I took a look, its spraying fine. I can even hear it spraying while someone else is pumping it. No delay.

Yes, the spark plugs are in correct order.

I found a open vacuum port, and plugged it up. Now we aren't back-firing as much. I took it about a mile down the road, and up a service road and back home, and it only popped twice. Once while taking off from a stop sign on a hill, and once on taking off from a stop light on the service road.

On a side note, I am having the catalytic convertor replaced and putting dual exhaust on it this coming weekend. Does anyone think this might help?

 

It sounds to me like it is a lean mixture, especially when you say you found a vacuum leak and plugged it, and that it doesn't pop back so much any more.

To adjust your idle mixture on the carb, get the truck to normal operating temp. Slowly turn the mixture screw in (clockwise) till the engine starts to bog down. Then turn the screw out until it starts to run rough again. Turn the screw in about 1/2 turn. You have now adjusted the idle mixture. However, this needs to be done at around 500 - 600 RPM, or you will get a false setting.

I think that 14* is a bit too far advanced for the timing. It is better to run it at the specified setting. Did you check the timing with the vacuum line disconnected? Is the vacuum line hooked to a ported vacuum port on the carb? In other words, is there vacuum when in the line while the engine is idling? If so, you have it on the wrong port. It should only have vacuum on acceleration, not at idle.

Have you checked the vacuum advance to be sure it is working and not binding or sticking? After years of use, sometimes the advance plate and pegs get grooves in them and they stick a bit, causing incorrect advance on quick acceleration.

There is even a deeper reason why it may be popping back through the carb. The engine may have worn valve guides. However, if you rebuilt the engine properly, that shouldn't be a concern.

Let us know what you finally do to remedy the concern.

Dan Harriman 84 F150 lwb
Orange, Texas

 

Alrighty, I actually have set it back to 10*, after I plugged the vacuum line. Sorry for forgetting to mention that.

I do not have a vacuum advance, its computer advance. Which everyone I've met says is weird, but its true. This 300 has computer advance. *shrugs*

I'll check the idle mixture again, and check those vacuum lines.

 

How bout the ignition system??? Coil, rotor cap, TFI mod if you have it.... poping or backfiriring is more of an ignition issue, not fuel....We removed the computer stuff and went back to the old fashioned coil.....