Topic: New Ford Owner

[Pablo2377]

Greetings all!
Long time Mustang fan, first time owner. I just recently bought a 1971 Grande. Fortunately there were quite a few mods to the engine already (edelbrock intake manifold and 4bbl carb, MSD HEI Distributor, and quite possibly an upgraded cam), so I've a good place to start.  Unfortunately for me I didn't do any of these mods to the 302. My main issue is that I have a backfire when I apply more than 1/3 throttle. I am getting ready to replace the coil, plugs, and wires as well as check the timing this weekend. Having come from a ...."gulp"....mopar background, I am also suspect of the vacuum system for obvious reasons. If anyone could point me to where I can find a schematic for this I would be greatly appreciative.  Better yet, if anyone could post one for me, that would be awesome.  Looking forward to keeping everyone posted on my labor of love.
Paul

[Soaring]

Welcome Paul.  I have a lot of those schematics in the FAQ section at the top of the classic section.  If you don't find it there, let me know. 

[Pablo2377]

Thanks!

[Jeff73Mach1]

It sounds like you are taking the proper steps.

Engines backfire through the carburetor when the timing has the spark occur while the intake valve is still partially open and through the exhaust when the exhaust valve is partially open.  Depending on which type of backfire, it is likely that the timing is off . . . or that some plug wires are misrouted and either going to the wrong plug or crossed and shorting through the wires.

Once you have the wiring verified you can set the timing.  Make sure and plug the vacuum lines to the distributor when you check the timing. With the timing close, put a vacuum gauge on the engine and look for signs of bad rings or valves, usually exhibited as low vacuum or a jump needle or both.

If this doesn't get you a smooth running engine and the plugs don't show you oily deposits or lean burn signs or anything helpful, I would run a compression test.

Another good idea is to pull the valve covers and check that you haven't had a pushrod bent or partially collapsed at the point where the end is swaged down.  If the cam has been changed, verify that the valves are adjusted so that when a valve is closed you can turn the pushrod with only minor resistance.

[Jeff73Mach1]

If it is a Holley Carb, check for a blown power valve too

[Pablo2377]

Just a quick update;
I just tuned up my '71 (new plugs, wires, and coil)....WOW! This is what I was hoping for. Got rid of the backfire under acceleration, and the performance is what I expected out of my pony.  Yhanks again for all of your input.
Paul

[Soaring]

Most of the time you will get the results you have experienced with a good tune-up.  Congrats.

[Pablo2377]

It's interesting,though. I knew that a tune-up would cure general performance issues, but clearing up my backfire issue was a pleasant enough surprise.  Learned a good lesson, as well as reinforced  the idea of starting on the obvious, and simplest when diagnosing a problem.