Tuning a Demon Carb

[DAVID GAGNARD]

What is your vacum reading at idle???? I'm guessing pretty low, maybe 10 to 12 or less....also guessing when you get on the gas, with very little vacum, it hesitates because as you open the butterflys the vacum drops even more, making off idle responce even worse......

I think your engine and driving style would be better suited to a carb with no more than 750 cfm rating.......

David

[ohno50]

You're right, the vacuum at idle is in the 12ish range and when I accelerate it goes up substantially. Thanks so much to everyone, I hope this thread helped some others too...I will update it with the conclusion once I get it tuned.

[DAVID GAGNARD]

Originally Posted by ohno50

You're right, the vacuum at idle is in the 12ish range and when I accelerate it goes up substantially. Thanks so much to everyone, I hope this thread helped some others too...I will update it with the conclusion once I get it tuned.

A quick and easy thing to do is hook a vacum gauge to the carb at one of it's ports and then adjust your idle to your normal idle rpm, then begin turning the 4 corner idle screws in/out, 1/4 turn at a time, till you get the highest vacum reading, all the while re-adjusting your idle rpm to your normal idle rpm,you'll have to do this a few times to get it, but,it's a start, then road test.....

Another thing, do this with the engine fully warmed up at operating temp.......

Carbs are not hard to tune, just takes a little time.......

David

[lovehamr]

To piggyback on what David said (which is spot on), on my Demon each of the idle screws are only about a quarter of a turn out. Remember that all 4 should be moved equal amounts for each change as well.

BTW, some little known facts about David: He's got a big dog, sleeps with his Bowie knife and once he cut off his own arm for a dare! Spot on David!

[DAVID GAGNARD]

BTW, some little known facts about David: He's got a big dog, sleeps with his Bowie knife and once he cut off his own arm for a dare! Spot on David!
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Steve: still laughing, dats a good one...........but, the dog died long ago, can't find my Bowie knife and it was a friends arm I cut off on a dare,not mine!!!!!!!!!!!!!

David

[lovehamr]

OH,.................................never mind...........:

[ohno50]

Here is an update on the situation...went out for a drive tonight and paid a little more attention to the vacuum gauge as I was driving. Here are some notes:
- when cruising, the vacuum gauge reads around 10-12
- when I take my foot off of the accelerator, the gauge jumps up to around 20
- under hard acceleration, the gauge drops to around 5 and THIS is when it starts to sputter until it "catches its breath" so to speak and starts to accelerate again and the gauge goes back up to around 10-12

Now this is all under the assumption that the vacuum gauge is operating and reading correctly.

What should I make of all of this? Is it safe to drive or will I do harm to the engine?

[DAVID GAGNARD]

when cruising, the vacuum gauge reads around 10-12

that's a little low, could indicate over carbed, could also be a by-product of your camshaft profile.......

- when I take my foot off of the accelerator, the gauge jumps up to around 20

thats normal.......

under hard acceleration, the gauge drops to around 5 and THIS is when it starts to sputter until it "catches its breath" so to speak and starts to accelerate again and the gauge goes back up to around 10-12

gauge dropping to 5 or less is normal under hard acceleration, sputtering, I don't know, could be a number of little things, could be one or two things....but does sound like a temorary lean condition...
again, I've seen this on engines that were over carbed, it can be fixed,it'll just take some tuning and trying different things to work it out.....

BTW: normally, your idle vacum reading will be the same or very close to your cruise vacum reading.......

David

[lovehamr]

Seeing that 5 as it starts to sputter still tells me acc pump issue. The power valve may also be sticking or just crap as well. If you have the means to test it I would, otherwise I'd just replace it with another rated at 5.5. Each of these circuits are responsible for supplying your engine with extra fuel during low vacuum/ high power operation. That 5.5 rating would insure that it was opening as your engine is starting to lean out. My bet is that you'll find your bog there.

Oh, David; "vacuum."

[mdross1]

Have always run Holleys on everything, carb of choice will always be the double pumper.The 460 in our car is basically what you have,and without hands on your motor it would be very hard to give advise without a few basic checks.Having a feel for what each circuit does in a carb and how best to adjust it makes it an easy chore,without that feel a headache.The 850 dbl pmp that was on our 460 was dailed so this motor is very crisp.Now running a 1000cfm Dan Davinci tuned Dbl pumper,clean idle and still crisp the way it should be.Having a tuning kit (jets,power valves,squirters,tools to check things) makes working with Holleys/Demons a treat.Otherwise ask around and find a good Holley person.Good Luck

[Jerry Clayton]

I've generally been staying out off carb tuning discussions because generally the poster doesn't want to listen but seems like you guys are carrying on in a dedicated civil manner so here goes a few guide lines---

A couple items about the demon---they have pink accelerator pump cams and should be around a 31 squirters---this works pretty good for a starting point.

They all seem to have too high a power valve and I will generally drop to a new 4.5 or 5.5 and sometimes on a BIG cam maybe a 3.5.

Jets, a general Jerry rule is sizes of Holley jets that = stated cfm of carb---800 cfm carb, #80 jets----I would probably do 78 pri and 82-83 sec for a start--primary has pv which = 3 -5 jet sizes

Demon carbs---there was a period of time where they were full of chips---this era probably turned more tuners away from these carbs and killed the company
Idle-eze---if you have an aircleaner stud that is 5/16 thread, you probably have an idle eze base plate---remove the stud and with a slender screw driver you can adjust an air valve that is located in the middle of the base plate---this allows you to keep the butterflies where they need to be over the transfer slots---Oh did I mention that the transfer slots usually have a burr inside the drilled hole?

Try to identify the carb number and if it has an Idle eze base plate----

[cdog]

Make sure the secondary is opening up in a staged fashion, as you can also position it to open up 1:1 ALONG WITH the primary which would dump a lot of fuel in at once under WOT. That would cause a rich condition.