Q&A / Tech

Mailbag: Tracking Down Common Causes of Sluggish Engine Response

Q: I recently rebuilt a Mustang 5.0L engine. I had it bored from .03o-inch over and installed a Ford Performance Parts E303 camshaft, Melling high-volume oil pump, Edelbrock intake, Holley 600 cfm carburetor, and a complete MSD Ignition system. The heads have 2.02-inch/1.60-inch valves, the headers are one-and-5/8-inch shorties, and the fuel pump is a Carter electric unit.

The engine has a very sluggish response, even when the transmission is not in gear. I’ve tried advancing and retarding the distributor, and put the lightest spring and stop in it. I’ve tried every carburetor adjustment I can think of, but nothing seems to work.

A: Your parts combination sounds good, and the engine should run extremely well. The sluggish response could be any number of things.

Start by doing a compression test. All cylinders should read close to the same. If compression checks out okay, do a leakdown test to make sure the piston rings have seated and there is no excessive blowby.

If the engine passes the leakdown test, start looking for vacuum leaks. Spray the base of the carburetor and the intake manifold rails with carb cleaner while the engine is at idle. If the idle changes, you have found the leak.

You should also check your spark plugs. If they have any oil deposits or oil on the threads, you may have a vacuum leak that is pulling oil out of the lifter valley into the combustion chambers.

If everything checks out okay, then go back and check the basics: cam position, distributor placement, and carburetor calibration.

This is another in a series of weekly Q&A Mailbag sessions with Summit Racing‘s tech department, in which there are hundreds more. Click here to see them all.

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10 Comments

  1. Gordon Keith says:

    I would also look for an overall rich fuel condition

  2. Raymond Hilfer says:

    I say, buy a Chevy:>):>)

  3. What does the exhaust smell like? Rich / lean, Also I have seen folks off on their timing set?

  4. did you degree your cam

  5. Did you put timing gears and chain a tooth off? Has your harmonic balancer slipped, thus throwing your timing marks off?

  6. Put a clean airfilter in it, and check the Cat’s, see if they’re plugged.

  7. Howard Elwell says:

    check the vacuum advance (if there is one)

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