Why Am I Getting Such Terrrible MPG?

[30something]

Here are the specs to my existing motor:

L20b
Duel Mikuni PH44 Carbs
280 ZX K&N Air Filtre
BRE Manifold
Nissan Comp Header
1980's 2L electronic ignition
Oil spray bar & oil feed line & guage line
280 ZX turbo oil pump
Shadbolt M780 cam 0.522 gross lift 304* duration
2000 Roadster outer valve springs
Isky inner valve springs
Adjustable cam timing gear
.73" 280ZX intake valves & steel valve seats
Nissan comp stainless steel exhaust valves with waisted stems
Ported head & welded and deshrouded chambers - U67 Head
280zx flat top pistons with fly cuts for intake valve clearance
Oil pan from an L16
1982 280ZX transmission
Shortened driveshaft
720 pickup differential
2 1/2" - 3" exhaust
2 chamber 3" Flowmaster muffler
1983 280ZX disc brakes (non turbo)
280ZX master cylinder
Blaster 2 MSD coil

I put $40 dollars of Chevron supreme plus gas in and made it only about 50kms. I did not put my foot into it either.
I have had these carburetors tuned by an expert named Paul who lives in East Vancouver and only works on old cars from Europe and Japan. He managed to get the car through Aircare and then retuned the car afterwards. I want to do an engine swap if it is worth it with a modern Nissan motor that has been done before? KA? SA? I have the ability!

[datzenmike]

The cam is a bit hairy to expect any good mileage. What differential ratio? 4.11 or 4.375 because these are bad for mileage too.

You don't quite say it but $40 to go 50Km??? That's about 7.3 gallons @ 1.20 / L so 4 MPG? Even if misreading the gauge by double 8 MPG is seriously bad. Check the plugs. If dry and sooty the carb(s) are flooding. Maybe fuel pressure is too high. Gas tank or gas line leak??? take a look.

[bertvorgon]

Paul knows his carbs, are you sure on your numbers????
On our Specialty race car, set up for road racing, we got 12 MPG and that was at race speed.
At that low of an MPG, I am amazed that it runs.

as Mike suggests..check all those things.

In my turbo motor, L20+ at full on 300 HP mode, I get about 5 Mpg, at take it to the moon RPM.

maybe a trip to Specialty Engineering would be in order on this one.

[30something]

I just had the motor apart last year because I had low compression in 2 cylinders. It ended up being that the head needed rebuilding. Had a look at all the bottom bearings, crank, cylinder walls, rods and cylinders and everything was good. At that time all the plugs, wires, cap and rotor were changed. As well, I added a MSD Blaster 2 coil. No fuel leaks. All properly jetted for the new 280ZX K&N air filter ( had 2 different size air filtres before taking in different amounts of air into each carb causing different jetting. Car was set up by a carb guru in east Van (Paul Silva recommended to me by Overseas Auto). My car is ultra fast and can be shifted at 7000 rpm. Put a Porche Boxter in my dust! I want to get more fuel efficient so I can enjoy the car more. I stopped driving the car because it cost way too much to put fuel in. I want to drive this car from March to October. KA? SA?

[30something]

Been told that the car has 9.5 to 1 compression!

[okayfine]

How are you calculating fuel mileage?

[510rob]

How are you calculating fuel mileage?

I put $40 dollars of Chevron supreme plus gas in and made it only about 50kms.

[30something]

In short, I was living at my girlfriends place in Pitt Meadows and we were moving back into my house when my renters moved out. Put a day permit on the car to move it from point A to point B. Pitt Meadows to 1st and Clarke in Vancouver to 1st and Clarke to Shaunessy in Port Coquitlam.

[okayfine]

Is the speedo/odo calibrated? Was the tank empty and you put $40 in gas in, then went 50 clicks and ran the tank dry? Etc.

[30something]

Speedo and odometer are working. Tank was dry when I filled it up and was running on fumes when I got home.

[dislexicdime]

Fuel leak?

[30something]

No, no fuel leak.

[datzenmike]

What do the plugs look like. At 4 MPG they should be black.

If over 7 gallons was put in what did the gauge read? Did it drop steadily over the trip. Could the gauge be wrong? Maybe there's almost 6 gallons still in the tank.

[bertvorgon]

Only way to check fuel economy proper, is fill the tank right up. Then, drive an already measured distance ( maybe drive a route with a modern car ), then go re-fill the tank up, making note of how many liters went into the tank. Then you can calculate your mileage. Maybe have a gas station at the end of your loop, or whatever. Do a reasonable distance, say half a tank, that will give you a number on average, of at least that traffic situation, stops at lights, whatever. Fuel economy can drastically change from traffic to highway, specially with a modded engine. Large cams with overlap are BAD for fuel, but, if Paul got it through AIRSCARE, it's not that bad.

Divide kilometers driven by .6214 = miles
divide liters by 4.545 = gallons imperial
divide miles by gallons = MPG.

For all intents, the 510 fuel tank if run dead empty is 10 gallons ( depending on what manual you look at )

[okayfine]

What do the plugs look like. At 4 MPG they should be black.

As will most of the rear of the car around the exhaust tip. It will also smell quite obviously of unburnt gas.

If over 7 gallons was put in what did the gauge read? Did it drop steadily over the trip. Could the gauge be wrong? Maybe there's almost 6 gallons still in the tank.

This is what I'm getting at.

OP, I didn't ask if your speedo/odo worked, I asked if it was calibrated. Tire size, rear gearing, trans changes can contribute to massive errors in your odo calibration, where 1 mile actual may be .7 miles indicated, or vice-versa.

Similarly, what does "on fumes" mean? Was the car surging, or was the gauge merely reading empty?

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You have to have an accurate baseline of data before you can claim you have a problem, or what size problem it is. We see this often with people reporting temperature issues, when they merely have sender or gauge issues that indicate a fault that doesn't exist.

Your engine is relatively hi-po and will drink fuel, but 4mpg indicates serious leakage (bad gas cap or flange seal and all your gas is going out the filler on left-hand turns?) or mixture issues. But, 4mpg getting L-series engines will run like crap and show it.