1993 Cobra W/F303 cam


New Member
Jun 27, 2004
I just replaced the stock cam with a f303 cam and got the timing set to 10degrees BTDC and when I first mash on the gas it backfires thru the intake. What could be the problem? :bang:
  • Sponsors (?)

Yes, the cam was properly installed this was the first time I replaced a cam but I had a friend from work help me with it he use to race in some pro class but he is not here today to help with timing but he said to set it to 10 degrees BTDC and lock it down and i did, so where is the most common place for vacume leaks?
There's your problem more than likely. The timing is controlled 2 ways. Where you set the static timing, and how much more the computer adds. The wire controlling the computer added timing has a device in it called a spout. When you pull the spout out, it disconnects the computer from the distributor allowing you to set the initial static timing. By setting it at 10 btdc with the spout in, your static timing is about 0 degrees btdc to 4 degrees atdc. That's why it's running so crappy - WAY retarded timing.

On Fox bodies the spout is a little device about 3" back from the dist. module on the dist. wiring harness. You'll see it - it has a little fuse-looking device that can be removed. If you remove it first, your timing is set now so far retarded that it probably won't start. So start the car; put the timing light on it and set the timing at about 20 degrees btdc; then remove the spout; now, use the light again, and set it at about 12-14 btdc (with the spout out); then plug the spout back in. The timing will jump up to around 20-24 when you do that. That cam is gonna want more than stock timing - I'd run as much as you can without detonation 14-16 if you can get away with it.

Try that and see if it helps.
Once the cam has been changed, idle speed is sort of subjective. Whatever rpm seems best for your needs. I'd set the timing around 14 btdc with it idling around 800 rpm. Idle speed on the fuel injected cars is controlled by the computer. Some people are successful resetting it by turning the throttle stop on the throttle body; others aren't. It seems for vary from car to car and ecu to ecu.

Are you certain the cam timing is correct? Did you/he use a degree wheel and dial indicator to confirm that dot to dot provided the proper valve timing?
We did not use a degree wheel we aligned the dots and used a straight edge in the center of the key way on the crankshaft and centered it with cam shaft and made sure the dots on each of the timing gears were along that straight edge.