DansYellow66

This actually pertains to a Ford but I figure the principal of setting the idle is the same. I have dual holleys on an engine and I've made several rounds from carb to carb setting the idle mixture by vacuum gage. All is good with that.

My question, is how best to set the idle speed screw so that both carbs are functioning about the same at idle - to make sure I'm not starving one end of the engine and running the other too rich. Set the front carb to run the engine a little slow and then bring the back carb up to bring the idle speed the remainder of the way to desired level? Or is it not that critical and just adjust it however?

MikeM

Off the top of my head....................

I believe I'd back the idle speed screws all the way off until the butterflies were closed on both carbs (engine off). Then tighten the idle speed screw until it just touched the screw stop, then give it another 1/4 turn. Do the same to the other carb. Start the engine (assuming it will run at that adjustment) and adjust the idle speed to whatever you like by turnig the idle speed screws the same amount, in/out. Go back over the mixture screws one more time followed by the idle screws as previously outlined and you're done.

DansYellow66

Makes sense. My first time with multi-carbs. Thanks.

midyearvette

good advice to use both carbs for idle settings....back in the day usually only the rear was used and at times this caused the front carb butterflies to stick closed....however you can rely more on the rear carb than the front if you like for idle settings but be sure the front is used to some degree too to avoid this problem
of course the bb fords back in the day had the holleys mounted backwards and..... i forgot where i was going with this.....

FastEddy

Imagine it's all one carb -keep all the idle bleeds in the same exact position (number of turns out), and turn them each a quarter turn at time in the direction needed. Move them all, see how it idles, move them all, see how it idles, etc. etc.

DansYellow66

You got it - backwards mounted carbs. Makes for interesting throttle linkage.

Frankie the Fink

I've heard it both ways from very competent carb guys on the dual WCFBs. Some say set the front/back idle equal on both carbs - others say use the rear carb for the most part and set the idle on the front carb just enough to crack the throttle plates. I do the former just to get a little better fuel distribution at idle but I've done it both ways and doesn't seem to make a big difference (or even a small one).

midyearvette

with the "fink"
be sure to use the front carb just enough to slightly open the butterflies...

DansYellow66

Has anyone ever had the opportunity to check vacuum on a car with a single quad versus dual quads? I wonder if vacuum tends to run a little lower with dual carbs. This thing (Ford) doesn't idle as well as my 427 Corvette even though the Corvette has a much bigger cam and higher compression. I do have unported vacuum advance on the Corvette and none on the Ford but the Ford is set up for 18 deg initial advance which is more than the Vette. Vacuum at idle is where it should be - 12 inches for a 231/236 cam at .050 lift. I probably just need to work on getting it dialed in a little better.

Ironcross

Now after the above suggestions have been tried you may want to try advancing the lead slightly.....It may clean up and run nice and smooth at a slightly increased/higher idle........If it responds as I suggested you could merely close up the butterfly's to get it back to a sensible idle.....The engine might like this but it may spark knock without good fuel if the compression is real high....... and then your back to square one.....but a good solid lifter idle is 800-850 with any fuel

DansYellow66

This actually has a hydraulic flat tappet cam but the lift is pretty high at around .565 to .575 (can't recall exactly from memory). I do need to get the initial timing set a little closer. It has a stock, black painted balancer and it's impossible to read the timing marks on it. I may be a couple degrees off. The distributor is set up for 16 to 18 degrees of initial timing but I'm going to have to paint a white stripe exactly on 18 degrees on the balancer to be able to read it with my timing light. Fords have to be different and put the scale on the balancer instead on on the timing tab.

Frankie the Fink

Most times dual quads are on performance cars with more aggressive cams (e.g. my solid lifter Duntov 097 283ci)....so comparing single to dual applications isn't totally "apples-to-apples". I would bet the majority of dual quad setups have lower vacuum.