74_rebel

So Iv been battling my carb for a while now and I need some advice, first a little backstory. Iv got 74 vette with a 400 sbc in it and I recently pulled a Holley 750 off of it. The reason being is that we were getting popping and the back barrels wouldn't open. Lightning hit us after a week of fiddling with it, duh this has vacuum secondaries and it's in a 4 speed car... Fast forward to Monday I get a hit on a craigslist ad for a demon carb. Guy doesn't no anything about it (of course he doesn't) but he wants $200 for it. I show up and it's a bit older, he thinks it's a 750, doesn't known the age but said it ran fine on his 347, ended up gettin it for $150. I get it home do a little research and find out its a mighty demon 650 (crap...) throw it on the car anyway and I can already tell it's gasping for air. So what I need is a little advice on what size carb would be best. I'm pretty set on a Demon and have found a few decent deals on the 825 and 850. Is 850 too large? Iv heard that you go 2cfm per HP, but it could be garbage.

So here's my engine specs

400 sbc
Stock Pistons and crank
Vortec heads 906 casting
Mild cam (.477 intake .480 exhaust) (comp cam part 12-242-2)
Edelbrock performer dual plane air gap (1500-6500rpm)
Headers if it matters

Side note I've got a small superchager (it's a camden but it looks to be the same size as a weiand/B&M 142 or 144) that I'm putting on in the spring so that needs to be taken into account as well.

chevymans 77

The Mighty Demon 650 flows over 750 cfm, they are rated different than other carbs.

It should be good for you mild 400

Neal

F22

You're throwing a lot of carbs at that car. I have a '74 that I'm running a Holley DP with mechanical secondaries and it's a 650. The engine is roughly modified the same as yours, with maybe a little less Cu In. It hauls butt and easily does burnouts with ease.

I doubt that it's gasping for air, because a Demon 650 or 750 should flow plenty well enough. Have you checked at what rate the secondaries kick in? Too soon and it bogs down. Try running it on the primaries only and see what happens.

Have you checked the distributor yet? What are you running for a distributor? If it's the original, you may have to replace the vacuum can with one designed for a far higher horsepower engine, then the 185 hp, L48 that came stock (unless you have the L82, but not much difference there either).

The vacuum can, that controls the advance, can have a big effect on how your engine runs, as well as how much mechanical advance your getting as well. Both are independent of each other.

Initial Timing is important! What is your baseline timing? 8 degrees? 12? 16? Are you running the vacuum advance off've the ported vacuum on the carb, or at the base, directly off the manifold. Again, a big difference in how it's set up. You'd be surprised at the difference it makes.

F22

We just delivered this custom '74 Short and Wide to our client. It has a very, very healthy 383, with an air gap and a Holley 750 carb, mechanical secondaries. Automatic, with a big stall converter, mechanical advance MSD and it is one hard runner.




This is with your foot, not even half way into it!!! Too bad it doesn't have a posi rear end, but look at the stripe it left on the pavement in the lower right corner. It wasn't hard to do in this beast!

74_rebel

It bogs right at the beginning, before the secondaries even have a chance to move. The distributor is brand new, everything on the engine is some form of new or refurbed, the block is magnafluxed and honed, the heads have been completely taken apart and dipped with new valves and seats, the distributor, water pump, intake, oil pan and pump, cam and lifters, pushrods, and piston rings are all brand new. The only thing that's not new is the carb.

garygnu

try a bigger squirter nozzle on the rear .or try a more aggressive accelerator pump cam .or install a bigger accelerator pump. or try a 1"open spacer if you have the room .

F22

The secondaries don't have to open at all, to get it to bog. There's an adjustment on the back side of a Holley on the passenger side. You'll see an adjustment screw that pushes down on a lever there, next to the secondaries. Give it a push and you'll see it push down on the lever and it will squirt a bunch of fuel instantly into your secondaries, open or not. Just watch it and you'll see. That's the likely cause of your bog, too much fuel! Back off the screw a few turns or better yet take it out temporarily. It won't hurt it. You want it to engage later, than sooner.

Let it run off the primary's first! There shouldn't be any fuel squirting out of the back secondaries until the engine is good and ready for it (at least 2,000 rpm or more. My brother is a master mechanic and showed me this.

Alwyn678

My Holley was malfunctioning and running off the Secondaries@ idle.

The Holley replaced a Demon 650.

Believe it or Not the Edelbrock performs better Than both!