M-Block 351M/400 Performance

Enhancing your stock M-block

Copyright 2002-2003 Dave Resch
All rights reserved.

This page explains how you can improve the power output of a stock M-block engine, with the stock cam and stock low-compression pistons, stock Motorcraft 2150 2V carburetor, and stock DuraSpark II ignition. If you don’t have money to spend on major performance upgrades, or maybe you just have an old engine with a lot of miles and you can’t justify spending the money to soup it up, this section is for you.

Now, we have to get down to some brass tacks here. I can’t tell you how many vehicles I’ve seen with a hideous mess under the hood — spark plug wires swollen from soaking up the oil leaking from the valve cover gaskets, carburetors covered in sludge and varnish, and so many missing and/or broken components it was a miracle the engine could run at all.

As obvious as it should be, I have to say that the starting point of any performance improvement is tuning up your engine and getting it up to at least factory specs for performance.

Even though factory power ratings were only in the 150hp ballpark for most 351M engines, if your engine has been neglected for a while or is in dire need of a tune up, it could well be down to 100hp or less. In that case, just getting it back up to factory spec would be a huge improvement.

If you’re starting from scratch, with a neglected engine in need of a tune up, here’s what I recommend you start with:

Basic M-Block Tune-Up Procedure

Step 1.

Clean the engine.

Steam clean or thoroughly de-grease the engine. It’s a lot easier (and way more fun) to work on a clean engine.

Step 2.

Check the ignition system.

Start at the spark plugs, and work your way back to the distributor. If it’s been a while, you may as well replace all the wear-out parts (spark plugs, spark plug wires, distributor cap, and rotor).

Appearance of the spark plugs tells a story. If you don’t have one, get at least a Haynes or Chilton manual with the page of spark plug color illustrations. Read the manual about what different spark plug conditions mean and what you might need to do to troubleshoot the problem(s).

Step 3.

Overhaul the carburetor.

Remove it, clean it, and rebuild it. Even if you’ve never worked on a carburetor before, you can probably overhaul a Motorcraft 2150 2V carb — they are that easy.

All you need is an aptitude for reading the exploded view drawings that come with the rebuild kits, some patience for working with teeny-weeny parts, a clean workspace where you won’t lose any of those teeny-weeny parts, and a sense of adventure. Follow the choke and accelerator pump adjustment instructions in the carburetor rebuild kit.

If you’re not so adventuresome, maybe you’ll be motivated by the desire to save the $200 (or more) it will cost for a rebuilt carb at the parts store.

Note: I'm including a carburetor overhaul in this tune-up procedure because many problems are caused by dirty or maladjusted carburetors, and fixing everything else but leaving the carburetor untouched is not a recipe for success.

For detailed information on overhauling and tuning the Motorcraft 2150 carburetor, check out this page .

Step 4.

Change all the filters.

  • Air filter element

  • Breather element

  • Fuel filter

Step 5.

Install a new PCV valve.

Just to be sure, especially if you haven't done it recently. A faulty PCV valve can cause horrible driveability problems.

Step 6.

Change the oil.

Old motor oil can adversely affect the engine's performance and efficiency.

Step 7.

Check all vacuum hose routing.

Make sure all vacuum hoses are connected properly. Check the condition of all hoses, connectors, and vacuum diaphragms. This is critical to eliminating vacuum leaks — a major, and surprisingly common, tuning problem.

If you don’t have an engine calibration label with a vacuum system diagram, the best option is to look for another vehicle similar to yours that has one and refer to it.

The ProjectBronco web site has a calibration label posted for a 1979 Bronco, 351M with automatic transmission, here .

Step 8.

Set the ignition base timing.

Set the base timing according to the specification on your engine calibration label. If you don’t have one, start with 4 degrees BTDC.

You can look up some general '77 to '82 M-block truck timing specs on this page .

Once you have the engine in a good, basic state of tune, you can consider moving beyond the factory specifications.

Next: Tweaking your stock M-Block

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