Just built a 350 SBC, bored .060, flattop pistons, 76cc heads, Comp 270 hydraulic cam, Edelbrock performer intake, blockhugger headers. Bought a new Road Demon carb, friend has one, works good, put one on a friends car 2 weeks ago, same cam as mine, works good. Mine is dumping way too much gas, mild black smoke, fouls plugs @ idle, put it up about 2-3000 rpm blows out heavy black smoke then cleans plugs will idle again. Went thru all their tuning tips, no change, called their tech line, waste of time. Last night took it apart thinking power valve might be bad, looked it over good everything appears ok. Runs good if dont let idle long just too much gas fumes. Primary jets are 70s. Is this normal? Blew up a muffler when started it the other day, dont want to run much as dont want diluted oil. Any suggestions? Thanks!
Float stuck or set too high...idle mixture screws out of adjustment...bad power valve...vent tubes plugged...excessive fuel pressure.... There's a list to get you started!
I once had a Chevy that sounded just like yours. Turned out to be killing its spark plugs! No idea why, but it went thru 3 sets pretty quickly. I had a Holley 750 on it, virtually brand new. Change all your plugs for new ones, and see what it's like. Cheers, Paul
I had the exact same problem the needle and seat was bad, had a small bur stuck in it, replaced it now it's fine. BTW Demon's tech line sucks, dont bother to call them unless you are out of options.
My demon was doing the same exact thing, but probably for different reasons (mine idles @2.5" of vacuum). The guy I took it to put power valve plugs in and jetted it up like 8 sizes.... seems like a band aid to me, but I'm no expert
I thought power valve too but neighbor running same cam no problems. Think I will try plugging it anyway. Thanks guys! Anyone else? Does jet size sound right? Neighbor wont let me take his apart, wonder why!!!
I had the exact same problem but with a speed demon, I tried everything, then one day after putting it back on after yet another adjustment I forgot to reattach a vacuum line and got a good vacuum leak and the thing finally idled right for the first time. I mentioned it to a guy at work and he told me to drill a hole in the primary butterflies. I was pulling my hair out over it so I figured I had nothing to lose. If you try this remember to start off small, you can always enlarge the whole. The guy at work said that there are quite a few stock carbs that come this way.
That jet size sounds good to me. I ran anywhere from 68-72s on my 350 and it ran good, the only weird thing is that it would stutter when I had 68s in there, but only on the highway. This was on a 750 cfm with vacuum secondaries.
I haven't had a Demon yet (plan on getting a Road Demon or Demon, Jr. later), but had a similar prob with my Holley 2300, turned out to be a stuck float. Whapped the top of the fuel bowl with the flat of screwdriver, then the adjustments became effective. Creating a leak is an interesting idea, but makes sense since yer running rich. Do you think the plug gap needs to be changed? Don't know if a carb change would need that, but changing from points to an Ignitor 2 unit called for a widening of plug gap. Hope this helps.
I just thought it was me...but same deal running 350 with 650 Demon and it runs black rich and sooty. Fumes you bet. I have the screws almost turned off to compenstate. Whoever said dont bother to call them was right. They were rude and unhelpful.I would like to hear more suggestions as well.
I was having my exhaust shop do some patch work on my Ford the other day when a guy came in asking about Demon Carbs. The guy that worked there was apparently into dirt track racing. He said that he got a smokin' deal on a Demon for a friend of his and couldn't tune it to save their lives, I don't remember everything he said, but he said he'd never buy another Demon. Sounds like you guys share that opinion.
I have recently blown up the second muffler due to demon carburetion issues. I am ready to start a barry grant demon sculpture garden and will donate my carb first.
I bought my demon (825 race demon, Drag race series) because they had so much adjustability in them. Unfortunately that adjustability seems to be in a range that is not applicable to a real car!?! I can't find ANYONE that can tune one, yet ALL of the car magazines run 'em regularly (funny thing, they always say "it was set up PERFECT, right outta the box!") So I'm guessing you have to be somewhere on the west coast to find someone to tune them.
Ohhh Another thing, not that they would actually check it, but should we email a link to this thread to the DEMON people???
Not a Demon expert but I know on the Holley there is an adjustment on the secondary shaft that props the blades open or not. They need to be cracked but not too much. Had one that was an eyeburner a few years back and after checking everything that has already been suggested I checked that adjustment. Brandy new carb, and that was the problem. On the Holley ya gotta pull the carb and look under the Vac pot for the secondaries.
I've been tuning on Holleys for 20 years and Demons since before they were Demons (they were "claws" for awhile, as in Gold Claw, Silver Claw, etc.). The same theories apply and it sure sounds like a stuck float to me. Fuel is not being regulated into the carb and it's not being metered at all. The needle and seat inside the float bowls should control this, and it should be set just short of halfway up the sight plug when the engine is running. With regard to the fuel transfer slot, this should be only slightly exposed at idle. It's worth the effort to pull the carb and flip it over to see how much of the fuel transfer slot is exposed by the throttle butterflies where you've got the idle screw set now. Yes, you can stretch the range of effectiveness on this by drilling a 1/16-inch hole in each of the primary buterflies, but this isn't normally necessary unless you'e got a big lopey cam incapable of providing a good vacuum signal to the carb base. I really think you've got a simple float bowl issue, which could be as simple as an external adjustment or as complex as a stuck float. You could simply have too much pressure to the carb (it's best at 6-8 lbs, max) pushing fuel past the needle and seat. These carbs do work, but they are most-effective in the hands of an experienced Holley/Demon carb tuner. They can be broken down into distinct circuits (idle, midrange, and wide open) and tuned circuit-by-circuit. They're better than they've ever been, but almost always require tuning once out of the box. Sure, the magazine stories show them being used out of the box on some applications, especially when the carbs are designed for use on the kind of engine being tested (Demon makes "crate" carbs for crate engines, and they are "right" as shipped for the majority of customers). Try not to get hyper-critical of these carbs...there are many adjustments and that means many opportunities for erroneous adjustment. If you don't know exactly what to do, do not take shots in the dark and draw straws...take the time to work with an expert who will not give up until it's right. Learn from what they do and ask why they're doing it. Most tuners aren't keeping secrets and once it's real close, even a rank amateur can keep it in tune. Setting floats and fine-tuning idle air/fuel screws is not hard, but needs to be done regularly. Jetting and power valve swaps should only occur once- when the carb is first installed. Naturally, engine component changes may require further tuning, but if you drop the cash for a good carb, don't be scared to find a really good tuner to set it up for ya. I learned from an expert, and it's helped me plenty. Scotch~!
Thanks guys! Very good reply, Scotch. Thanks for the jet info Deyo. Float level ia aboutmid sight glass will pull down make sure everything ok needle/seat area. Luck of draw I guess, friends car with same cam Demon great right out of box. Need a good full day to fool with this, maybe I can sort out.
Yeah, you have good points scotch, but without trying to sound liek too much of a jerk, with those big ol' sight windows it's pretty easy to tell that the float systems working properly. I really think there is something else up with these carbs...
brxter, I've seen that trick too, but it's usually for pretty highly tuned motors that draw a lot of air at low revs. A virtually stock tune 350 like Bellms shouldn't need such radical changes to the carb. Maybe tweaking the small screw hidden underneath, that holds the secondaries open a touch, would be enough to smooth things out. Cheers, Paul B I still vote for changing the plugs though. Sometimes the simple things are the answer
I know this ain't the recommended way to adjust the float, but I do it static w/the bowl off. Turn it upside down and adjust until top of float (now on the bottom) is level/parallel with top of bowl (also on bottom) - forget measuring, drill bits, graduated paper "L"s. It works and is damn close. Your jets don't sound out of line to me. Don't plug your power valve except as a complete last resort. DO check the secondary idle transfer slots as recommended above. Finally, the only thing that hasn't been mentioned (that I saw) is to check the air bleeds on the top of the carb. Spray some carb cleaner in those puppies & ensure they're clear by cleaning with something non-metallic (DON'T damage/enlarge them). If one of these is partially clogged, it can wreak havoc on your mixture & foul your plugs in no time. When you're done - get new plugs! Good Luck!
Do you have any other similar 4 barrels around that you know are good? How bout the other friend you just hooked up? Try another carb to make sure that there's not something wrong with the motor. I just fixed a brand new speed o motive crate motor (383stroker). Had a demon and was running way too rich. I couldn't tune it no matter what. I pulled the lifters and found that the first one closest to the distributor had about 1/2" chunk worn off of it. I pulled the cam and the lobe was fubar. I pulled the heads (dart aluminum) and found that a few of the valves weren't sealing also. Lifters were solid so we changed over to hydraulic with a cam and did a valve job. Runs like a stiped ape now. Demons are awesome.
morrisman, i didn't mention the engine that i has working on was a 440 chrysler that has a fairly large cam, but when i say i tried everything i mean everything and its easy enough to try by just creating a vaccume leak
WOW... After all this, I'm wondering now more than ever what kind of carb to install on my SBC, just rebuilt .40 over with extreme energy compcam 262 and a 350 crank. It was a 327.. I was leaning towards the Road Demon but can't help but wonder,,,,,,,,,,, why this seems so difficult???
Demon just came out with a new baseplate device called "Idle-Eze"...it's a screw-controlled airway to a central vacuum point under the butterflies and allows you to fine-tune the bypass air without screwing up the .020-inch adjustment required by the transfer slot. It's basically doing the same thing as drilling holes in the primary butterflies, but it's an adjustment rather than a permanent modification. So, if you get the Idle-Eze baseplate and adjust it per the instructions, you should be able to clear up your idle richness condition without any further headache. By doing this without altering the transfer slot dimension, the whole carb should work better. I anticipate the Idle-Eze baseplate will become standard on Demons someday soon. Luckily, it's an easy upgrade. The adjustment is via screwdriver right down the air cleaner stud hole. I'm looking forward to getting mine. My Demon-equipped 383 is good, but it's still a tiny bit rich at idle. This should clear it (and my watering eyes) right up. http://www.barrygrant.com/demon/default.aspx?page=newsarticle&articleid=IdleEZSolve Scotch~!
Sorry to dig this one out again. but i'm wondering if any of you guys are still running these carbs on your motors...
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