mighty demon runs rich on passenger side

madmaxx · #1 (**permalink**) 03-01-2014, 12:59 PM

Not a carb expert. Have both butterflies with only .020" showing. The passenger side exhaust is noticeable richer than driver. All 4 idle mixture screws adjusted the same. Other symptom is car idles at 1050 and over 3 minutes pulls back to 800 rpm. Other than that it runs like a champ. any suggestions appreciated.

i think its a mighty demon, it has the idle eaze on the air cleaner hold down but it doesn't appear to do much when i adjust. air bleeds have been blown through with gum out.

Pman1961 · #2 (**permalink**) 03-01-2014, 01:15 PM

Definitely need more specifics on the engine as is this a new setup or existing that has developed this issue. My engine with an aggressive cam would pull back almost exactly as yours. Turned out my engine was idling in advance. When the fans kicked on the the rpm would drop to initial advance sending the rpms lower. My solution was to take some of the curve out of the distributor and ultimately mine did best when I locked it out. As far as the air/fuel idle mixture I've seen many methods for adjustment. My engine performs best if the mixture screws are adjusted individually and not all the same. You have to alternate from side to side several times until you hit the spot that works best. You could start by leaning the rich bank and richening the opposite bank for balance. But again...we need more specifics.

blown871 · #3 (**permalink**) 03-01-2014, 01:16 PM

The Demon is not a bad carb... but the first thing I did is reach out to Patrick at Pro Solutions.
Took the new carb out of the box, adjusted the idle. CHANGED the car for the better.
Love the new carb.

Pro Systems - The Worlds Fastest Carburetors

olddog · #4 (**permalink**) 03-01-2014, 02:01 PM

One bank rich, I would look for avacuum leak on the lean bank. Especially if it is a dual plane intake.

madmaxx · #5 (**permalink**) 03-02-2014, 08:57 AM

wow fantastic suggestions!!!!!!!!!!!!! its a demon 850? on a roush 427R. It has the vic jr style single plane intake. Interesting note on distributor advance. My timing is set at 32 max at 4k rpm, if i remember i think it 16 at "idle" What is idle rpm? 800, 1000??

I have never rebuilt the carb, is it possible its dirty? History of the car is I owned for about 700 miles. I don't mind the rpm pulling back at idle just don't want to run the passenger side rich.

In the end the only carb adjustment which has a significant effect is the butterflies. I seam to have a lot of adjustment with the mixture screws with no change in performance. Manifold was removed and reinstalled with new gaskets, new carb gaskets, phenolic spacer installed to prevent gas from boiling out of carb. It exhibited the aforementioned symptoms before the manifold was removed and after reinstalled. manifold was removed for polishing.

Car runs like a beast, it is the fastest I have ever had. I have two others with Roush 427sr. The "R" street manners at idle are a little rougher but digestible. The payback above 3500 rpm is much greater!!!

My last one had a holley 4150 and was for the most part flawless as far as idle quality, mixture etc. I never touched it.

Quote:

Originally Posted by olddog

One bank rich, I would look for avacuum leak on the lean bank. Especially if it is a dual plane intake.

Chilibit · #6 (**permalink**) 03-03-2014, 05:28 AM

If you are judging that one side is rich by the side pipe---perhaps it would be possible to measure the exhaust header temps on that side to see if they are consistent. If they are then I like the vacuum leak approach. If not, then perhaps it is something else.

madmaxx · #7 (**permalink**) 03-03-2014, 11:07 AM

Not many places for a vacuum leak and could not find any. Intake gaskets replaced and it was doing it before and after the replacement. Not really concerned about it, just messing around.
Probably adjust the idle mixture screws and see what occurs. With a single plane all plenums are open to the heads at the same time so for some reason more fuel likes to go to the passenger side.

madmaxx · #8 (**permalink**) 03-03-2014, 11:09 AM

I guess butterfly adjustment is irrelevant. the primary side of the carb is on the same shaft so its not like one could be closed and the other open. I read the pull back in idle is a result of a rich condition. I get my vacuum gauge out and see what I can come up with on the mixture screws.

Chilibit · #9 (**permalink**) 03-03-2014, 11:24 AM

Incomplete/inefficient combustion would mimic an overly rich mixture. What do the plugs look like?

madmaxx · #10 (**permalink**) 03-03-2014, 02:15 PM

Talked to Demon today. He suggested to check vacuum at idle and determine steady value. Divide number by 2 and check power valve size. Due to my cam profile he feels the power valve may be letting fuel in at idle. Simple enough to check and a power valve is like $10.00.