Finally getting back to work on the car after 3 mo. the question i have is the car has a fresh rebuilt 350 with a new dragon fire distributor a holley 750dp freshly rebuilt the car starts runs great great throttle response when sitting but under a load she wants to fall on her face.could this be due to not enough vaccum advance.or is the carb???car is 73

 

depends on what your timing is set at now.

 

It's far more likely to be a carb fault than ignition.

Have you tried running it with the vacuum disconnected and plugged?

 

Statistically, that's quite the opposite.

 

I mis-read the OP, I think without more info it could easily be either.

 

There is nothing in the carburetor that changes going from no load to under load condition. There are changes in the ignition system.

If it revs up fine in neutral but has a bog or hesitation in gear, look to the ignition system.

 

1973 vacuum advance should be connected to a straight vacuum port, ie: one pulling vacuum all the time.

 

How did you set the timing? Where is it set?
What are the specs on the fresh 350? Is it stock or did you add some performance parts?
Poor performance under a load is typically not the vacuum advance, because vacuum levels drop under a load so the vac advance is inactive.

 

I think you need to rethink this statement.

 

Why would I? As I stated previously...

There is nothing in the carburetor that changes going from no load(in park/neutral) to under load(in drive/gear) condition. There are changes in the ignition system.

Ergo, if the engine revs fine in park but has a hesitation in gear, there is not a problem with the carburetor.

 

lilypad:
I had the same problem in my 68 w/holley 4160 and what I found that one of the primary jets was almost closed, I clean it and put back and the car run like it should.
Avner

 

I have to agree with Roger, the carb conditions between stationary and full load are very different. The amount of fuel an engine needs to rev cleanly on idle is a fraction of the fuel it needs under laod.

 

Then please explain to me, if you can, what is different in the carb from no load to under load.....
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The conditions may be different, but it is not as a direct result of the change.

The carburetor is basically a fuel reservoir to provide fuel to the engine. The engine is going to draw out the amount of fuel is wants, only restricted by the jets/metering rods.

I will say again, there is no direct change in the carburetor due to putting the car in gear. Having spent over 25 years remanufacturing carburetors for a my livelihood, I do know a little something about this.

But let's get back to the OP.

He has a hesitation. The only thing in the carburetor that would cause this is going to be the accelerator pump. If it's working when at a no load condition, it is going to work in an under load state.

 

I gotta agree with Roger and Golden here. Given the limited amount of information from the OP, it sounds like a fuel issue (going lean?).
An ignition issue under load (breakdown) quite often results in a backfire or erratic RPMs, rather than a smooth "fall on the face".

OP, we need more details.

 

This is not fuel.

 

Maybe Roger and I have misunderstood you.

The amount of fuel required to rev a car from say idle to 5000rpm is a fraction of the fuel needed to drive the car hard under load.

So are you suggesting that the accelerator pump, emulsion tubes/metering rods and main jets all work at maximum capacity on a stationary engine?

 

Nitronick I'm not saying you are wrong but I think the statement you made was wrong.
The OP said he had a "new dragon fire distributor a holley 750dp freshly rebuilt" . "Rebuilt"- If Lars rebuilt it your probably right the problem is in the distributor.
But we all know that it barely takes a "spit" from the accelerator pump to allow the engine to rev while sitting. A simple accelerator pump adjustment could fix this. IMO

 

First off, we really don't know the EXACT problem the OP is having. He has said "she wants to fall on her face", which in my experience of dealing with these types of problems, he means you hit the gas, it hesitates and then goes. He may mean something else, but I have a feeling that is it.

Now we're putting time into the equation. Calculate the amount of time it takes to get this car we are talking about from 0 to 5000rpm under load and the fuel it uses getting there. Now take the same car, rev and hold it at 5000rpm for the same length of time. I'll bet you use close to the same amount of fuel.
You see? The carburetor doesn't care about load or no load. It is there to provide fuel. It is a reservoir with metering facilities to say "you can only have this much".

The pump shot is the same in our instance above. At any given time, there is a finite amount of fuel in the pump reservoir. The amount used is in direct proportion to the amount of throttle travel. The reservoir will be replenished when we back off of the throttle.

The jets are a static size while in use. Same with the metering rods. The power valve is what makes adjustments in the metering rod location when the available engine vacuum changes. In our revved up engine above, the metering rods are going to be pulled out of the jets quite a bit, because we are at full throttle. The only thing varying is the amount of time they stay out.

You can make some changes in these items while the car is not running, but as I said above....the carburetor is a reservoir. Nothing changes between no load and load.

 

As I have said before. If the accelerator pump works in neutral, it is going to work in gear.
If the pump were the culprit, it would "fall on it's face" under both conditions.

 

I don't agree.
But I do agree we need more input from the OP.