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Old 05-31-2010, 03:39 AM
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78 350 runs rich

trying to tune this stock 1978 350.....i thought it was the carburetor that was the problem. the engine was running rich so i thought a new demon 625 carburetor would help. replaced the cap and rotor. coil and wires. intake is performer edelbrock.

i put a new timing chain on it and set it at anywhere from 4 to 10 advanced with vacuum advance unhooked. fuel pressure is 5 to 7 and carb floats are centered in the sight glass. power valve was checked and is working well. engine seems to have power at higher rpm but just wont idle. when hitting throttle engine rpm raises fine and vaccum drops. when going back to idle vacuum hits 20 then drops to 15 then fluctuates at 18 and 20.

the vacuum gauge fluctuates between 18 and 20 dipping down to 15 and raising back up to 18 and so on. engine runs rich. has 4 corner idle adjustment. i tried to do all 4 but ended up closing off the back and trying to get it with just the front. right now it likes to at least run the best at 1 turn out on each side. engine is missing but not a solid miss and i cant hear where. there is a bad sticking lifter but this was going on before that started. motor may be worn out. compression check was only done on 2 cylinders so far and was at 120psi. what are some possibilities.

when dropping down into gear motor likes to stall unless idle is set high. plugs are new and have been cleaned 2 times due to the rich mixture. this engine just does not want to run right.

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Old 05-31-2010, 04:43 AM
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Well, it seems like your carb is obvsiously causing a secondary ignition problem. The miss is likely a combo of overly rich idle, and fouled plugs...
Holley style carb normally recommend the floats be set with gas sitting right at the bottom of the sights; NOT half way up, that is a good start for you. Too high float levels will certainly cause an overall rich condition.
Also, demon carbs are known for running rich, likely you will have to adjust the jets; however I would start with the idle circuit.

So you had the stock carb on a stock motor and it suddenly started running rich? what did you change? did you rebuild anything besides the new manifold?

I believe the timing should be set at 8*BTDC
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Old 05-31-2010, 09:35 AM
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Not a big fan of re-using fouled plugs that have been cleaned. I would try a new set of R45TS AC's in there.
Being your lifter tick was going on before the problem started, the problem might be a result of that. (burnt valve, carbon build up on the valve and seat)
Are you sure you used the right gasket on the intake? Your vacuum readings are all over the place. Probably caused by rich condition, vacuum leak, poor valve timing. Hook your advance hose up to full time vacuum, lower your idle speed, and re-set your A/F ratio to best highest reading and no farther. 10* is ok on a stock 350. I have mine on 10 and it runs fine.
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Old 05-31-2010, 04:01 PM
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def the carb, the floats are prb too high, set the floats to where gas baerly trickles out of the site plug!! and if i remember rite if it has the 4 corner A/F screws, all 4 has to be set 4 it to run rite as well...??
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Old 06-01-2010, 02:31 PM
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Is your Vac Advance directly to the carb?
I ran into a similar issue when I had the vacuum advance line teed with others and not directly to the carb.
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Old 06-02-2010, 12:55 PM
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Advance vac is hooked direct to distributor. I will set the floats at the very bottom of the site glass. The demon set up guide said they should be half way up the sight glass window. I remember the holleys used to just flow out. Thank you that might be the problem
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Old 06-02-2010, 09:13 PM
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I read somewhere that there are multiple lines on the site glass of the BG; but that for street setups, you set the fuel at the bottom one.... Give it a try, adjusting floats has got to be the easiest adjustment on a center hung float carb
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Old 06-04-2010, 12:12 AM
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adjusted the floats at the bottom so far it still runs rich and wants to die at idle. i have to turn the idle adjustment screw up to keep it running. havnt replaced or cleaned the plugs yet. that could be the another issue. screws are turned out 1 full turn a piece for all 4 corners. will try that and go from there. i did manage to set the engine on fire adjusting the back float.
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Old 06-04-2010, 03:54 AM
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HOLY JESUS! Sorry about the fire dude.... So did turning the idle screws out give you a steady idle? I know you said it wanted to die, but then said you turned the screws out 1 turn... that fixed that?
Next, its still running very rich you say? I know you said that the power valve was checked and still working well what size is it? at what level of vacuum does it kick in?
If you were able to stablize the idle, how are the vacuum readings? are they steady now?
B4 you keep workin on this carb, you wana check to see if you have vacuum leaks (though, I believe that usually causes a lean condition...)
Did you set your timing to 10* advanced initiail without the vacuum advance hooked up and then reconnect the vacuum advance?

Sorry about your fire again, hope nothing got too burned up..
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Old 06-05-2010, 09:45 PM
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Setting the floats that low made the engine not want to run at all. In upper rpm its fine. Power valve checked out good. Vacuum hasn't changed
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Old 06-06-2010, 01:48 AM
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maybe the power valve is too big from the factory. it is a 6.5......also what do you know about the idle ease set up on a demon carburetor. looking at other forums i see the idle circuit is horrible on these carbs. no one seems to come up with much of a solution in all of the threads. some people even say go down in jet size. i think mine is 78/70
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Old 06-06-2010, 04:01 AM
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Okey Dokey.... Let's get into this:

The reason most people don't have luck with this idle circuit is because it's likely most people don't know how to adjust the idle circuit in a Holley style carburetor; other than idle screw adjustments and correct transfer slot exposure.

First thing for you to check: How much is your transfer slot exposed? You want it to look like a square which should be .020" exposure under the butterfly. Make sure this is how it looks on both pri and sec. side of carb.
Next, you have a Speed Demon with the new Idle-Eeze setup. What this does is allow more air/ or less air into the carb without having to adjust the fast idle screw which moves the butterflies from their optimal position/transfer slot exposure.

Judging by what you've said about adjusting your floats, your experiencing a lean idle. You can try to reduce the air coming into the motor by screwing in the idle-eaze and see if that helps.... I doubt it. But actually, since this is a stock motor.... maybe it will. give it a try!

Your power Vavle is undoubtly the right size. With a stock motor pulling upwards of 15-18" of vacuum, a power valve coming in at 6.5" if anything would come in too late, but that's not really true bc ur running rich


Now that all that's out of the way, and DO TRY THAT FIRST!!!, next I will explain how an idle circuit it truly adjusted: with idle air bleeds (IAB), and idle feed restrictions (IFR).
CAUTION!!!! Small changes make big differences. But, luckily you have a Speed Demon which has adjustable IAB (the rest of us with cheap Holleys have to drill our's out and pray to God!! lol). They're sort of like jets for the idle circuit.... but not really.

IAB's (located behind the boosters on top of the carb: little brass holes), when you shrink them: the circuit gets richer, and responds more quickly.
When you make them bigger: the circuit get leaner and responds more slowly.

IFR's, ( which are in the metering block) are exactly like jets.
When you make them bigger the idle & transfer circuit get richer.
When you make them smaller, the idle & transfer circuit go leaner.

Look both these things up extensively b4 adjusting and also write down every adjustment b/c this gets confusing.


This would be less difficult if you would have bought a road demon for a more stock setup. Ordinarily, I would think you needed bigger air bleeds (since you have a small cam with high signal), but if you have an already lean mixture, perhaps you need to go the other way.


I CAN NOT STRESS THIS ENOUGH, do not attempt any IAB or IFR adjustments until after you have checked thoroughly the initial adjustments I mentioned.

By the way, you said it's running fine in the upper RPMs? does that mean no more black smoke? Could be a good start... just need to fix the idle circuit and you could be all good

Last edited by dumbmechanic; 06-06-2010 at 04:06 AM. Reason: typo
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Old 06-06-2010, 12:11 PM
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i have a road demon, dont think it has the idle eze
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Old 06-06-2010, 12:18 PM
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HA! they don't call me dumbmechanic for no reason... lol Yea you don't have it. Take out the air cleaner hold-down stud on the carb (in the middle) and look down it: if you see a screw in there, you might just have it... If not, nope! you ain't got it.
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Old 06-09-2010, 03:00 PM
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Lets clear up some bad information first..... the Road Demon does not have an idle-eze so no need to look for it and as the op stated our recommended float setting is half way in the window.

Next.. is the base timing at 4 or is it at 10? That carburetor calls for it to be in the 10-12 range and if it is swinging 6 degrees back and forth that needs to be corrected.

Some good tips were also given in that we dont recommend reusing fouled plugs either and also try runing your vacuum on the constant port. Try the hotter plug range as well.

Also check the voltage and make sure the distributor is getting full current,check the spark at the plug and ohm test the wires if needed.
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