Carb issues

Kagan · Mar 24, 2008

Hi,

I bought a Road Demon (525 cfm). We have put it on my 65 Mustang. I have set the float levels to below mid point and very close to the bottom line, I have set the 4 idle-mixture screws to the 1.5 turn from the closed position. I have set the idle screw after seeing a very high RPM idle when I started the car. The current situation is;

1) I have black smoke from the exhaust. (at idle and when I push the gas pedal. When I accelerate hard, there is too much black smoke)
2) The engine runs a bit rough at idle.
3) It stumbles for a moment when I push the gas pedal. (Then it starts to get better)

The car details are as follows:

289 cu in original engine
C-4 Transmission
Original intake manifold
Double exhaust with no modification (The thickness is 2 inches)
There is a spacer between the intake manifold and the carburetor
I have a electronic distributor (the brand is Proform)
I have NGK platinium spark plugs (They are brand new, we put them at the same time with the carb)

Previous carb was an Autolite 4300 and I know it was big for this setup, that's why I changed it. But I did not have any black smoke problems at idle. I just had some minor black smoke when I accelerated hard.

Please help me and get me through these problems.

jcode68 · Mar 24, 2008

A couple of things come to mind:

You should pull a couple of plugs to confirm the carb is too rich. This will show black soot(sp?) on the ceramic part of the plug.
Did you set the plug gaps properly or just install them out of the box?
Do you know what your fuel pressure is running at? What type of fuel pump are you using?
Is your vacuum advance hose connected to the timed spark port on the carb or full manifold vacuum?
Is your choke working properly?

With some of these answers, we should be able to help you hone in on the problem.

D.Hearne · Mar 24, 2008

Well you wasted your money with Platinum plugs for starters. And you don't need the idle circuits on the secondary side, you can screw the adjusters all the way in and close these circuits, that'll help the idle mixture. The stumble can be too much slack in the accellerator pump arm to lever clearance, or your timing could be retarded too much. (set the initial timing at 12* BTDC with the vacuum advance plugged, if it has this)

mrmustangman357 · Mar 24, 2008

when you ease into the gas or at cruise, does it have black smoke? Sounds like the carb might have a power valve in it set too high (or blown) but check the other stuff first.

mrmustangman357 · Mar 24, 2008

does gas drip out the boosters or accelerator circuit at idle?

Kagan · Mar 25, 2008

Hi,

1) I will go to my mechanic tomorrow and will check the timing.

2) There is no problem with the spark plugs. My mechanic checked them.

3) The fuel pump is an original one, no modification. It was working with no problem before I changed the carb.

4) Vacuum advance (If I know correctly, it is coming from the distributor) is connected to the carb.

5) How do I check whether the choke is working or not?

6) I will shut the secondaries. But how do I check the accellerator pump arm to lever clearance?

7) While cruising I dont know whether I have black smoke or not for sure. If I have it, it cannot be seen fromö the side mirrors. But I see smoke when I push the gas pedal. I will check this one too. How will I check the power valve?

8) I did not see any gas dripping out. We checked these.

woodsnake · Mar 25, 2008

Does your carb have a manual or electric choke?
If it is a manual choke, and it is warm weather, you should leave the choke plate wide open.
Even if the vacuum line is connected from the carb to the distributor,
Is the vacuum advance working? I.E. can you get your advance to hold vacuum?
is the actual vacuum hose itself in good condition, not stretched,cracked, or dry rotted?
Was there an instruction booklet that came with the carb with instructions about 'ported' or manifold vacuum for the vacuum advance?
Do you know when your timing gear set was last replaced, or is it original to the car?

woodsnake · Mar 25, 2008

I found this info for demon tuning......

http://www.barrygrant.com/fromBarryGrant/Demon Instruction Manuals/King Demon Manual.pdf

D.Hearne · Mar 25, 2008

The vacuum advance needs to be connected to the nipple on the side of the meterting block, not at the bottom of the carb.(or at least that's where it goes on a Holley, I assume the Demon is the same) There's an adjuster screw and nut where the pump lever contacts the accellerator pump arm to take the slack out of the linkage. Too much slack lets the throttle blades open without having the pump add fuel to the mixture, this creates a lean stumble when you hit the pedal. When I said you wasted money on the platinum plugs, I didn't mean there was something wrong with em, only you bought plugs you didn't need, std plugs are fine for what you're running.

Kagan · Mar 25, 2008

I checked the link. It was useful. The vacuum is connected to the side of the carb.

By that adjuster screw; there is one nut and bolt on the drivers side, near the primaries side. I can push it down and the idle kind of decreases. It is also somehow connected to the point where the throttle linkage connects to the carb. Do you mean that nut and bolt?

On the other hand, about the plugs, if they fire it up better than the originals why not use them?

About the vacuum related questions; I dont know when the timing gear set has been changed. There is no problem with the hoses. But the way, the carb is electric choke. The vacuum advance is working. There was no booklet about 'ported' or manifold vacuum for the vacuum advance. If you tell me what to look for, what to check I will do it tomorrow first thing.

D.Hearne · Mar 25, 2008

Kagan said:
I checked the link. It was useful. The vacuum is connected to the side of the carb.

By that adjuster screw; there is one nut and bolt on the drivers side, near the primaries side. I can push it down and the idle kind of decreases. It is also somehow connected to the point where the throttle linkage connects to the carb. Do you mean that nut and bolt?

On the other hand, about the plugs, if they fire it up better than the originals why not use them?

About the vacuum related questions; I dont know when the timing gear set has been changed. There is no problem with the hoses. But the way, the carb is electric choke. The vacuum advance is working. There was no booklet about 'ported' or manifold vacuum for the vacuum advance. If you tell me what to look for, what to check I will do it tomorrow first thing.

That's the screw and nut I was talking about. There should be no more than .010 clearance between the pump arm and the lever the screw is mounted on. In otherwords, the instant the throttle shaft moves, there should be fuel squirted out the nozzles in the center of the throttle bores, under the choke plate. Platinum plugs are meant to be used with the latest electronic ignition systems which fire twice in each combustion cycle, there's no benefit using them on other systems. Just trying to save you money here.

Kagan · Mar 26, 2008

Hi,

I am back from the mechanic. We have done everything you told. Now it is not giving black smoke at idle or when I push the pedal down. Everything seems ok now. Thank you all very much for your help.

Regards,

Kagan

D.Hearne · Mar 26, 2008

Kagan said:
Hi,

I am back from the mechanic. We have done everything you told. Now it is not giving black smoke at idle or when I push the pedal down. Everything seems ok now. Thank you all very much for your help.

Regards,

Kagan

Glad to help our Turkish brothers "across the pond"

Kagan · Mar 27, 2008

D.Hearne said:
Glad to help our Turkish brothers "across the pond"

hehee. Thanks. I will be back with more questions later

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