Technical Too much advance too soon (vacuum)

Carb and distributor are set and idle is fine, dist. advance is ok but when giving gas (fast) vacuum makes too much advance and engine suffocates (wrong word but you get the idea).
What´s the cure?
It still has original dist with points.
-54 Chevy 235

 

If the engine dies when the accelerator pedal is pushed down, then vacuum advance is not the problem. The vacuum advance only works as RPM's increase and there is vacuum from the engine. Not the case when the pedal is first pushed down, there is no vacuum at that point.

Perhaps accelerator pump.

 

When giving gas I can see the whole dist rotate anticlockwise a lot (and it´s not loose).
When I use my other hand to prevent the dist not to move, engine revs better.

 

I must be missing some thing but the whole dist. should never turn when properly bolted down.

 

That doesn't make any sense, the distributor should not move
so let's get some basics here.
What are you working on?
Do you have a vacuum guage?
Do you have a timing light?
Have you done a piston stop test and verified your timing marks?

 

Here, have a look. Not my engine but they are all like that.

 

1954 Chevy 235 Stovebolt.
Yes, yes, yes and yes.

 

Here is an other one. It´s like it should be, vacuum is external on these engines.

 

Can you see a shot of fuel when you operate the carb? Some of these early inline carbs use a check ball in the "shooter" down inside the carb. They're prone to sticking and also make cold starts difficult. Also, is that advancing or retarding on a rev? Some will retard to avoid excess timing when loaded (more throttle) and then it comes back as vacuum increases.

 

The distributor is designed to rotate, as described by Bummer.
Perhaps the source of the vacuum connection is incorrect, it should be connected, at carb base.
NOT at the intake manifold fitting.

 

By "shot" do you mean the burst or spray of gas from the accelerator pump?

 

Yes, it´s in the carb on its original fitting.

 

My money is then on a faulty accelerator pump, maybe thanks to Ethanol fuel.
You may want to check the flyweight springs, as well, for a lack of tension.

 

My next question was supposed to be about the springs but you beat me to it. I don´t think it´s the pump, springs may well be faulty. Is there a way to measure the tension (prppably not) and are those sold somewhere?

 

I don't know of a "gauge reading" for the springs, but will get a part number for you, on the springs.

Stick or Powerglide ?

 

Jack up the factory timing setting a few more degrees and be sure the squirter in the carb is working and has a healthy squirt. My '49 cad's factory setting is 5° @ idle, but it runs a lot better at 10°.

 

As I discovered, on the '54, it makes no difference, whether stick or P.G.
GM Part # for springs are 1906766, quan***y of 2 needed.
OVERALL FREE LENGTH of spring is .729 inches.
One size does NOT fit all.

 

Sounds like the accelerator pump isn't working. While your at it.Hookup a mighty vac to your vacuum advance can with dist cap off . Make sure all is working well. Vacuum cans are cheap , the last 2 numbers of the part number will tell you how much degrees that can will pull.

 

I'm sorry I ***umed it was a newer small block. I should have known not to do that.

 

Thanks Bob.


Sent from my iPhone using The H.A.M.B. mobile app

 

You are welcome, but not finding the springs online.

 

99% chance the inside of the distributor is fine. We set the timing by advancing until spark knock is heard, then back off until barely audible under full throttle.
Looks like you gave up on the Zenith swap ... the Rochester most likely has slop in the pump linkage, shrunken leather on the pump piston, scored bore in pump barrel, or all the above.
You still need to adjust the valves ...

 

I did not give up on the Zenith, I just thought I´d better to fix this first "just in case".

Just came back from the garage after removing and opening the carb.
Seems like (as you noticed and mentioned) looks like it´s the acc. pump leather. Seems like it doesn´t pump as it should. Very weak and occasional.
Other thing I noticed; fuel level seems too low. Where abouts should it be? I try to post a video clip on how it behaves...

Quick notice; There is small valve on the acc. pump. I can hear it rattle while shaken but can it be the problem? If it doesn´t open and close properly then the pump can´t do what it should, pump. Right? When pump is manually pushed, it doesn´t give proper spray.

 

Here´s something that confuses me.
In the pic there is a hole in the gasket to the rail where the fuel should spray from while acceleration, right?.
Mine hasn´t, there is a hole beside it. How can acc. pump work when there is no way for the fuel to go to the right place??

Carb by Bummer posted Jan 5, 2017 at 3:43 PM

 

Stick your finger over the big ball & spring & give the pump a shot .. see the fuel shoot from the side of the venturi at 11 o]clock in pic "A"? There isn't a separate discharge nozzle on a Rochester B.

 

So it´s not even meant to go to that rail? Are there holes on the wall of the venturi where it´s supposed to spray?

What´s the purpose of that rail then?

 

Ok, time to move on.
Got the old squirter working (put back the old acc pump and checked the levels) and also checked the timing/advance, now it sounds and revs ok.

 

Good for you. Sounds like it was the accell pump circuit afterall. Have fun...

 

Pretty much combination of 4;
acc pumps valve was stuck, so it did not make enough pressure to pump properly) the tiny hole where it should spray was clogged, octane selector wasn´t advanced enough (!) and faulty gasket. Happily, I had a spare one still.