New old carb..

[jvike]

Hi, I put on an Edelbrock Preformer RPM Air-Gap intake on my 340 last evening. I also swaped the Eddy 750 for a BG Speed Demon 750 that I got cheap on ebay. I have checked the carb and cleaned it inside out before I put it on. I didn't expect it to work out of the box, but I can use some pointers where to go next. The car stumbles of idle, and if I rev it high, or floor it on the street the idle will stay at 2000 rpm idle. The idle-mixture screws are pretty much out too. So my I guess I'll have to change the jets. Any other Ideas? I'll check the floaters again..

[Moparal]

sounds like it may need a bigger squirter size

[Chryco Psycho]

make sure you use a thin gasket under the carb & do not overtighten the carb on the intake , also make sure the carb is not hanging up on the gasket below the carb & keeping the idle high , make sure the float levels are set correctly , if it is low it will bog off idle , if the floats are st & it still bogs you need a larger squirter [Holley will fit]

[moper]

a 750 speed demon i stoo large. The air bleeds are not sized right for a smaller short stroke engine. There's a real good book.."How to tune and win with Demon Carbs" from HP books. They are not liek hlleys. They can be a PITA. I'd swap the Edelbrock back on and sell that Demon...lol

[440Charger]

Check this out:
http://www.corvette-restoration.com/Tech_Papers/BG_Carb_Setup.pdf

[67Vette427]

750 is pretty big. It shouldn't do what its doing though, just run rich.

[Chryco Psycho]

typically a large carb will not run rich , on a small engine it will have less vacuum signal & will not pull as hard on the jetting or idle circuits , it should be lean

[patrick]

    I've been learning a lot about this in the last month or so. I alway's thought bigger is better. Makes sense right? I wanted to put a Holley Street Avenger on my 340. 870, 770, 670 cfm. I wanted at least 770 cfm for my 73 Rallye. But was talked down to the 670 by my buddies at the Challenger Message Board. Just enough is perfect!!!! Everything need's to work together in your engine. Depending on what your specs. are, you might need a 2100 cfm, But that's a Hot, Hot, Hot, 6pack 340. You want thing's to run snappy and have clean looking spark plug's, no foul looking one's. What you have in the engine, you should compliment with the carb. My engine is HOT, and I went with the 670. My ego took a beating but, I think it's the right choice. No hessitation's, no bogg's, No problem. Still eat's 5.0 Mustang's for dessert. Keep us posted, Pat

[74 340 4speed]

My 340 loves the 750 holley vacuum secondary thats on it.  No bogging/hesitation or anything.  Just 

[Rob C]

Good thing they talked you out of that monster carb for a correctly sized unit.

The bigger carbs, as said, normally do not run rich, but they can. Ethier way, it can be a pain to tune up for a good response allmost all the time. It is possible though, just a larger pain than a smaller carb that is easier to tune without major headaches and tail chasing.
That throttle response of the smaller bore carb is so nice compared to a more than likely squishy feeling of the bigger carb.