Mechanics injected 3.9 Hi-Po engine that was installed in 1989.
The car has one electric fuel pump mounted under the car on the left side of the rear quarter of the chassis.
The car runs great but when I wind up the engine, it just goes flat at about 4400 RPM; as though the valves are floating.
I have a new distributer, wires, etc but this has perplexed me forever. I think the engine may simply not be getting enough fuel.
My pump is likely the original and it appears to work perfectly. What I do not know is if there was ever a second pump on the car, perhaps mounted near the engine?

 

[decoupe]

Is this a new issue? I wonder if you are getting enough fuel for a souped up big bore and running into a lean mix at full throttle. Are you still carbs? Did you make any adjustments to timing or advance? Lots of set up questions.

You could get a lot of answers with a wide band air fuel gauge for $300 that reads your fuel mix in real time or take it to a chassis dyno outfit and have them do a few runs to see what your baseline set up is and what kind of power you're getting. Then you could adjust the jetting or increase the fuel volume as needed.

I'm sure others will have more specific questions and advice but finding out what you have is always a good starting point.

Doug

 

[gazzol]

Everything that decoupe said but also that I had a similar scenario years ago after converting to L-jetronic, mine ran out of breath at 4800 rpm. It turned out that the electronic ignition module was faulty, I was getting 12 volts to the coil with the ignition on but only getting 4.8 volts to the coil when the engine was running, that had me scratching my head for quite a while before I found it.
Also carbs or injection? If injection what inlet manifold are you using? If its the stock "bunch of bananas" its a bit small for an engine of that size and it will act as a restriction and limit both peak power and RPM.

 

[sfdon]

Fuel delivery measured at the return line after the fuel regulator needs to be 2.2 liters and make sure you are steady on the fuel pressure gauge needle at WOT. A shaky needle means a lean engine as does less than 2.2 liters of delivery.

 

[michael silverman]

All:
Thank you for your response. The car was set up at Metric Mechanics in Missouri back in the 80's so I assume it ran fine at one time. I even spoke to Jim Rowe (founder) who remembered the car.
The car runs through a Motronic injection system with a Datsun Z brain in the glove box.
I am not a mechanic, but my local guru, who advised me to start this thead, will follow your lead.
The low voltage situation seems to make some sense since the car was professionally set up years ago with no changes made. It sat in storage for several years prior to my purchase.