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Car was running fine then came up to a light and it didn't idle so well anymore and it also likes to backfire out of the carb when first starting out from a stop. Actually it doesn't really idle at all anymore. I have a Mighty Demon 850 by the way. My buddy has a King Demon and says that the fuel/air screws are always backing out and he has to redo them quite often. Is this my deal or what else could it be?
 

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Carb guru's

I had the same thing happen, although I have stock hollys. Check for water in the gas/tank. I got a big slug after a fill-up and it would not idle and it backfired and bucked all over the place. Sucked out all the water and haven't had any trouble since. Good luck, look for the simple things first.
 

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Besides water, what else could it be? Could I have twisted the distributor a bit? Or with the carb coming a bit loose could some of the settings have gone out of whack?
 

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... backfire out of the carb when first starting out from a stop. ...
This sounds like a too lean condition, or accel pump. Water can do that. If you can, get under (a lift is best) and pull the fuel line from the tank and let it drain into a glass jar for a few seconds or so. If you see water, you found your problem. You could also drop some gas additive which absorbs water if you can't drain your tank.

Again, sounds like a too lean problem. Yes, timing can do it also!

How does it run after coming off idle?

Good luck!:thumbsup:
 

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Lean conditions show themselves like that. Also, vacuume leaks can cause this too. Cup your hands over the carb when its running, and if you cant choke it out all the way to stall, then you have a vacuum leak somewhere. You can spray water around the intake and if you find a leak the revs will temporarily go up as the water seals the leak. As far as the lean condition, try to push the accelerator pump down with a little screwdriver while the car is running. Do not tough the throttle, just push the accel pump plundger down as see if it revs immediatly then tries to stall. If not, your pump is not working right. Then, check the float level, 9 times out of 10 a low float level will be the the culprit. When you give it gas, if the bowl doesnt fill fast enough it leans out an POP, right up through the carb.

Good luck,

Dude
 

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Addin some stuff

How check float level, besides I already checked for vacuum leaks by spraying brake cleaner around the carb gasket and no idle change.
 

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I dont know about a Demon, but with Qjets, there can be circuits internally that can leak vacuum and cause strange things. I had one that did not adjust at all when I turned the idle mix screws ALL the way in. Some internal problem. Anyway, Read the Demon wed site FAQ, or perhaps someone else with a Deamon could advise us how to check the float level, and where it is supposed to be set...

Was your Demon 850 "bolt on", ready to go out of the box? If anything, I would guess that would be set too rich. Ill see if I can find the Demon site for some info, and Ill post the link...
 

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Dude your in luck, the Demon appears to be very easy to adjust NOT like a Qjet, as the Demon has sight glasses...here the link for your carb

http://www.barrygrant.com/fromBarryGrant/Demon Instruction Manuals/MIGHTY Demon Manual.pdf

I recommend checking the float levels, adjusting the accel pump, and tuning the mix screws just like the manual says. If this doesnt correct the problem, and you have no vac leak and spot on timing, then send the carb back.

BTW, is this a new problem, has the car run fine up until now? Or has this always been the case with this carb?

Ive also seen wiped loabs on a camshaft do this, but I would not think that until a carb problem is ruled completely out.
 

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By checking float levels do you mean to watch the sight glass when I goose it to see if the gas level falls down too low?
 

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My first reaction to your first post is that your distributor clamp loosened up and your timing has moved. That has happened to me before, with the same symptoms.
If not, check your plug wires--are they dirty & greasy? Old? Touching each other? (Burnt?) Could be spark jumping. Look at them in the dark--spray a fine mist of water from a spray bottle on them. You'll see the arcing if that's it. Same for coil & connections....Fouled or bad spark plugs, causing dieseling/misfiring? Pull 'em and look at 'em. They always have something to tell you, be it rich, lean, no fire, fuel contamination. (You're not running Champions, are you? If you are, start there: Throw them just as hard as you can on the floor and go get some NGK or Bosch plugs!)...Distributor cap--old? Cracked? Dirty/greasy/deposits on the inside? Again, spark jumping. Crapped out/arcing points? (Capacitor going south--Didn't look to see what year/kind of ignition you have, just thinking out loud...)
While it's running, pinch off the various vacuum lines with needlenose pliers close to the carb & as well as the ones to the manifold to see if you have a vacuum leak. There are lots of vacuum lines on a 'Vette, and thus lots of opportunities for leaks.
Check the fuel filter.
Then the float bowls for water, or sediment which could be clogging the jets. I'm not familier with Demons, but if they have a sight glass, you should be able to see the fuel level when it's running. When you goose it, the level may drop but will quickly come back. If not, check the needle (after you check the filter) for binding or blockage.
Air/fuel needles backing out? Wow, that's a new one to me. But that, I would think, would be a gradual change, not a sudden one.
If your "carb came loose" you may well have a vacuum leak from a gasket now damaged, or even a warped carb flange if a bolt was over tightened.
How is your charging system/battery? If you're running out of electricty to properly operate the ignition system, similar symptoms can occur also.....

Check all the easy, simple "yes/no" stuff first.

Good luck,
John
 

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Crafty, there is a horizontal line cast into the boss that holds the sightglass in the fuel bowl. You want your fuel level to be even with that line. Car should be on a level surface when you check it.
 

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First, check the level at idle, it should be where Fredd Mann indicated, if not correct it. Then, accel it and make sure its recovering properly, instantly, if not, it could be trash in the bowl.

I have a sneaky suspicion that you need to increase your float level. At idle, make sure the level is right, then test it out.

If at idle, the level is where the manual and all says it should be, then increase it some, then test it. If it helps a little but does not solve the problem, increase it some more.

If that is not it, let us know...
 
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