My setup is in my sig. When I have it nailed, around 5,000 RPMs, my motor gives me a bunch of backfires. It sounds like it is under an incredible amount of load. The plugs look good. I can't remember what jets I have in there now, but I know they're not way too big or way too small. The timing is good. It is very strong up til the point that it backfires. Any ideas on what this could be? Thanks.

 

What distributor do you have in it?
Have you checked your fuel pressure at the carb when it is backfiring?

 

I pulled a couple of jet sizes out of my primaries and I ended up with a high rpm (5000) backfire through the carb. I use a wide band band o2 sensor and it shows a lean condition when it starts to get rough and then smooths out about the time the vacuum secondaries are drawn in and the mixture starts to richen up.
Sounds like you could be lean either through fuel mixture or low pressure as suggested.

 

I have an MSD Pro-Billet distributor (mechanical advance). I don't have a fuel pressure gage in the car. It has an electric fuel pump, so I wouldn't think it would be an issue, but you never know I guess. Should I try over-jetting it a bit?

 

You can try it. Over jetting the back barrels won't help if it's running out of gas.
I'm still voting for low fuel volume.
You can eliminate the ignition by disconnecting the linkage to the back barrels. The front barrels by themselves shouldn't be able to overdraw a low volume supply. If it goes above 5000 without backfiring, it ain't the ignition...

 

What kind of volume should I have? In other words, if I were to disconnect the fuel line at the carb measure the amount of fuel poured out for a minute, how much fuel should I have with this setup? Thanks.

 

That depends on how much the engine uses at WOT. Also the delivery into an open container will tell you how much it supplies at 0 psi. Usefull for showing a stopped-up fuel filter and not much else.
The best thing to check is the amount of pressure that is on the carb when the engine is consuming gas at the highest rate. At the dragstrip, it's the fuel pressure at about 1/8th of a mile.
I trust the electric fuel pump is not pulling fuel thru an in-tank pump that's shut off, right and it's not one of those $20 square ones either.

 

I have an in-tank fuel pump with a pressure regulator set at about 7 PSI. I did end up doing the open container test, and I got about 1.1-1.2 quarts in 30 seconds. I don't really have a way right now to check the pressure at WOT. All I have is a gage attached to the fuel line under the hood at the carb.

My timing is set at 16 degrees base (36 degrees total). I pulled one of the plugs again, and they don't look as good as I originally thought. They look a bit too light.

I'm not sure when this all exactly started. It was sometime last year. This is the first time in a year that I've really driven the thing again. It completely falls on its face at about 5000 RPM when it starts backfiring.

I did install 1.6 rockers last year, but I seem to recall it not being an issue right after I installed them. I think my next step will be a new set of plugs (I need them anyways) and then I'll try increasing the jet sizes. Any other help would be appreciated. Thanks.

 

I've hardly had any time to work on the car at all lately, but I stopped by a local speed shop. He's thinking the valve lash could be a possibility, since i just installed new rockers a year ago and haven't touched them since. I set them with the motor off, so I'm thinking I'll try to set them with the motor running this time.

Anyone think this could be the potential culprit?

 

i would definietly say that it is a carb tuning issue that may be partially due to timing.

i had the same thing on my holley, and i retuned it for the most part and also put 93 octane in (instead of 87 and backing off timing) and it went away.

i am not at all familiar with demon, but for me (holley), i changed the accelerator pump cam (more aggressive), reset idle, float level, and the needle screws, changed to premium and no issues.

i would say you have a lean issue at high RPM's based on your plug description (pics would help everybody significantly more) and the carb backfire (traditionally a sign of being excessively lean).

if you have the $$$ for a wideband, they are unsurpassed when it comes to tuning, or you could use a stock o2 sensor and read with a meter as steven has on his website: http://www.hardtail.com/techtips/carbtuning.html