Backfiring F

[ats72fj]

I searched but did not find my specific problem.

Heres the info: 72 FJ40 with F engine. New Rochester 2bbl carb. Did not seem to be running at full potential so I have since done the following: New feul filter, new plugs, wires, dist. cap, checked points. I can't get the srew out that holds the condesor on yet so I have not changed it. I adjusted timing to manufact. rec. 7 deg btdc.

The problem is that since I have adjusted the timing, which was the first thing I did, it is bacfiring out of the carb. It only backfires after it warms up and the choke opens. Specifically, it only backfires when I give it a lot of gas, let off quickly, then gas it quickly. It sputters and then POW!

The timing is set diffently than it was when it didn't backfire. But, I was trying to set everything to factory specs.

It starts right up and runs good other than the backfiring. Any ideas? I need help!!!

 

[Poser]

Is this " backfiring " comming out the carb, or out the tail pipe?

 

[folsom50]

Since it's backfiring out of the carb, try to advance the timing a couple degrees. See if that helps, and if not, there might be a vacuum leak that's causing the problem.

 

[65swb45]

It only backfires after it warms up and the choke opens.[ QUOTE]

These are both vacuum related, so start there. As the engine warms up, all the metal expands, including the valvetrain. Your valves may be too tight and, after they warm up, you are not getting a full seal at flashpoint. This causes flash to travel backwards thru the intake and out the carb. You may need to loosen the valves.

Choking the motor will increase the starvation of the cylinders for air/ fuel mixture, and the increased demand will temporarily negate the backflash. As the choke opens, and the starvation decreases, the backflash can make it further upstream.

Try the valves first. Then do what folsom50 said.

Mark

 

[ats72fj]

Thanks for the help guys. One more rookie question. How do I check the valves and adjust them if necessary? Thanks again.

 

[red66toy]

Here ya go. https://forum.ih8mud.com/showthread.php?t=20801

 

[Pin_Head]

Backfiring or spitting out the carb under acceleration is a pretty classic sign of lean mixture. This is consistent with the fact that it doesn't do it with the choke on. It is hard to believe that ignition timeing plus or minus a few degrees would influence this.