Rich Demon carb

Melzer · #1 (**permalink**) 07-31-2002, 05:55 AM

Have a Demon carb on my 427 center oiler. From what the Demon people told me, it is a 625 (has the red disk) with elect choke.

The motor is running very rich, pulled the plugs - they were carbon black at the porcelin and tip. Fuel level in the bowls is halfway at the view windows, which I believe is correct.

A 625 carb on a big motor seems undersized. The motor runs strong, however - but I must be losing horsepower running this fat.

Any comments?

Thanks

Hotfingrs · #2 (**permalink**) 07-31-2002, 06:09 AM

Put in a lower power valve....you probably have a 6.5 in there which is what most come with.....put in a 2.5 and this should help. also make sure that the choke is open all the way, and IMHO 625 is way to small for that engine.

Melzer · #3 (**permalink**) 07-31-2002, 09:48 AM

Jack......

I probably should have indicated that especially at idle the gas fumes could choke Godzilla. Does the power valve affect the idle circuit? I thought the Demons were noted for their exceptional performance and flexibility.

The previous owner had tried several carb setups, but found the Demon to be above the others that were tried, (I don't know what the other's were).

If this small 625 carb is leaned out, what will feed the motor? Fuel is being delivered with an electric fuel pump currently, Demon sits on an Edlebrock Performer RPM aluminum intake. Cam is solid lifter, fairly potent lift - around .572 I think, don't know the duration, idles at 1,350-1,400.

What would a larger cfm carb do for the power band on the street?

Thanks.....

Hotfingrs · #4 (**permalink**) 07-31-2002, 10:16 AM

Do you have a fuel pressure gage between pump and carb also a regulator?? You should be running 7 1/2 - 8 lbs tops. Lets start there. Let me know. Also the PV doesn't effect idle. What are the jet sizes also...

Tongue Pirate · #5 (**permalink**) 07-31-2002, 11:59 AM

I had a demon on my big block chev. The idle circut was very sensitive. If your turning your screws out 2.5 turns it may be too far. Are you speaking of the red disc on top of the vacuum secondaries diaphram? If so my carb was an 850 with the same, so if this is the case you may have a larger carb than you think. Mine was too large for my application & I got a smaller holley free from a friend. I would have bought another Demon. Great Carb!!!

Chad Johnson · #6 (**permalink**) 07-31-2002, 12:04 PM

Melzer,

Demons have four corner idle adjustment; have you adjusted those for best idle? A 625 Demon is called a "Road Demon" and does not have all the adjustability of Speed Demon, Race Demon or King Demon. The Demon book says you should be running a 750 Speed Demon for your size engine and weight.

Good luck with tuning your carb

Melzer · #7 (**permalink**) 07-31-2002, 10:00 PM

Hotfingrs...

Yes, I do have a fuel pressure gauge at the carb. I will check the pressure as soon as i get a chance, probably over this weekend.

One thing I didn't mention before, there is an NOS setup on the motor. I don't use it, the plumbing is disconnected at the bottle. The rest of the lines are still intact however.

I don't believe that this is a contributing factor to the richness.....do you think??

A Snake · #8 (**permalink**) 08-01-2002, 01:02 AM

Just a thought for you. I haven't tried a Demon myself, but I can tell you this: I had the Carb Shop trick out a 750 Holley double pumper for my 427 side oiler. They got it to flow 910 cfm, did four corner idle got rid of the choke horn and race prepped the floats. Once I figured out how to get the idle set properly, the engine ran beautifully. What I'm saying is this; unless you don't plan to run at more than about 3500RPM that carb of yours, if it really is a 625 is a bunch too small. To get rid of the fat idle, try doing what I did and screw the idle screw closed (with the engine shut off) then in half turn increments, starting with 1/2 turn start the engine and see what it does 1/2 turn at a time. Bip the throttle to clear out excess fuel. If it won't idle at 1/2 turn try a full turn and so on. When you get close, with the engine running, turn the screws both directions to find the cleanest idle. the cam you described didn't tell me enough. Lift and duration are only part of the picture. You need to know what lobe center the cam is ground on. 108 degrees or what? It makes a big difference in how the cam acts. If you have a relatively narrow or short lobe center, the cam will be pretty bumpy sounding and hard to find a reasonable idle speed. Also, as I don't know about Demon carbs, but a lumpy cam will give you fits trying to make it idle. The vacuum signal is too erratic. Most folks do the best they can with the idle mixture screws and finish with the curb idle screw.

Hotfingrs · #9 (**permalink**) 08-01-2002, 04:36 AM

THE NOS could be a source of your problem. Most systems run a fuel line to provide extra gas when the NOS is activated. Not knowing what setup you have, I'd check into that first. If you don't plan on using the NOS I'd take it off and get rid of that variable, then we can work on adjusting the carb. I still would need to know what jet sizes are in the carb...also you really need to use a vacuum gage when you adjust the idle screws...

Jack21 · #10 (**permalink**) 08-10-2002, 10:58 AM

Some additional things to look for.

Assuming, no additional fuel is being added from NOS lines or fuel leakage from acc pump channel at float bowl/throttle body gasket (had to O ring the Holley to get rid of this).

Small carb on big engine could have primary curb idle opening primarys into the transition slot. Too much fuel, not enough air.

Carbs come out of the box worst case optimized for intended use. Running too rich is OK. Running too lean is not OK. Hydraulic roller cams pull a lot of vacuum, and the carbs have to be leaned out. Solid, lumpy, rollers may be the other way around, but you won't know until you put a vacuum gauge on the engine.

Your 625 is set up for a 300 - 350 sized engine with a mild hydraulic cam, making 250 - 350 HP.

Adjust curb idle so primary butterflys are at, or just under the transition slot. Adjust idle mixture to 1 1/2 turns both sides. Hook up a vacuum gage to the carb manifold vacuum line, and try to start engine. Chances are it won't start without some blipping.

Start opening the secondary butterflys to add more air to the mixture. You don't need much. With Holley, I replaced the slotted screw with a setscrew so I could adjust on car, engine running with a small allen wrench. As you open the secondaries, the RPM will pick up, and the engine will lean out. The Holley book has you drilling holes in the primaries also. I haven't done that.

Vacuum gauge reading. Power valve should be at least 2 points under the lowest idle vacuum. This keeps the power valve from opening with the engine idling. Demon has screw in air bleeds, and you may try using larger ones.

That carb is way too small for that engine/cam combination. Go 750 - 800. Put 625 Demon on a 302.

Melzer · #11 (**permalink**) 08-10-2002, 02:36 PM

Hey guys.....

I really appreciate all of your helpfull comments. Been very busy lately and haven't had a chance yet to work on the carb settings.

As soon as I can I will post a response with results.

Thanks again to all of you.......



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