Again I'm having the same problem. 496 BBC pulling 11-12 mm vacuum at idle. Some hiccups from off idle to about 2200 rpm, then cleans up. Only does this under load, not when in idle. I've adjusted my timing in increments of 2 degrees from 18 initial up to 26 initial with a small amount of improvement as the timing was increased. But at 26 my total is over 42 and I was a bit concerned about that. Combo is in my sig. I did notice that this got worse when I changed from my Brodix modified dual plane manifold to the Weiand. Problem also gets a bit better when the engine is warmed up (185 degrees). I called DUI Performance distributors today and they said they did not have a vac can that will engage at 8-10 mm. Does anyone know where I can get a can that will engage that low, but fit an HEI? Heres a stupid question, but when one changes manifolds (especially dual to single and visa versa) does the carb need re-jetted? ( already has #84 primary and #92 secondary jets, 6.5 power valve from Pro Systems). Again, above 200-2300 rpm, the motor runs excellent. Just at the lower rpm. Maybe just life with a bigger cam and single plane? Although this happened no matter what manifold was on it (Eddy, Brodix, Weiand). Definately worse with the Weiand. Maybe it is a carb problem then? Sorry for the long winded post. Thanks guys!

Patrick

I had the same basic problem with my 454, the car would have an off idle stumble under load but would clear up above 2000. Also, after driving the car for 20 to 30 minutes the problem would away. Ended up having fouled spark plugs, with new plugs the engine ran great but started acting up again in a few weeks. Still haven't figured what's causing the plugs to foul. Good luck! If you get it figured out please update us.

My plugs are not fouled. Its not really a stumble. Its like a bunch of little hiccups. One almost has to experience it to see what I'm saying. I've dealt with off idle stumbles before and this is not that, which made me think of timing first. It gets better when the car is warmed up, but does not completely go away. Not sure where to go here.

There are numerous post on this forum that cover this. The very first thing you need to do is remove your carb without touching the idle screw. Look at the bottom of the carb on the primary side and you will see a slot in the bore where the butterfly closes. With the throttle plates closed, that slot should look like a little square, about .025". If it does not look like a square, but looks elongated, then your throttle plates are too far open at an idle and your transition circuit will not funtion properly. That will cause the kind of problem you are having.

You need to add some extra air from somewhere besides the front throttle bores. Sometimes there is an adjustment screw on the rear throttle plates that keeps them from closing too far, so they don't stick in the bores. You need to give that a turn to open the secondaries farther to let in more air for idle. Most carbs have some kind of adjustment that allows you to admit more idle air. You just have to find it.

After you adjust to front to look like squares, put the carb back on and see if it will idle. If it doesn't, you need to open the rear butterflies until you get it to idle. With each adjustment of the rear air, you need to reset the idle mixture with the idle mix screws. Don't be surprised if this takes a while to do.

You can't ever get your car to run right without the transfer circuit working properly. It's one of those things that must be right before you try to do anything else. (Google: Carburator Transfer Slots) Here's a start:

http://www.gnetworks.com/v4files/barrygrant/IdleEze%20with%20Images2%20(3).pdf

I pretty sure that with that cam and almost 500 CI, you will need to add idle air from somewhere besides the front butterflies. Check with the carb maker and see if you carb has something like the BG "idle air adder" screw.

Thanks for the info. My carb came from pro Systems (basically a Holley modifed HP.) with holes drilled in all 4 throttle plates. Do you think I still need more air in there for idle? I'll take the carb off tonight though to check out what you mentioned. One other thing is that the car does like to "deisel" on after turning the motor off. That could be a symptom of primaries open to far at idle right?

kirkwooden,
I took the carb off and indeed the transfer slots were about 1/4 inch exposed! I loosened the idle control screw so they now look like little squares (by the way the idle screw is now as loose as it will go. The throttle blades will not completely cover the slots entirely. Does that sound right?). Anyway I found the screw that controls secondary plate opening. Since my idle control screw is completely loosened and the transfer slots are little "squares", am I correct in assuming that I will control the idle rpm with the SECONDARY plates and 4 corner mixture screws?

Yes! The transfer slots are necessary for a smooth transition from the idle cirduit to the high speed circuit. When the slots are exposed at idle, they cannot perform the function they were designed for. You can't back out the primary idle screw all the way or the throttle plates can stick in the bores. You must use the screw for a stop. When you get it so it will run (idle) with the front blades shut, you will (should) be able to turn the idle mixture screws and they will actually do something.

The transfer circuit supplies most of the gas you need for cruising. If it's not working, you usually get surging from lean mixture. It feels like something just isn't quite right. For the transfer curcuit to work, it has to start out as two little squares. Even if your engine seems to run right with the slots open farther than little squares, it will run better when it's set right. It is just the way carbs are designed to work.

I have never had a preference of one carb make over another. All are adjustable and should deliver equal performance for equal CFM. I think the Carter/Eddy is the easiest to work on as far as getting the cruising good. I like the super accurate jet stepping and large float bowls of the Holleys. The small primaries and multi venturi stack of the Q-jets give great driveabiliy and mileage when set up right, and the secondaries are infinately adjustable. So really, they are all about the same. If someone were to give me a good Thermo-Quad, I'd try to use it on something. They were a great idea that just wasn't made quite good enough. The plastic they were made from left something to be desired, but the design was good. If you have never heard of the ThermoQuad, here's a quote about NHRA factoring:

MOPAR MUSCLE MAGAZINE 1998
Making its debut as an O.E. carb in 1971 on high-performance 340 engines, the Thermo-Quad quickly gained respect on the street and dragstrip. (...) With the secondaries at attention, a Thermo-Quad was a treat to hear, delivering a fender-rattling moan at full song, sounding as though it’d suck the hood through the engine. The NHRA slapped the `71 Thermo-Quad fed engine with a 25-hp factor increase in Stock Eliminator over the otherwise-identical AVS 340 of previous years.

Something to think about!

BB489, to answer a question in your original post, yes, you need to re-tune your carb when you go from single plane to dual plane, or vice versa...each intake will require different metering requirements from the carb. They work quite differently.

back off the screw till there's just a square showing on the transfer slots, reinstall the carb, and reset your initial timing as I'm sure that has changed with a lower rpm seen at startup, now tune your idle circuit. It needs to have a clean burning idle and a rich fuel enrichment curcuit to work with that single plane. I shoot for the highest vacume and rpm with the mixture screws then turn them in 1/8 turn to lean it out just a tad and keep the plugs clean and ready for the pump shot and transfer to the power circuit.

Sounds like a fuel problem to me.

Alright.
So I took the car out for a spin last night after getting the transfer slots right and the problem is virtually gone!! Yes!!! I did not have time to reset the timing due to it being pretty late. However I did note that even with the transfer slots as little squares, I could turn the idle mixture screw on the primaries all the way in with no notable change in idle. I didn't turn both sides in, just one (4 corner idle). Same with the secondaries. I didn't notice any vacuum leaks and the vacuum guage was steady betweeen 10-11. My idle is 1100 rpm in idle, and 850-900 rpm in gear. Car runs 100% better though. Seems to have a bit more punch too. Any thoughts? Also I noted my power valve is a 6.5 and my vacuum again is 10-11. Should I increase it to and 8 or 8.5 or is ok where it is? Thanks a bunch!!

Patrick

After reading JohnnyO's comment, I realized you have a single plane manifold. If you are switching from a duel plane, you may have to go up a half dozen jet sizes to be rich enough on the mains. Also, if you have a bad sparkplug or a leaking intake valve, it will have a much greater effect on performance compared to a duel plane. Always something to consider on a real street engine. I'm sending you a PM.

Here is a good carb tuning page: The part where it says,".004", should be ".040". (Didn't think I'd send it without correcting it, did you?)

http://www.jason.fletcher.net/tech/carbtuning/carbtuning.htm

How much mechanical advance is built into the distributor and is it adjustable? Your powervalve should be fine at 6.5. Turn all four corner mixture screws to 1/2 turn out and start the car, then adjust all out 1/8 turn untill your highest idle rpm and vacume is reached. Your initial timing and idle fuel curve are the first things to tune, then the main jet and enrichment circuit. Total advance will like to be between 36-42 depending on what fuel you have and the burn characteristics of the engine. Don't forget to readjust your accelerator pump arm after the idle stop screw is turned.