Demon Carb still rich
Thread Starter
Racer
Joined: May 2002
Posts: 280
Likes: 0
From: Poughkeepsie New York
Demon Carb still rich
Have followed all the baseline adjustments as outlined by BG and Lars.
Carb still rich. As I tweak the mixture screws (turning in) car will stall.
It is running very strong, but its rich at idle. RPM=850, floats btm line,
ck'd butterflies, both @ .020 before tweaking. I dont understand why its still rich? As I lean it out, I'm getting an off idle stumble. Any suggestions. Wish I was in Chicago, that sounds like a good time!
Carb still rich. As I tweak the mixture screws (turning in) car will stall.
It is running very strong, but its rich at idle. RPM=850, floats btm line,
ck'd butterflies, both @ .020 before tweaking. I dont understand why its still rich? As I lean it out, I'm getting an off idle stumble. Any suggestions. Wish I was in Chicago, that sounds like a good time!
Safety Car
Joined: Jul 2002
Posts: 3,770
Likes: 3
From: St Louis MO
If the BGs work like the Holley's, which I'm fairly confident they do, the idle mixture screws do not change the emulsion characteristics of the idle mixture, just the volume of the mix that reaches the cylinders. The emulsion is the mix of gas to air in the mixture. To make your idle mixture (emulsion) leaner, you need to add more air or subtract some fuel. Most times people subtract fuel by adding a restriction in the fuel idle circuit in the carb. If you're lucky, this can be accomplished by changing and idle feed restrictor in the metering block. If you're unlucky, you have no idle feed restrictors but all is not lost. You can add a restricition in the idle circuit by using small small small wires in an idle passage. This will cut down on the amount of fuel in the emulsion, thus leaning out your idle. Simple, huh?
The reason you get the stumble is because the idle mix screws are too far in, not letting enough mix get to the cylinders, creating a lean stumble.
Brett
The reason you get the stumble is because the idle mix screws are too far in, not letting enough mix get to the cylinders, creating a lean stumble.
Brett
Racer
Joined: Feb 2005
Posts: 258
Likes: 2
From: Boise ID
???
the idle mixture screws do not change the emulsion characteristics of the idle mixture, just the volume of the mix that reaches the cylinders. The emulsion is the mix of gas to air in the mixture. To make your idle mixture (emulsion) leaner, you need to add more air or subtract some fuel.
Safety Car
Joined: Jul 2002
Posts: 3,770
Likes: 3
From: St Louis MO
Originally Posted by danno12345
I thought all the air/fuel mixing was done in the barrels... Where does the carb get air into the idle circuit before the idle mixture screws??
Brett
Racer
Joined: Feb 2005
Posts: 258
Likes: 2
From: Boise ID
Originally Posted by Brettmc
Idle Air Bleeds....emulsion air for the main circuit comes through the High Speed Bleeds. You can tune the idle circuit by replacing the air bleeds but they'll have to be the removable type...unless of course you want to drill them to size...
Brett
Brett
Ahh. I'm gonna have to do a little more studying. I've got an off part-throttle lean stumble, but I think I need to play with the accel pump cam.
Thanks!
Dan
Pro
Joined: Sep 2002
Posts: 690
Likes: 1
From: near Memphis TN
Originally Posted by conway62
What about downsizing the primary jets?
Reducing the fuel pressure?
Using the Idle Eze baseplate?
Reducing the fuel pressure?
Using the Idle Eze baseplate?
Fuel pressure - doesn't effect fuel mix (unless his fuel pressure is so high it is overriding the needles.
Idle Eze baseplate - ? Must be a Demon carb thing, never heard of it.
If he is rich at idle he may have several problems (vacuum leak, idle circuit to large, primary throttle open to far at idle, accelorator pump opening at idle, float too high, float leaking, power valve opening, bad carb) and there are several solutions (find vacuum leak, put wire in idle circuit to make narrower, drill idle air bleed, close primary throttle blade, open secondary throttle blade, drill holes in throttle, adjust float level, change power valve, change carb).
Edit - It sounds like you may want to lean out the idle with the idle mix screws and then play with the accelorator pump to get rid of the stumble. That's assuming nothing else is wrong with the carb to cause the rich condition.
Last edited by SpyderD; May 6, 2005 at 02:51 PM.
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Racer
Joined: Jun 2004
Posts: 423
Likes: 0
From: Okiehoma
Here is what the speed shop where I bought my Demon carb from told me to do. Pull the bowls off, take the wire twisties that come with a loaf of bread, pull the wire out of the twisties and stick a piece of wire in each circuit, bend over one end to keep in place, reinstall bowls and adjust. I can adjust mine with a vaccuum gauge to pull 18-19 psi. The carb idles great, runs great, by far the best carb I have owned. Not saying this will work for everyone, but sure worked for me.
Le Mans Master
Joined: Oct 2000
Posts: 6,563
Likes: 2
Same exact thing happened to me with a Demon on my 502. After f*cking with it non-stop for weeks (including jets, power valves, and wires in the idle circuits), I yanked the sob off.
It ran way too rich, and stumbled badly at off-idle...........Expensive POS, IMHO.
just my own opinion.......
It ran way too rich, and stumbled badly at off-idle...........Expensive POS, IMHO.
just my own opinion.......
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
interesting...sounds like you have covered most bases....your set up is not that different then mine and i do not have these problems.....have you gone through all the ignition timing adjustment and are you running ported or direct vacuum to the vacum cannister? if you are running ported try going to direct ...this should pick up your idle RPM and maybe then you can adjust both transition slots on the butter flies down some to reduce idle and perhaps this will let you run the idle mixtures out a little to increas a/f mixture...if not come and watch Lars in NJ in the fall he will be tuning my 650 speed demon
Burning Brakes
Joined: Apr 2004
Posts: 847
Likes: 0
From: Des Allemands LA
What size carb. engine jets, power valve cam stall all of that do you have? Kind of hard to make any realistic suggestions with out knowing what you got. I just put a 750 mighty demon on my 355, cam has 255 deg. duration @.050 and .565 int /.575 exh lift. I had to screw idle screws in all the way and then backed them out about 3/4 of one turn this all happened slowly as I checked/adjusted timing and idle speed after adjustment. This along with about a quarter turn of the idle-ez absolutely woke my motor up. I had a Holley 750 dp on before that I constantly fiddle-farted with changing acc. cams, squirters, jets, float levels, jet extensions and even considered sending it to the new Pope to have it blessed but could never get all the off idle hesitation out of it.
Instructor
Joined: Nov 2004
Posts: 194
Likes: 0
From: Appleton Wi.
Now...my new speed demon has the problem of...off-idle hesitation, but runs rich throughout the rest of the fuel curve. I have an A/F guage hooked-up. The guage tells me that as soon as I start to clutch, it goes lean. Then everywhere else it is rich. When I adjust the mixture screws so most of the stumble goes away, then it starts to load-up at idle, and wants to die. From what this thread is telling me, I have to richen the idle circuit by putting in smaller air bleeds? Then lean out the primary and secondary jets? Am I on the right track? Please help!!!
TJ
TJ
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
Originally Posted by TJ76
Now...my new speed demon has the problem of...off-idle hesitation, but runs rich throughout the rest of the fuel curve. I have an A/F guage hooked-up. The guage tells me that as soon as I start to clutch, it goes lean. Then everywhere else it is rich. When I adjust the mixture screws so most of the stumble goes away, then it starts to load-up at idle, and wants to die. From what this thread is telling me, I have to richen the idle circuit by putting in smaller air bleeds? Then lean out the primary and secondary jets? Am I on the right track? Please help!!!
TJ
TJ
Thread Starter
Racer
Joined: May 2002
Posts: 280
Likes: 0
From: Poughkeepsie New York
Running zz4 with LT4 hot cam. Speed demon 650 with mech secondaries. Now the stumble is much less, opened mixture screws, but still rich. will try playing with primary and secondary throttle adjustments. (thought they should be adjusted exactly same with baseline @ .020.), but will try primary throttle adjustment first....seems to be less sensitive than secondaries when adjusting for idle. Thanks.
Drifting
Joined: Sep 2004
Posts: 1,959
Likes: 1
From: North Bend Ohio
[QUOTE=TJ76]Now...my new speed demon has the problem of...off-idle hesitation, but runs rich throughout the rest of the fuel curve. I have an A/F guage hooked-up. The guage tells me that as soon as I start to clutch, it goes lean. Then everywhere else it is rich. When I adjust the mixture screws so most of the stumble goes away, then it starts to load-up at idle, and wants to die. From what this thread is telling me, I have to richen the idle circuit by putting in smaller air bleeds? Then lean out the primary and secondary jets? Am I on the right track? Please help!!!
TJ[/QUOT
Tip in hesitation is an accelerator pump problem. You're not going to cure it with the idle system. Adjust the accelerator pump so that any throttle movement moves the pump. Reset the idle screws to the best idle you can get. Make sure the timing is correct (between 12 and 18 degrees initial advance). Make sure the air bleeds are clean. Do you have a book on Holley carburetors? If not now is a good time to get one. Check the vendors.
BigBlockk
Later.....
TJ[/QUOT
Tip in hesitation is an accelerator pump problem. You're not going to cure it with the idle system. Adjust the accelerator pump so that any throttle movement moves the pump. Reset the idle screws to the best idle you can get. Make sure the timing is correct (between 12 and 18 degrees initial advance). Make sure the air bleeds are clean. Do you have a book on Holley carburetors? If not now is a good time to get one. Check the vendors.
BigBlockk
Later.....
Safety Car
Joined: Jul 2002
Posts: 3,770
Likes: 3
From: St Louis MO
Usually an off-idle stumble is cured by pump shot occurring sooner. However, if you move the throttle just a hair, not enough to move the pump arm enough to shoot any significant amount of fuel, you should get no stumbling. To me this is the real test of stumble-less part-throttle operation. At this point, with a barely moved throttle and no significant pump shot, the carb should deliver fuel through the transfer slots...enough fuel for the engine not to stumble. It takes careful adjustment of the primary and secondary throttle plate screws for this to happen. With that said, what is occurring is an interaction of 2 of the carb circuits. In total, there are 5 circuits. You HAVE TO know how these circuits interact to effectively tune a carb. That "get a Holley book" comment up there is a dayum good piece of advice because you can't just make a change in one circuit without knowing how or if it will affect another circuit. Here's a start:
http://search.barnesandnoble.com/boo...84089143&itm=4
http://search.barnesandnoble.com/boo...84089283&itm=4
Both of those are excellent choices. They taught me alot...
Brett
http://search.barnesandnoble.com/boo...84089143&itm=4
http://search.barnesandnoble.com/boo...84089283&itm=4
Both of those are excellent choices. They taught me alot...
Brett
