demon carb too rich?

[plydus340]

Well, I've been running the mighty demon 850 w/ annular boosters for about two months now. Boy, big differnce from the 750 edelbrock. My problem is I have had to replace the boche platnums once, and now it sputtered again the other day. I know we have a good thread for tuning, I'm just scared to touch the dang thing. I noticed the front sight glass is about 2/3's full. the back is about at the halfway mark. Hard to start first thing, but then after warming up seems fine. Good throttle response when running good. Good acceleration all the way through ( unbeleaveable). I think it is running rich. I have heard race cars run rich and I dont want to mess it up before sunday for my first run at the track. I have been tempted to mess with it but I need some encouragement. I also want someone to tell me I didn't get too big a carb. I went by the chart on the BG website and picked the mighty demon. I put the 3" exaust with 3" flowmasters dumping out in front of the rear tires. I have a six pack cam w/ torquer 2 intake. Dont mind changing plugs if that is what it takes to keep the performance. I have a buddy next door who can help change jets if needed. O.K. I'm shutting up now, thanks for any and all responses in advance...

[pishta]

Any idea what jets are in there right now? stock 80/88's? I would try 70/76's as this is still on a relatively mild 340. 340's like larger carbs, and you only lose a few low end ponies with a largeer carb. It is about a 15 minute job to change the jets.

[Kevin Garceau]

sounds like your floats are way too high.... I usually run them to the bottom half on everything I drive... also you may need to put an extension on the bowl vents. If you have them set that high and are driving it around on the street, if you get on it for a very short period of time yu can dump the bowls through the vent tubes into the carb, will be very rich when you let off...

Jet changes...very easy to do. Be careful with the BG though, they strip the bowl screws very easily.....

[ehostler]

Your problems are why I would have bought from a pro, like FBO Systems.

Don gets tons of information from you, before he recommends a specific Demon. Then he sets it up for your application and ships it out. Once you get it bolted on, you just fine tune it and away you go. If you do have problems, after that, he will provide tons of post sales support.

[plydus340]

pishta,
Didn't you get the update? The duster is now a 71 440 hp outof a GTX. my engine builder rates it at 500 hp. I still go by plydus340 cause thats what it had when I joined moparchat. Trust me the duster is not mild. I still have the small valve heads (2.08" In. 1.74" Ex.). Balanced, blueprinted, and line bored, ect... . Surely all the work I've done justifys the 850. You know what I think it is? I suspect the unilite. I think it advances too much, and doesn't burn all the fuel. Think I'm on to something, or am I showing my stupidity?

[ehostler]

If you suspect the advancement of the unitlite, you should get yourself a dial back to 0 timing light. Then you can see what your advance curve is looking like.

What coil are you running? If the coil isn't throwing enough spark, that could be your problem as well.

From the sounds of it, I would agree that your secondaries are too rich. Have you read the Demon tuning thread in the tech archives?

[jeffco]

the stock jetting for a Mighty Demon 850 is 85 pri with a 6.5 power valve and 93 secondary blocked, run the float level in the lower 1/2 of the glass. If you have a rich idle go over the base idle spec again, Demon carbs get real rich and can get an off idle stumble if the throttle plates are improperly set. The hand book gives a good method of setting idle and the four corner idle mix, but for more exact info go to the Demon section in the archives and use Don's method. If you have a newer model that has the idle compensation screw, that is used to increase idle and not the curb idle screws on the pri and sec throttle shafts, hope this makes sense, good luck, jeffco

[fastmopars .inc]

not positive, but i believe there are two things going on in that engine.
1) that mighty demon is calibrated for a big block chevy. a big, heavy, lethargic short stroked p.o.s that burns a lot of fuel makes a lot of heat and goes no where. the idle air bleeds are probably making the car run very rich at idle, and to compensate you probably just opened up the throttle blades a bit to raise the idle speed. this would explain the hard starting. it probably lugs a little bit when idling in gear, because of the less than nominal vacuum due to your more open throttle blades.
the lower vacuum also means your power valve might be opening quite a bit sooner than it should. again, this would cause an overly lean condition but because its on acceleration it would be hard to notice.
2) the carb is not too big but it is too race-oriented. it is designed to work with a big, lumpy cam, big ports and big intake / exhaust. you will probably need to change the cam on the accelerator pump, definitely adjust the air bleeds and probably put a milder power valve in it. turn your needle and seat down as far as it will go so you can just see fuel at the bottom of the plastic site, but get a fuel pressure guage you can watch while driving to make sure your not running out of fuel. that carb would normally be put on a big cubic inch motor with 250+ duration @ .050, an idle speed of 11-1200 rpm and lots of initial timing to fire all the fuel. you could try recurving your distributor but i would sway away from this if you don't know what your doing. you could put too much advance in it and it will start to pop and run really hot.

what i would do is: take the carb off, and set all the screws back at their default setting's. the paper in the throttle blade to measure opening angle for idle, the paper in the accelerator pump to set that up, turn the idle mixtures all the way in and back out the proper number of turns, and adjust the needle and seat how it says in the demon tuning section. put it back on and your problems may just go away, but chances are you will spend lots of time messing around before you figure out that you were going in the wrong direction the whole time.

thats why i stick with holley's, if it ain't runnin right its pretty easy to figure out the problem, with those demons they can create a whole wackload of, well demons.
my 2 cents

[mgreen2994]

plydus you answered your problem when you asked for help lol you said your front sight glass was 2/3 full and your back glass was 1/2 full demons are about the same as holleys back your needles and sets down to let the fuel come up till the fuel just about comes out the hole lol the clear plugs arnt there to see your fuel the reason your motor is hard to start is because its flooded and its having to clean its self out before it can start i run and 850 on my 318 and it does fine i just run smaller jets 74s in the front and 72s in the rear but for the 440 i'm sure it takes more like 80s

[BDS 871 Cuda]

Originally Posted by plydus340

pishta,
Didn't you get the update? The duster is now a 71 440 hp outof a GTX. my engine builder rates it at 500 hp. I still go by plydus340 cause thats what it had when I joined moparchat. Trust me the duster is not mild. I still have the small valve heads (2.08" In. 1.74" Ex.). Balanced, blueprinted, and line bored, ect... . Surely all the work I've done justifys the 850. You know what I think it is? I suspect the unilite. I think it advances too much, and doesn't burn all the fuel. Think I'm on to something, or am I showing my stupidity?

I don't Think a .480 lift cam with advertised 292 dur. will get you 500 HP.
What is the duration at .050 lift? I'm going to guess under 230 deg.
I would install the 2.14 intake and the 1.81 exaust with a lot more cam.
Let's say .550 to .570 lift with dur. @ .050 around the 240 to 250 range.
A far as your Unilite goes, I would not waste any more $ on it.
THREE LETTERS... MSD!

[pishta]

Oops, didnt mean to underrate your mill! Well, 273/500+ still run cleanest on 14.7:1 , 13-15 ideal. Your mixture really relates to your cam profile and ignition. Give Don at FBO a call. Even though you didnt get it from him, he'll probably ballpark you close enough to forget about it.

[dave571]

Ed's bang on the money. Don/FBO may have been of some help here.

The Mighty Demon is spec'd for engines with 240 to 260 @ 50 duration.

The annular booster model is more made for chevy's.. I will admit I don't know why, that is, but usually the regular one is used on a mopar.

An 850 is too big(arguably). An 850 holley, no problem, while an 850 demon is pushing it. It's not so big that your screwwed.

The right carb for you was probably a 750 speed demon, which would have flowed and performed worlds better than the 750 eddy when tuned right.
That said, you can make this one work reasonably well if you follow the demon tuning guide to the letter. The mighty demon is pretty tunable. Your car is light, so it will be more forgiving than a B body or the like.

Yes Demons can have "demons" but this is a great example of why. Most Demon owners buy the wrong series, and size of demon.

A guy is running a 750 eddy or older holley, and looking for an upgrade. See's the features of the Demon(billet construction, quick change features, std metering blocks on both ends, idle ease screw, larger float bowls, yadda, yadda, yadda) Sounds good. So what size? Well running a 750 now, want more right? Get's the 850. Reality is, that on the same flow bench, a 750 demon will flow more than the eddy or holley. So right off the bat he's got too big of a carb, and when there as many series of carb as demon offers, he get's the wrong series too.

Easy mistake to make. Good news is:
1) that I think you will be able to tune it out. At least you're not trying to run an 850 on an SB chevy.
2)If you can't tune it out, those carbs have extremely good resale on ebay. Sell it off and get the right one. The speeder is cheaper, so you might even be able to pocket some cash.

[ehostler]

Originally Posted by fastmopars .inc

you could try recurving your distributor but i would sway away from this if you don't know what your doing. you could put too much advance in it and it will start to pop and run really hot.

A unilite is one of the easiest distributors to set the advancement on. As long as he is using a dial back to 0 light to determine where he is now, he can determine how much advance you needs to add/subtract. Then it's just a simple adjustment, using the Mallory Degree Key, to set the advance to where it needs to be. Then hit it again with the dial back to 0 timing light, to confirm the max advance hasn't gone beyond 36º BTDC

[ehostler]

I will admit that Don/FBO Systems may be a little more than other places; however, you get what you pay for. You get the correct carb for your combo, it's properly jetted and course tuned for you, and then you gets all of the post sales tech support that you will ever need. Don has been known to provide tech support, from the pits, between rounds at the track.

[ehostler]

Even after reviewing the info on the BG/Demon website, I feel that the 750 would have been a better choice. Most likely a Mighty Demon. I say that as my 440 is mostly stock and Don told me that it is borderline between needing a 750 Speed Demon and needing a 750 Mighty Demon. With lots of tuning, the 850 can be made to work.

I have read alot of that in the mags. They (unknowingly) buy the wrong Demon and spend days tuning it, before it will work right. Had they just bought the correct Demon to begin with, most of them would have spent less than an hour in tuning.

Your cam specs also appear to be misleading. What is the actual duration at .050? I think that you've listed the advertised duration.

[dkn1997]

Originally Posted by dave571

2)If you can't tune it out, those carbs have extremely good resale on ebay. Sell it off and get the right one.

that's the best advice given so far. I was given some erronious advice for my mild 440 by someone (not don) I was talked into an 850 speed demon. Car never ran right, even after I sent the bowls out to don for him to check. I sold it on ebay, got most of my money back, and bought a 750 mighty from don. without touching it (cam change coming soon) car ran worlds better.
In the end, I was only out about 100 bucks, so it was not the end of the world.

[70Barracuda]

Boy, it seems clear Don's the guy to deal with! BTW, I have a 750 Speed Demon "that Don will go thhrough" for sale.

[plydus340]

hey guys,...
I still haven't touched the duster this week. I have been doing overtime this week. Anyway, some great replys here. First I will set the fuel levels to half way in the sight glass, change the plugs, check the wires, make sure their not burnt, make sure the batterys not down, ect... .
I need to tell you guys that the pistons are gas ported. I'm not sure, but I heard they use more oil when you do that. Is that true? Now yall know this car is very fast when it runs right, right? I plan running test and tune this weekend.
The heads have been mildly worked. The edges were rounded by the builder. I don't know " advertized duration" is? I have the cam card, somewhere... .
Thanks for all the replys. I'll let you know how she runs this weekend!

[ehostler]

The fuel level should be at the bottom of the sight glass, not the middle of the sight glass.

[dave571]

Originally Posted by plydus340

.... First I will set the fuel levels to half way in the sight glass, change the plugs, check the wires, make sure their not burnt, make sure the batterys not down, ect... .
..... I plan running test and tune this weekend....
Thanks for all the replys. I'll let you know how she runs this weekend!

I look forward to hearing the results.

While you are doing the plugs, ditch the bosch platinums.

The Electrode is smaller than on conventional plug. Very bad combo on a cammed up performance ride.

You'll get 200 opinions on what conventional plug is better, but I'd go with an NGK.

[plydus340]

Guys, it was the charging system. Started it up yesterday, drove it a bit. Took it to my buddys garage to work on it. The fuel levels were at the bottom of the sight glass. Checked the battery, and it was a little low. Drove it a while to get it charged up. Went to the hiway to try it out, it ran like it was supposed to. Go figure. Mopar, heal thyself. Well, the bad news is its raining. No test and tune today. I should have checked the battery first. It's a wal-mart and has been replaced once already in 6 months. I still wonder how the fuel levels changed themselves. Anyway, thanks for the input fellas. I'll let you know what it runs when I run it...

[pishta]

My Demon had a constant problem with flooding, turns out the O-ring that seals the needle seat was a Holley piece (im guessing) because it sheared off a slice when it was assembled ans was causing the needle not to seat. It would seat sometimes, but not others. Very frustrating until you know exactly what it is. As for float settings, there are 3 lines cast into the bowl, the lower one is used for street applications, I would baseline the level to the middle line, NOT just visible at the bottom like a 3310 holley. It has a much smaller sight plug diameter that would show the same level as the larger sight glass at the first line, get it?