Posted By: MrBelvedere2
setting up dual quad tips&tricks - 07/17/12 04:17 AM
Hey guys, I just got my dual edelbrock 500 cfm carbs on my 383. They were tested each after a rebuild on the single 4bbl manifold, and worked great. I put my offenhauser manifold on, and mounted the carbs, running a 1:1 linkage (non-progessive). I fired it up and it wants to idle very high, timing was set with a single carb right where the engine likes. It acts like a vacuum leak, and it does run hot as well. I tried playing with them (idle adjust, mixture, etc.) while the engine is running but to no avail. The idle screws were all the way out. Any tips to get them tuned up and to sync together? I retarded the timing which brought the idle down but not sure the engine will like it that retarded. thanks for any help.
Posted By: bb74swngr
Re: setting up dual quad tips&tricks - 07/17/12 05:39 PM
Hey Larry. Is there a reason why you are not using progressive linkage? I have a dual setup with Eddy instead Offy intake, progressive linkage. Seems like it would be difficult to get the idle circuits happy on two carbs at the same time. Could you unhook one of the carbs and try to tune it that way first. The other carb butterflies would have to be completely closed while tuning. Just a thought...
Posted By: dynamite
Re: setting up dual quad tips&tricks - 07/17/12 06:46 PM
You said it ..It sounds like a vacume leak to me ..!!
Posted By: TooMany62s
Re: setting up dual quad tips&tricks - 07/17/12 07:36 PM
Did you by any chance forget to plug any of the unused ports. Particularly the ones at the front and rear of the bases.
Posted By: mickm
Re: setting up dual quad tips&tricks - 07/17/12 07:41 PM
i agree, it sounds like a vacuum leak. fix that first.
and if the manifold gives even distribution from the rear carb, i'd run progressive linkage as well. run off the idle circuit on the rear carb, and have the front butterflies all the way closed and the idle screws all the way in. that way you kick in to the cruise circuit and the WOT.
if you don't go progressive:
i have this vacuum tool, i can't remember the name, i'll find it later, that is meant for tuning multiple carbs on one of the foreign models, (can't remember which one either!). i took a 100 count CD case, and cut a hole in the top the same size as the base of this vacuum tool. i plop the CD case over the air horn of the carb, and the tool on top, and then i can read the vacuum that carb is pulling. that way i can set the throttle plate openings evenly on both carbs. works nice and is very quick.
i'll try to remember to get the name of that thing, or PM me if i don't respond back. busy couple days ahead of me!
Posted By: MrBelvedere2
Re: setting up dual quad tips&tricks - 07/19/12 02:55 PM
Thank you for the replies so far very helpful. I was wondering this as well.
I had the manifold powder coated clear, but they did not mask off the intake suface and ports or the carb mounting surfaces. Even though they are smooth and I used gasket sealer, could this be where it could be leaking vacuum?
Posted By: MrBelvedere2
Re: setting up dual quad tips&tricks - 07/20/12 02:43 AM
Yes I did make sure the surfaces were clean and I am using the paper gaskets as well as a new valley pan. I'm just wondering since the mating surface is powdercoated it could be causing it to leak?
Posted By: MrBelvedere2
Re: setting up dual quad tips&tricks - 07/20/12 02:32 PM
Thanks for the info I will try to sand it off
Posted By: 70Duster440
Re: setting up dual quad tips&tricks - 07/20/12 04:38 PM
Posted By: Cudajon
Re: setting up dual quad tips&tricks - 07/21/12 12:14 AM
Sounds like a vacuum leak OK, but before you go tearing it apart. Close the front carb all the way down so that it is running off the rear carb only. Make sure the idle screw is all the way out so the butterflys are closed down for sure. Might disconnect the linkage to the front carb so you don't start fighting the front carb while adjusting the rear carb. Trying to sync both carbs to idle is very frustrating and requires work with the springs and metering rods. Thats why progressive is SO much better. It takes a lot of engine to handle two carbs on straight linkage and be streetable. The fact it is overheating is also an indication of a lean condition. I find a wide range O2 sensor to be a lot more valuable than a carb synchronizer for adjusting multiple carbs. It will tell you if your lean.
Posted By: Lee446
Re: setting up dual quad tips&tricks - 07/21/12 02:03 AM
When you get your vac leaks fixed, go to
www.diamondbackengines.com for dual quad tuning tips, very informative!
I like to the have the bottoms at .000 and the tops opened up a tiny bit, say .001 to .003 so the manifold will squish the gasket tight at the bottom, sealing it up real well 