problems with holley on half-bridgeport.
 

ive got a 12a half-bridgeport with an rb intake and full rb streetport exhaust. it also has a 650 holly vaccume secondary carb.

the problem im having is the idle air fuel mix screws are turned all the way in and the car still runs rich. this leads me to believe that the jets on the idle circuit are too big. ive got timing set by "ear" which means ive turned the dizzy both ways until the motor runs its best at idle. i might be 3-5 degrees off but i dont believe this is causing my main problem.

also, the secondarys are not opening up at full throttle. the adjustment screw is backed all the way out for the vaccume diaphram on the secondary butterflies. turning the screw in is supposed to slow down the responce of the secondaries, and turning the screw out speeds the secondaries up. either way shows no difference. none of the linkages are binding and i have no vaccume leaks.

the throttle responce is excellent but im afraid that my stock 12a limited edition pulls stronger than this half-bridgey.

i would also like to know what the average idle rpm should be on a half-bridge. it idles steady at about 1500 rpms with that characteristic "brap-brap" any lower and it falls on its face.

Sounds like the primary jets are too big. Unless you know alot about Holleys, it's going to be really tough getting the secondarys right. You really need a mechanical secondary carb with the half-bridge. I don't think the engine is going to build enough vacuum to open the secondarys on the carb you have. You can try getting a secondary spring kit and putting the lightest spring in first. There are ways to make the carb you have mechanical secondarys, but it will make driveability hard.

dont know if this makes a difference, but, both sides of the intermediate plate are the only ones bridged. not the front of the intermediate and the back plate like it is supposed to be. some hack mechanic worked on it before me and told the owner that that was how it was supposed to be done. will this cause drivability problems? i am so close to saying "eff" it. im just trying to help this guy out.

That would cause some tuning issues. I've never heard of anyone bridging the center iron, and not the front and rear. I really don't know what to tell you at this point.

One person I know of did on a 2nd gen. Ported the wrong irons, he tore it back apart.

Put a webber, mikuni, or dellorto on it?

Is this a Holley modified for rotaries or is it an off the shelf Holley from a parts store? check your jetting sizes for the primaries and secondaries. you may also need to change the spring where the secondary diaphram is to the weakest tensioned one. Spring kits are sold at high performance shops.

Beat me too it, diaphram may be bad or weakly sprung. Possible vacuum leak? Float level too high....jets too big...

As mentioned....sell the Holley to a V8 guy and buy a Weber 48 IDA. THis will work and can be modified for all apps. They can even be enlarged to 51.5mm for big bridges. Holleys will just give you massive headaches...both mechanical and mad vapors!

Yes... the first question has to be... is the carb off the shelf stock or rotary modified by a known carb modifier??? if stock sell it... 650 is really to large for a 12A anyway, a 600 would be better for the street especially since it is not a full b-port anyway...
do not really mean to be harsh but with only the primaries being B-ported and secondaries not... either have it torn down and replace the intermediate housing and have the ends b-ported or have all of the ported the same... otherwise redrill the manifold for the stock carb (the stock carb with the thick spacer it will fit perfectly across the holley pattern) rejet it and deal with it till you can afford to have it done better...

Now if you are stuck between a rock and a hard place try this... if the idle needles are having zero effect (starting point is 1-1/2 turns out and both are moved equally at all times for now), then using a flashlight, look down the primary throttle bores, if you see fluid dripping out of the boosters onto the throttle plates then the throttle blades are open too far and you are feeding from the main metering circuit, you will need to close the blades down to bring the idle circuit back into the game.
I am sure at this point you probably are not able to get enough air into it for it to idle so you will need to introduce a controlled air leak into the primaries runners, (or drill a small hole just in from the top edge of the primary throttle plates of your new carb which is a common mod for some applications), otherwise the air bleed can be done with a short piece of hose and a fish tank adjustable air valve plugged into one of the ports on the carb that has vacuum... now you should be able to adjust the idle mixture a little bit better and not effect the other tuning issues you are most likely having...

If you can get to the point of an acceptable idle and part throttle operation then we can try to work on the secondaries not having enough air flow through them to open all the way...

idle for a "normal" combo port can be as low as 900-1100...