zupmanZ28

I just got the demon 750 (mech sec), and it is rich, all the time. On the highway just cruising my A/F is almost all the way rich. At idle it's rich, and at WOT it's still rich. I think i need to jet down the primaries and see where that gets me, but i'm new to the whole carb tuning thing so i'm very open to suggestions.

The only tuning i've done so far is to set the idle air screws, and the float level, as per the instructions. Fuel pressure is between 7 and 8 psi (could be the prob?). I've got a FP regulator so i can tone it down if need be.

Also, i have noticed a slight bog at times when i floor it at low rpms (below 2500). When this happens the A/F shoots way lean. What is a possible solution for this?

the holley jet kits work with the demon carbs right? I'm gonna go pick one of those up today.

thanks for the help
Eric

Jester

Well offhand I'd say it's time to actually start tuning it a little bit. You should be ok with 7-8 psi..but turn it down to about 5 for now to be sure thats not affecting things.

The bog off idle is a result of your accelerator pump needing tuning. And yes, Holley jets will work.

onebad82z

iTrader: (7)

You need to tune the accelerator pump circuit on the Demons.Beleive me I had the exact same problem when I first got my motor running with my Demon 750DP.The pink pump cam is nowhere near agressive enough for a healthy street motor..at least on the Demon anyway.Try the orange pump cam that comes in the Holley cam kit..and yes as Jester said the Holleys jets do work..I bought the master kit that has from 64-99 in it for $30..well worth it.If that doesnt entirely solve it move up in squirter size...I am sitting at a 42 primary squirter right now..the bog is gone but the initial rev is slow/lazy..best way I can describe it.Convential holley numbers are no basis for a Demon..I have tried squirter sizes that usually run mint on a Holley but are nowhere near optimal for the Demon...at least for me anyway.My motor seems to love a ton of fuel right off idle...but even my jetting is 76/83 and still lean a tad...whereas with a Holley I was running 68/75.I don't know what that is all about but that is my experiences to date.

onebad82z

iTrader: (7)

Did you put a power valve in as per Demon instructions?If it is too small from Demon (6.5) and opening she will go rich at all times also.I know when I finally swapped mine for the correct one about 90% of my driveability issues were solved.That is another area to tune that helps out tremendously.

zupmanZ28

as of right now the carb is pretty much at out of the box tuning. That master kit thing was the one i was planning on buying, but where can i get the acc pump cams? They don't come with the kits do they? They sell the jet kits right at Advanced, but i don't think they have cam kits. I may be wrong though...

thanks for the help guys

Eric

onebad82z

iTrader: (7)

Accelerator pump cam kit is a seperate kit from the master jet kit.If Advance can get you the jet kit they should be able to order you up the pump cam kit also..ask.It will run you around $15-17.

zupmanZ28

I got a jet kit today, and took the front bowl off the carb. it had 76 jets in it, so i went down to 72's, and it leaned it out a little bit. It seems to be running better but it's hard to tell cause i couldn't drive it (rain).

Then something happened. I snapped the throttle all the way open from idle and it backfired. bad. Fire came up through the secondaries. i shut the car off and the venturies stayed ignited until i blew them out. What in the hell caused that to happen? I'm going to Advance again tomorrow for the cam kit, don't by chance have part number for that do you? They are kinda slow over there and i don't really wanna be sitting there for a half hour while they try to figure it out...

Thanks

Eric

onebad82z

iTrader: (7)

Pump cam assortment kit #510-20-12...$19 in Jeg's.I have had carbs backfire..but never with flames that kept burning..have had the ball of flames..but they went out right away.Sounds like there was quite a bit of raw fuel during the backfire that ignited..too high of float levels is a guess?

zupmanZ28

hmmm. float level, didn't even think to check... i'll take a look in the AM....

thanks for the PN...

Eric

fazool

I use a speed demon 750 DP on a big block chevelle. It's been a bitch of time getting it to run correctly for me. What helped the most was an article in Car Craft written by Barry Grant himself on how to tune the carb AND how to setup the car to work with it. Gotta say, he knows what he's talking about. See if you can get your hands on the article - it may even be available from CarCraft.com, but I'm not sure.

Hey, that backfire thing sound suspicious. It could be your timing too. If you have things way to advanced or maybe the spark plug wires are swapped? Just guessing.

BTW, barry grant said the best thing you can do is to run as much initial timing as possible - and adjust the timing curve appropriately. Worked great on my big block.

Some other tips for the Demon:
1. check the float levels. Too high, and the car goes rich (fuel spills into the venturi and no amount of tuning the idle screws will help).
2. Make sure the idle transfer slot is not completely uncovered (throttle blades open too much at idle).
3. When changing jets, you should only move 2 numbers at a time. It's sensitive the change. 76 to 72 is a big jump.
4. Are the air fuel adjustment screws completely turned in (too rich)?

good luck

zupmanZ28

thanks for the info on that Car craft article. To answer your questions;

I had to eyeball the timing, because my timing light is still at my buddies house, so it could be off, but i don't think so. i normally eyeball it, then check it with the light and i'm seldom off more than a degree. As for the wires being mixed up, again it's possible, but i doubt that it's either. i'll get the timing light back today and check both.

The idle transfer slot looks like a small square, as is showed in the instruction manual.

The idle screws are turned roughly 1 3/4 turn out

I think my problem may just be in the float level, i still have not rechecked that, and it seems that a lot of my problems could be related.

I know about the jet increments, but you don't understadn how rich this thing was. It seemed to be a little better with the 72 jets.

I wanna buy a Distributer curve kit so i can play with that. Right now i get full advance around 2500 rpm (i think, not positive though).

Tomorrow i plan on getting into this thing hard, cause i'm takin it to the track that night...

thanks again..

Eric

onebad82z

iTrader: (7)

Great heads up on that Car Craft article fazool..that article has helped me more than enough times when setting up my Demon.I forgot all about it for reference..if anyone thinks it is a good idea I can try to scan the article and post it here for others to use.Let me know..that one is always on the reading block.Also I highly recommend the "how to win with your Demon" by Ray Bohacz.That book is another great source of info on the Demon line..got mine from Amazon.com for $15.Read it 5-6 times now and still pick up little bits of new info every time I read it.

zupmanZ28

i just checked carcraft.com but couldn't find anything. bad82z, would you mind either posting it or sending it to me?

Little update, the secondary float level was high, so i adjusted it down, and i dropped the fuel pressure to 7 psi. It seems to be running smoother, MUCH smoother, but i still get the bog, assuming from the accelerator pump. I'm going today on my way to work to pick up a cam kit.


As of right now i've got another problem though, either tranny fluid or oil is getting on the exhuast and smoking... Not good. I'm gonna change both tomorrow and look for leaks. If its oil though its most likely the rear main, cause it's not leakin out the Dist hole, or the back of the intake. Guess it could be valve covers.... ehhh it never ends...

fazool

Yeah, it's been a while since car craft posted an article that comprehensive and usefull. I wish they did more of them. Actually, that whole issue of the magazine was primo!

Back to the demon - Glad to hear that the float solved most of your problems. You don't need a ton of fuel pressure - maybe 5 psi or so, I forget the recommendation. If it's too high, you blow right past the needle and seat.

As for that bog - I have the same problem - sort of. Mine is a studder in the acceleration from "off idle". It got significantly better by running more initial timing and slowing down the overall timing curve (would you believe I still run dual-points?). If your problem is the same as mine, then yes changing to a more agressive cam should help. You could also increase the shooter size or go to a larger accelerator pump.

onebad82z

iTrader: (7)

Demon recommends between 6.5-7psi.I run 6 currently.I will work on trying to scan the article in tonight for everyone.On the accelerator pump circuit..I have gone as large as a 42 squirter to kill the bog along with the orange pump cam.Still a little lazy on the initial rev but no bog finally!Next step I am swapping for a 50cc kit.Again I stress the power valve..make sure you have the right one for your combo.Swapping the stock one for the correct one for my motor (9.5" PV) made the biggest tuning change I have done.Driveability improved 100% all throughout the range.

zupmanZ28

6 psi huh? I wonder what dropping another # would do for me. The car is running absolutely beautiful right now with exception of the low RPM bog. A/F is right where it should be and everything.

I even found my fluid leak, it was just a loose screw on the valve covers...

good day today...

Eric

onebad82z

iTrader: (7)

Hey guys...I tried scanning that article and no good.There is color blocks around the text on the pages that my scanner can't make clear enough to read the text within them.It is Car Craft issue October 2001.I emailed the mods to see how to go about posting the article on the magazine articles section of the board for all to use.Don't want to get in trouble for posting it without permission..I owe enough money