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1969 Nova SS502
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131 Posts
Discussion Starter · #1 ·
Here is the set up:
502 with Hyd. roller comp cam (Big Mutha thumper) AFR 305 heads, Mighty Demon 850 with 3.5 power valve and jetted at 80-83,Air Gap intake, MSD 6AL, coil and dist, the plugs are gapped at 50. The inital timing is set at 15 and vacuum at idle is 8-9.
The problem is that the motor is very rich, plugs are black and when I first start the motor it will hardly run and pops through the exahust if it has set for several days. The motor clears up after I run it around 2000 rpm for a few minutes. When I drive the car it smokes and really pours the smoke when I open it up, very rich smoke. Jetted down 3 times so far. I pulled the carb and replaced the power valve that was a 7.5 with the new 3.5 and found no leaks. I also adjusted the butterflys to the correct opening. The floats are set just below the center mark and this is where things get strange. After the car sits for a day or so and I look through the site glass I can see no fuel in the bowls, the float is below the window. I ran the car up to temp, pulled the carb with fuel in the bowls and set the carb on a stand and let is sit there for 24 hours. Today I go out and again the fuel level is below the site glass but no fuel leaked out of the carb onto the white paper I had below it. The fuel has to be going some where because when I fire the motor it is almost flooded with fuel which I think makes the backfire through the exhaust. Where is the fuel going??? I never had this kind of problem with a Holley and may have to ditch this new Demon.
Any ideas?
 

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NAVY VETERAN
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971 Posts
hmm maybe level is set too high
bowl vents evaporating the fuel.
is the level while running at 1/2 the sight glass?
I know holley carbs require just dribbling out the sight glass and secondary side is set 1/2 way up the sight glass.
remember that the demon is NOT a holley even though it looks similar.
so setting it up as a holley migh cause all sorts of problems.
 

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1969 Nova SS502
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131 Posts
Discussion Starter · #3 ·
With the motor running the fuel is just below the center of the site glass. There are 3 lines on the side of the carb next to the site glass and Demon says to set it in the middle line and mine is just a hair below the center line, but after it sits the fuel level drops out of site.
 

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Hotrodders.com Moderator
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2,783 Posts
carb

Way to big of a plug gap. That should be around .035".

I need to know what plug and heat range.

Does the distributor have a vacuum advance?? If so are you setting the timing with it un-hooked??

Does it have a electric choke on it???

Keith
 

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I had a very similar problem with mine. I could get it to run with changing the advance but would'nt idle, get it to idle but would'nt drive,,,, Talked to demon service and they took it back to check it out. Sent it back with the metering blocks replaced.
 

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1969 Nova SS502
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131 Posts
Discussion Starter · #6 ·
The plugs are NGK R5671A-9 and the reason I gapped them at .050 is because in the MSD installation guide they suggested the gap to be from 50 to 60 for compression up to 10.5. I do not have a vac. advance MSD and the carb does not have an electric choke.
 

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I had similar problems. I even sent my mighty demon back, and still same problems. I got my rich idle fixed by increasing initial timing to 26* and 36* total with vac. advance disconnected. With the vac. advance connected to manifold vac. it was at a steady 41* at idle. Also hooked up a pcv valve.

BTW. I'm running all MSD ignition on a solid roller 11:1 aluminum head 406.
 

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tune

redride said:
The plugs are NGK R5671A-9 and the reason I gapped them at .050 is because in the MSD installation guide they suggested the gap to be from 50 to 60 for compression up to 10.5. I do not have a vac. advance MSD and the carb does not have an electric choke.

You need to get the initial timing in the 18 to 20 range. Gap the plugs around .035" I know that MSD has something different in there paperwork, but it's one of those deals where there wrong.

Install a new set of plugs and try it out and let me know if it gets better.

Keith
 

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IMHO you answered your own question in the first post: ditch the Demon carb. After cleaning the junk out of the bowls, re-bushing the sloppy throttle shafts, jetting , re jetting, tuning on chassis dyno, running with and w/o vac advance, advancing timing, etc., I could not get my Demon to behave at idle or part throttle, always pig rich. Ran O.K. at WFO. Had a 4150 DP custom built by Pro Systems in Michigan.One of the few guys out there with a wet flow bench.I called them upon a friend's recommendation, gave them the particulars about my engine and drivetrain. In my case, for less than the cost of a factory 4150 HP Holley, I got custom metering blocks, drag floats, a wet flow calibration (much different than typical dry flow measurements) and a 24 hr tech line just in case you need help. They claim their settings to be accurate to within 1 jet size and send along a second set in case you need 'em. Best money I ever spent on a carburetor. Plugs run light tan, throttle response is immediate. Even though I changed to a cam with 14* more duration and higher lift my small block idles much better and 200 rpm lower than it ever did with the Demon. If you like continually messing with your carb, the Demon is your huckleberry. Otherwise my suggestion would be to give Pro Systems a call.
 

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Demon Carb Idle

My Road Demon had a way rich idle - until I shut down the "four corner" thing. Might work OK with an open plenum - no good with the 180° style manifold. Screwed down the rear idle mixture needles snug and closed the secondary throttle.

Also had unstable float setting - the rubber tipped needles were sticking due to either (1) alcohol or (2) bad gas. Things were good - then bad - then good - that quick - terrific pain. Trouble went away when I installed .130 steel needle and seat assemblies.

With those two items under control - it tuned OK
 

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1969 Nova SS502
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131 Posts
Discussion Starter · #12 ·
If I go up any more on the timing the starter gets heat soaked and is hard to start once warm. I know about shielding the starter but my point is with my old Holley (which I sold on Ebay) did not have that problem at the same timing setting. Guess this one will go on Ebay also and I will go back with Holley, I am tired of jacking with this carb!!!!!!! I will try the bringing up the timing and changing to new plugs and .35 gap before I give up but I do not have much faith in this carb at this point.
 

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WFO
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5,030 Posts
redride said:
I do not have much faith in this carb at this point.
As has been said earlier- Demon carbs are "different". I have no empirical experience w/them- some swear by them, others AT them.

There is a tech from Demon that frequents this forum. I would suggest giving it a couple days to see if he responds w/a recommendation. Or send the carb back to them.

It just seems like you will take a bath if you sell it w/o getting any use from it.

Good luck.
 

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Tuning my Speed Demon VE 650 has been time consuming, but I'm pleased with it. I've found mine to idle & perform best with the Idle Ease all the way closed, the idle screws out 1 1/8 turn. I've gone down 2 jet sizes all the way around. And mine needed the yellow holley accelerator pump cam with the original 30cc pump & .031 squirter, in order to prevent a stumble of idle

I had a .060" plug gap with the MSD 6a, but didn't notice a different when I went down to .040"

But it all just comes down to tuning.
 

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Technical Support Barry Grant
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1,500 Posts
k-star said:
You need to get the initial timing in the 18 to 20 range. Gap the plugs around .035" I know that MSD has something different in there paperwork, but it's one of those deals where there wrong.

Install a new set of plugs and try it out and let me know if it gets better.

Keith
To the original poster... this should be done first before you do anything else in the way of adjusting the carburetor.

Once this is done the jetting needs to be changed back closer to its original spread.

Unless you are at an altitude jetting will not clean up a rich idle and pulling jet out to try such will make it lean at wot.

Is this an annular or down leg booster carb?

I think you actually have a couple of different issues going on here so 1 fix will not cure them both. Correct the timing and plugs as Keith posted , change the jetting back and then readjust the mixture screws.

In regards to the float level mystery the floats should be set as mentioned so the level is half way in the windows. As far as what is happening when you shut it off and go back to restart it sounds like the fuel is percolating and boiling out into the plenum overnight and then when you go to start it all that fuel has to be burned. Try putting a heat shield under the carburetor. GM makes them for this very problem as well as a number of other aftermarket vendors.

Now.... if after making BOTH of these changes and it is still rich at idle then it may need the ifr's shrunk but lets not get too far out yet.
 

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Technical Support Barry Grant
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1,500 Posts
merced said:
I had a very similar problem with mine. I could get it to run with changing the advance but would'nt idle, get it to idle but would'nt drive,,,, Talked to demon service and they took it back to check it out. Sent it back with the metering blocks replaced.
If it came in with a rich complaint the ifr's were probably made smaller before it was returned to you which is standard practice.
 

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demon carb mystery

When I sent the carb back it took 6 weeks total. I wasn't pleased with the time but it's still a project at this stage. The good news was I put it on and filled the bowl ....Yeah cranked it a bunch... and it started ran and with only idle tuning. best of luck Ed
 

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1969 Nova SS502
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131 Posts
Discussion Starter · #18 ·
It has down leg boosters and today I put in the new plugs, set at 35 and timing is now set at 20 initial. I will give this a try when I get back from my trip and let you know and go from there. Are you saying to go back and jet it the way it was when it was blowing smoke because it was so rich?
 

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1969 Nova SS502
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131 Posts
Discussion Starter · #19 ·
Ok, now the timing at initial 20, gaping the plugs at 35, jetting the carb at 85 pri and 90 sec. adjusting the air fuel to get the best idle while warm and putting in the heat shield. Today when I went to start the car it was real hard to start and to keep running. It was acting like it was not getting enough fuel. After doing all this should I check my vac again to see if it changed at idle? The reason I ask this is because I changed the 7.5 power valve to a 3.5 because my vac. reading with the old timing was between 8-9.
 

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My Speed Demon was doing the hard to start spluttering smoke out the back,
being way to rich.
I had set it up according to the manual that came with the carb, but then I came across a printout I made of this, Addendum to Speed Demon & Mighty Demon Instruction manuals.....
The carb manual states the rear butterflies should be set the same as the fronts uncovering the transfer slot.
In the addendum it states for engines that idle below 1000rpm the secondary butterflies should be set to the bottom of the transfer slot.
It should not be visible from below, only being exposed with any movement of the butterflies.
 
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