attention cuda66273

[plumebody]

Hello, I'm having trouble with my holley carb and was wondering if you could give me some advice? Carb is a new 750 hp vaccum secondary, has 4 corner idle, down leg boosters, ect. Motor is a 383, 9.3 to 1, mp, 509 cam, weiand dual plane, 1 3/4 tti headers and full 2 1/2 exhaust. Converter is a 2800 stall unit and 3.91 sure grip out back. Car is a 1970 challenger and is all steel. The car has a serious hesitation at launch. It almost stalls out and then comes alive and goes great. The plugs are black as can be. I'm currently at 74f and 82r for jetting. Vaccum in gear is 5hg and I installed a 3.5 pv. I did try bigger shooters and it didn't help. Next I thought about the pump cams. What do you think? Thanks in advance for your time. P.S. I did read the sticky up top and it helped alote . Good work.

[cuda66273]

Whats the cam duration at 50?
Whats the initial timing at Idle?
Do you have the pink pump cams on it?
Have you adjusted the pumps to the .015 spec?
Did you use the paper setting for the idle speed screws?
Where are the floats set?
How much fuel pressure have you got at the carb?
Is fuel dumping from the vents?
What pump are you using?
Are you using a pressure regulator? Whose?


Let's get you going...

[cuda66273]

Go back to the original squirters...that's not the problem.

[plumebody]

Thanks for the response. Duration @50 is 248. The timing is set at 36 degrees, full advance. No vaccum advance. I have the clear pump cam on currently. This is the stock piece. Yes, I have used feeler gauges to .015 setting. Idle speed was ajusted with a vaccum gauge. Turned them till the highest reading then when it dropped off, turned it back a little. Floats are set at the bottom of the sight plug. Fuel pressure is unknown as I don't have a gauge. Its a carter mechanical street/strip pump. No regulator, I think this was factory set for 7 psi. Fuel is not currently dumping at the vents. Although at one point, when I loosened the nut for ajusting the floats, it did happen. I think that was because the floats were to high though.

[cuda66273]

What is the initial timing at idle?

[cuda66273]

Stay with me here

[cuda66273]

If your not gonna stay around I can't help you...now I gotta go I'll check back here later.....

I want you to re-read the Demon tuning post on adjusting the 4 corner idle screws. You are using the Eddey carb method and it won't work on a 4 corner system.

You may also want to bring your initial up to about 18-20 degrees and see if the throttle response picks up...don't drive it just see how it repsonds to you cracking the throttle.

You can also put your fingers over two of the air bleed holes at a time and see if it want to stall out or pick up.

The black soot tells me your way to rich on the idle circuits and you need to get off the transition slots or open up the air bleed circuit.

The manuifold vacuum seems real low...I have a similar duration and mines up around 7-9 are you checking it at idle in gear?

[cuda66273]

I see your back....I got a couple of minutes

[451Mopar]

Quick question, Have you tried running without the aircleaner?
If the air filter top is very close to the top of the carb it can mess with the bowl venting.

More initial advance like cuda66273 mentiond is almost needed with a 383 running a big cam.

Have you re-checked the float bowl level?
If the float level was too high, you need to adjust the float level down and use up the extra fuel in the bowls, then slowely re-adjust the float level back up to the bottom of the sight plug hole.

[cuda66273]

As I just said to Mrs Cuda...if these New Yorkers could type as fast as they talk that car would be down the road smoking the tires off itself and I'd be done in the shower and working on the trim on her new countertops

[plumebody]

I guess I don't know how to check for initial advance. Can someone help me with that? I thought if I set my timing at 36* all in at 2500 rpms that was it. Is the initial advance idle to 2500 in my case? Yes, vaccum was taken in gear. Yes I did try with out the air cleaner, no change.

[cuda66273]

Your initial advance is your at idle setting...so start her up and let it idle...if you have a dil type timing light turn the knob untill it reads zero on the crank...look at the dial and that is your initial.

If your using a regular type timing light read your timing tab and tell me what the advance is at according to the tab

[cuda66273]

This is a perfect arguement for having your distributor curved on a machine by a professional.

[451Mopar]

I think I run about 16 degrees initial timming which still makes the engine start up pretty easy, and limit the total timming to about 34 degrees, so the advance in the plate would be about 18 degrees (the less advance in the plate, the more initial advance you can run.) Sometimes the amount of advance is marked on the plate, but in distribitor degrees which is 1/2 the crank degrees, so a 10 marked on the plate would be 20 degrees at the crank.

[BakedMopar]

I have this same problem and I tried two different carbs with the same results. I am looking foward to a racer brown cam. I hear that they can make just as much power and better drivablilty. Cuda66273 also says that they are very high quality camshafts.

[gsmopar]

Sorry to interrupt, but I have to say that Cuda is one hell of a guy to help us out like he does! If you ever visit AZ the first round is on me!

Later,

Greg

[451Mopar]

Cuda, How Much to curve a MP electronic distribitor?
I don't know anyone with a working distribitor machine around here so I have been using the Mallory Unilite and Accell Billit Proof distrobitors because they are easy to change the advance on (compare the the MP distribitor.)

[cuda66273]

Thanks Guy's
I try and help as much as time allows....sorry I've not been around for a couple of days but duty's at the shop have been keeping up late and at it early and i just haven't had time to get to my email.

The amount of initial advance in a distributor is directly related to cylinder pressure at idle, carb type, manifold...etc.

As we all know the more duration in a cam the lower the cylinder pressure at idle due to the overlap of the intake and exhaust valves. The lower the pressure the less violent the explosion or burn in the combustion chamber so it simply takes longer to burn all the fuel so we need to ignite it sooner in order to keep a clean combustion chamber.....more advance.

Other contributors are the intake manifold design and it's ability to cleanly deliver atomized fuel to the valve without causing puddles or droplets to form, as these droplets are difficult to ignite and puddles of fuel are drawn into the CC as the flow increases but can't be atomized or mixed with O2 as this is done by the carb....result is a puff of black smoke and stumble as you start to accelerate into the intermediate circuits or transition and often misdiagnosed as a carb malfunction.

Carb selection is critical on our warmed up Mopars, the carb reads a manifold vacuum signal created in the manifold by the duration of the cam. If you've ever put a vacuum gauge on the vacuum port and watched the needle as the engine idles you'll see it bounces around 2-3 inches, this is actually changing the air pressure differential between the above and below throttle plate pressures. A stock type carb reads this as a variable amount of fuel required and systematically dumps and shuts off the fuel flow as the vacuum changes increasing the problem and eventually "Loads" up the manifold with unburned fuel puddles and droplets and subsequently stalling the engine...we've all seen or experienced this condition I'm sure.

So how do you get the right tune up?

1. Select the correct carb, it must be able to read the vacuum signal correctly and be able to respond correctly to the requirements of the engine at idle and also be able to repond to fuel and air delivery requirements at the rate the engine picks up RPM.

2. Enough initial timing to burn the fuel at lowered combustion chamber pressures at idle and be able to read the somewhat erratic idle speeds.

3. Timing curve must come in at a point when the compression ratio, engine speed and load can burn the fuel efficiently without pre-ignition or “Ping”.

4. Fuel octane level must be able to remain stable at the cylinder pressures created by the cam as they build through the RPM cycle.

5. The spark plug must be cold enough not to retain heat to the point of becoming a glow plug and cause pre-ignition.

6. Fuel delivery must be adequate to supply the correct amount of pounds of fuel to support the HP and Tork the engine is developing (BSFC) Brake Specific Fuel Consumption.

7. Clean combustion chambers, this includes carbon buildup and proper preparation of the head casting to eliminate casting roughness and possible hot spots.

8. General ignition system condition, correct phasing of the rotor, good plug wires routed correctly, the correct cap and rotor (does yours have copper contacts?) a good ignition box (is yours orange?) and of course the ability to make it right and keep it right....that, hopefully I will be able to help you with.

Of course I'm sure there's lot's more opinions and advice to follow read, learn and remember.....

Got a specific problem...my email is in my signature line.

Need some advice on some parts or component changes..the calls always free......

Someday I will write that book ...LOL
D