Popping out the exhaust

[dixie outlaw]

I need help, I have a sbc bds 871 with a bird catcher on alky 80a pump 44 nozzles and a 75 main jet. It's runs clean at 8% and at 15% but when I go up to 24% it pops out the exhaust and blows blue flames out. Can anyone tell me what is causing this? btw it has a msd 7al2 plug gap at 23 31 degress of timing ngk-10 plugs

[tafc]

sounds lean. whats the barrel valve at?

[Big Blue Oval]

absolutely, I set my barrell valve once and forgot to jam the nuts on the linkage and while I was sitting in the car with it running it started popping and blowing fire because it was vibrating the linkage loose to the lean side.

[dixie outlaw]

The barrell valve is set at 78% it leaves and starts popping about 75 to a 100 feet out sounds good at idle

[Big Blue Oval]

Do you have EGT's? From what you are saying, it does this when you step up the overdrive (boost), this says that it is getting lean. When you say 44 nozzles, are you running 44's in the hat and ports, or just in the ports?

[tafc]

idle is barrel valve, on the run is main jet ,so you are LEAN on the run.

[dixie outlaw]

It only has hat nozzles, Should i go up to bigger nozzles when over driving the blower more?

[dixie outlaw]

Battery was dead on the egt's so I can't help ya there.

[Big Blue Oval]

Hmm...I have heard of only running hat nozzles, but honestly, thats not my bag of tricks....MIKE C....where ya at?

[Mike Canter]

Don't want to bust anyones bubbles here BUT leaness causes blowback up through the blower. A popping out the exhaust and blue flame out the exhaust is caused by unburned fuel being dumped out of a cylinder into the exhaust and igniting from the heat or (depending on the type exhaust) another cylinder firing into the exhaust. The other thing that can cause it is to have the spark plug wires crossed or to have a bad distributor cap with a crack in it. Check your firing order and the inside of the cap carefully. Then I would try a new set of plugs gapped down to .015-.016 and see if that fixes it. Also check you coil tower for cracks or carbon tracks. If the plug wires are bad the the increased cylinder pressures at 24% is causing the spark to go elsewhere. The other possibility is that the 7AL2 cannot produce enough spark at 24% OD because the cylinder pressures are too high. Closing the plug gap may help that problem. If done of that works then time for an ignition upgrade. Has it ever worked at 24% before????

[Big Blue Oval]

Nope, you're right Mike, I wrongly assumed he was talking about at idle.......

[Eric David Bru]

I agree with Mike C.

EDB