Norsky, this may need to go into a different forum.
I am posting this as a kind of lesson learned on modifications. As many of you already know, I am planning on putting a 454 fuel injected engine in my '69. I am doing a little prep work right now which includes exhaust work. ( and a breakdown here and there )
first boo-boo. I let a mechanic talk me into putting a new Holley street avenger 770 on my stock 427/390. The carb is WAY TOO big. I lost all my bottom end power and torque. The car barely runs till it hits 3000 rpm. Of course from there up to redline it is a blazing banchee. Goes from 70 to over 100 in an amazingly short time.
second boo-boo. In prep for the engine, I installed large tube shorty Hedman headers, 3 inch tubes and the only round muffler that summit sells that is not advertized as a glass pack. results...lots of noise and worse, lots of backfiring ! I talked this over with a local speed shop owner that has a pretty good reputation and he told me that the backfiring would go away once I got the other engine installed because it will breathe better.
The moral of the story....well, hell I guess there is no moral other than it is something not to do .

Jeez, you go from trying to get your motor back from a "nitwit", to now bad mechanical advice. When I meet up with you at Hershey I'm not just gonna buy you a beer, I'll buy you a case. I feel for ya' man.
Sarge

Well, it is a 'General' lesson learned. But it will probably be more at home in the 'C3 Fuel, Emission Control, and Exhaust Systems' forum where there seems to be more threads related to carbs-n-pipes. Can't wait to hear how the 454 runs out.

guess that shows you that bigger is not always better, i tend to go on the consertive side when making changes. yes a larger carb would allow the car to be sluggish as the air flow will get slow as it flows through those big opennings and the fuel will drop to the bottom of the intake making a poor fuel mixture untill enough air can get flowing like 3000rpm and up.. a smaller carb is much more responsive because of that. thats why Q-jets are spread bore carbs. same with the big headers the exhaust will flow slower out because of the large pipes and won't cause enough vacuum to help evacuate the exhaust in the cylinders. this will cause the engine to loose low end power but have great power at higher rpm. because most of your driving is in the lower (5000-RPM) smaller headers will flow faster and help pull the exhaust from the next cycle. yes you will loose some power 5000 and up but how often do you wind it that high. At lease i don't anyway

I spent some time talking to a local speed shop owner. He explained another problem that was caused with the carb that I just never thought about. He said " The factory intake has two sets of holes ( large and small ) to accomodate the Q-jet. The holley has four holes in the bottom but they are all the same size. Even using the 'correct' adaptor plate will result in (at best) minor problems with both gas and air flow into the manifold".
Anyway, the biggest problem I have now is the car runs fairly ok till about 2500 rpm. At that point it starts to backfire through the exhausts. If I hammer it, it may give one backfire and continue to accellerate. If I hold a steady rpm ( 3000, 4000, 5000 ) OR take my foot off the gas, it backfires fairly steadily. I realize this is a stretch but could this be a lean condition or maybe a couple sticky or misadjusted exhaust valves.

Hi Glenn,

Since I am sure you have checked the common problems, and there was no backfiring before the carb and exhaust change, I would think the valves are OK. Could it be that the oversized carb, combined with the oversized exhaust has slowed the exhaust speed sufficiently to collect unburnt fuel in the header? This might explain the backfire condition. It may be time to ebay the carb.

Alex

i have a 74 454, when i bought it, it was dirt slow, it wouldn't even light up the extremely hard tires. Pretty sad for a heavy front end big block. Anyway, i then installed 1 7/8 super comp hooker headers with 3 inch collectors and a 750 barry grant carb. And then replaced the rusted out exhaust with true dual 3 inch all the way back. And WOW, what a huge difference it made. It not only helped my high end, but when i stomp on it now from a stand still, low rpm or not, it burns the tires up to 40mph no problem. I didn't notice anything but improvement for my bottom end and i went bigger everywhere. Anyone else do this????

Glenn, sounds to me like because the car is breathing more freely now, you will need to adjust the timing and jets and everything else, when i changed everything over, i gained at least 50hp's. Let me know what it was, so i can look out for the same problem.

The car is now running just great ( except for a backfire when I take my foot off the gas ). We rejetted the carb ( up 2 ), reset the front float, readjusted the idle adjustments, removed the vacuum advance and set timing to 36 degrees. The car takes off with no bog-down from a start. It runs great at high rpms and it even starts out in 4th gear with no pinging.
The backfiring seems to be coming from a blown ( or not seated ) header gasket over number 3 cylinder.